CBSE Class 9 Chemistry Structure Of Atom Worksheet

Assertion & Reasoning Based MCQs

For question numbers 51-60, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Both assertion and reason are true, and reason is correct explanation of the assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Assertion is false, but reason is true.

Question. Assertion : Cathode rays get deflected towards the positive plate of electric field.
Reason : Cathode rays consist of negatively charged particles known as electrons.
Answer. A

Question. Assertion : Anions are larger in size than the parent atom.
Reason : In an anion, the number of protons in the nucleus is less than the number of electrons moving around it.
Answer. A

Question. Assertion : Thomson’s atomic model is known as ‘raisin pudding’ model.
Reason : The atom is visualized as a pudding of positive charge with electrons (raisins) embedded in it.
Answer. A

Question. Assertion : The number of electrons gained, lost or shared by the atom of an element so as to complete its octet is called the valency of the element.
Reason : Elements having the same number of valence electrons in their atoms possess different chemical properties.
Answer. C

Question. Assertion : For noble gases, valency is zero.
Reason : Noble gases have 8 valence electrons.
Answer. A

Question. Assertion : The size of the nucleus is very small as compared to the size of the atom.
Reason : The electrons revolve around the nucleus of the atom.
Answer. B

Question. Assertion : Electrons moving in the same orbit will not lose or gain energy.
Reason : On jumping from higher to lower energy level, the electron will gain energy.
Answer. C

Question. Assertion : Bohr’s orbits are called stationary orbits.
Reason : Electrons remain stationary in these orbits for sometime.
Answer. C

Question. Assertion : The distribution of electrons in different orbits or shells is governed by a scheme known as Bohr-Bury scheme.
Reason : Electrons are filled in the shells in a stepwise manner in increasing order of energy of the energy shell.
Answer. B

Question. Assertion : In Rutherford’s gold foil experiment, very few a-particles are deflected back.
Reason : The size of the nucleus is very small as compared to the size of the atom
Answer. B

Exam Questions NCERT Class 9 Science Chapter 4 Structure of the Atom

Question. Give the postulates of Dalton’s atomic theory.
Answer.
(i) Every element is composed of extremely small particles called atoms.
(ii) Atoms of a given element are identical, both in mass and properties. Different chemical elements have different kinds of atoms; in particular, their atoms have different masses.
(iii) Atoms cannot be created, destroyed or transformed into atoms of other elements.
(iv) Compounds are formed when atoms of different lements combine with each other in small whole number ratios.
(v) The relative number and kinds of atoms in a given compound are constant.

Question. Give the mass numbers of A and B, What is the relation between the two species?
Answer. Mass number of A = No. of protons + No. of neutrons = 6 + 6 = 12u
Mass number of B = No. of protons + No. of neutrons = 6 + 8 = 14u
The species A and B are isotopes, as they have same atomic number but different mass number.

Question. Name the three subatomic particles of an atom.
Answer. (i) Electrons (negatively charged particles) which revolve around the nucleus.
(ii) Protons (positively charged particles) which are present in the nucleus.
(iii) Neutrons (having no charge) which are present in the nucleus.

Question. Give four characteristics of isotopes.
Answer. All isotopes of an element consist of the same number of protons inside their nuclei. Hence, they have the same atomic number.
(i) All isotopes of an element consist of different number of neutrons in their nuclei. Hence, they have different mass number.
(ii) All isotopes of an element give identical chemical reactions.
(iii) Isotopes of an element have same electronic configuration.

Question. Write the conclusions drawn by Rutherford for the following observation during his scattering experiment :
(i) Most of the alpha-particles passed straight through the gold foil.
(ii) Some alpha-particles getting deflected from their path.
(iii) Very small fraction of alpha-particles getting deflected by 180°.
Answer.
(i) Most of the space inside the atom is empty.
(ii) It indicates that the positive charge of the atom occupies a very little space.
(iii) All the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

Question. What is the difference between Rutherford’s atomic model and Thomson’s atomic model?
Answer. Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits.
There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nuclei. Thomson proposed the model of an atom to be similar to a christmas pudding. The electrons are studded like currants in a positively charged sphere like Christmas pudding and the mass of the atom wassupposed to be uniformly distributed.

Question. Describe the essential properties of the atomic nucleus. Compare these properties with the properties of electron.
Answer. Nucleus is small positively charged centre located in a very small space. An electron is a very small negatively charged particle with well established charge to mass ratio. The charge on electron forms the smallest unit of charge on atomic particles.

Question. Which is much closer to the nucleus of an atom out of K and L shells?
Answer. K shell is much closer to the nucleus of an atom.

Question. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?
Answer. Isobars.

Question. Name the radioisotope used for examining the circulation of blood in the body.
Answer. Na-24 is the radioisotope used for examining the circulation of blood in the body.

Question. Who discovered proton?
Answer. Goldstein discovered proton.

Question. Name one element, the nucleus of which does not have any neutron.
Answer. Hydrogen.

Question. Write the names of three elementary particles which constitute an atom.
Answer. Electron, proton and neutron.

Question. Which study led to the conclusion that atoms are not indivisible?
Answer. Study of static electricity and the condition under which electricity is conducted by different substances led to the conclusion that atoms are not indivisible.

Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom? What is the valency of this element? Name the element.
Answer. The maximum numbers of electrons that can occupy K and L shells of an atom are 2 and 8 respectively. Therefore, if K and L shells of an atom are full then the total number of electrons in the atom would be 2 + 8 = 10 electrons. So, the valency of this element is zero. The element is neon (Ne).

Question. State the charge and mass of a neutron.
Answer.Neutron has no charge and its mass is equal to that of a proton.
(i) Most of the space inside the atom is empty because most of the a -particles passed through the gold foil without getting deflected.
(ii) Very few particles are deflected from their path, indicating that positive charge of the atom occupies very little space.
(iii) A very small fraction of particles was deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a small volume within the atom.

Question. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? And (ii) what is the charge on the atom?
Answer. (i) Atomic number = Number of protons = 8
(ii) The charge of the atom is zero, as total numbern of positive charge is equal to total number of negative charge. Number of protons = Number of electrons 8 = 8

Question. What is the limitation of J.J. Thomson’s model of an atom?
Answer. The major limitation of J.J. Thomson’s model is that it does not explain how positively charged particles are shielded from negatively charged particles, without getting neutralized.

Question. Name an element which has one electron, one proton and no neutron.
Answer. Hydrogen atom (1H1) has one electron, one proton and no neutron.

Question. Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Answer. Two.

Question. From the symbol 16S32, give :
(i) Atomic number of sulphur
(ii) Mass number of sulphur
(iii) Electronic configuration of sulphur
(iv) Which of the two elements given would be chemically more reactive? S, Ar
Answer.
(i) 16
(ii) 32
(iii) Electronic configuration : 2, 8, 6.
(iv) Element S, having atomic number 16 is chemically more reactive than element Ar of atomic number 18. It is because the outermost shell of the atom of element S has six electrons only and has to complete its octet, whereas the outermost-shell of the atom of element Ar is completely filled up, i.e., its octet is complete and thus it shows little
chemical activity.

Very Short Answer Type Question (VSA)

Question. Atomic number is defined in terms of protons and not in terms of electrons. Why?
Answer. An atom may lose or gain electrons, but the number of protons remains constant till the same atom exist.

Question. What will be the charge on an atom with mass number one and atomic number one?
Answer. The atom will not carry any charge because the atom contains one unit negative charge in the form of electron and one unit positive charge in the form of a proton.

Question. In Rutherford’s model of an atom, fast moving alpha (a)-particles were made to fall on a thin gold foil. State two properties of a-particles.
Answer. (a) Alpha particles are positively charged particles.
(b) They are doubly charged helium ions having a mass number of 4 (consists of 2 protons and 2 neutrons).

Question. What characteristic feature is seen in the configurations of chemically inactive elements?
Answer. Chemically inactive elements have 8 electrons in their valence shell except helium which has 2 electrons in its valence shell which is the maximum capacity of K shell.

Question. What would you conclude from the observation that cathode rays rotate a light paddle wheel placed in their path?
Answer. When a light paddle wheel is placed in the path of cathode rays, the blades of the paddle wheel begin to rotate.
It shows that cathode rays consist of material particles having mass and velocity.

Question. An oxide of an element Z has a formula Z2O₃.
(a) How many electrons are there in the outermost shell of an atom of element Z?
(b) Write down the formula for the chloride of Z.
Answer. (a) An ion of element Z has the formula Z3+. Hence, it has 3 valence electrons.
(b) ZCl₃

Question. Neutrons can be found in all atomic nuclei except in one case. Which is this atomic nucleus and what does it consist of?
Answer. In case of hydrogen atom, there is no neutron. It consists of only one proton.

Question. Find valencies of the elements having atomic numbers 10 and 15.
Answer. Atomic number = 10        Atomic number = 15
Electronic configuration                  Electronic configuration
= 2, 8                                          = 2, 8, 5
Valency = 0                                  Valency = 3

Question. Why are anode rays also called canal rays?
Answer. Canal rays are positively charged anode rays. The canal rays are called so because they pass through the holes or the canals in the cathode.

Short Answer Type Question

Question. (a) If the number of electrons in an ion is
Answer. (a) (i) Atomic number (Z) = No. of protons = 9
(ii) Charge on the ion = –1
Here, one electron is more than proton. So this one extra electron attains –1 charge on the ion.
(b) No. of electrons in M²⁺ ion = 10
Atomic number of atom M = 10 + 2 = 12
No. of protons in atom M = 12
Mass number of atom M = No. of protons + No. of neutrons
= 12 + 12 = 24
The element M with atomic number 12 is magnesium (Mg).

Question. The atom of an element has 9 protons, 9 electrons and 10 neutrons.
(a) What is the atomic number of the element?
(b) What is the mass number of the element?
(c) Name the element and give its electronic configuration.
(d) Predict the valency of the element.
Answer. (a) The atomic no. of element = No. of protons = 9
(b) The mass no. of element = No. of protons + No. of neutrons = 9 + 10 = 19
(c) The element with Z = 9 is fluorine (F). Its electronic configuration : 2, 7.
(d) The valency of fluorine is 1 and is calculated as 8 – 7 = 1.

Question. Element X has a proton number of 7. It also has seven neutrons.
(a) Deduce the number of electrons and the nucleon number of X.
(b) Represent X by writing the chemical symbol, including the proton and nucleon numbers.
Answer. p = 7, n = 7
(a) Number of electrons = Number of protons = 7
Number of nucleons = number of p + number of n
= 7 + 7 = 14
(b) ¹⁴₇X

Question. Justify the statement ‘atomic number of an element is equal to the number of electrons in a neutral atom only and not in anion’.
Answer. In neutral atom, No. of protons = No. of electrons
= Atomic number
An anion is formed by gain of one or more electrons by an atom. Therefore, anion contains more electrons than neutral atom or in other words number of electrons in anion is greater than atomic number (the number of protons).

Question. Give two points of differences between isotopes and isobars.
Answer. Differences between isotopes and isobars:
Isotopes                                                         Isobars
(i) These are the atoms of               (i) These are the atoms of different elements.
the same element.
(ii) These have same atomic           (ii) These have different atomic numbers.
number.
(iii) These have different                 (iii) These have same mass numbers.
mass numbers.
(iv) The chemical properties           (iv) These have different chemical and physical properties.
of isotopes are similar but
their physical properties
are different.

Question. An element has two electrons in N-shell.
Identify the element.
Answer. An element can be identified with its atomic number (Z) which is equal to the number of electrons in its neutral atom.If there are 2 electrons in N shell, it means K, L and M shells are completely filled. Two electrons can be accommodated in K shell, eight in L shell and 8 in M shell since outermost orbit cannot have more than 8 electrons, 2 electrons go to N orbit.
Hence total number of electrons = 20
K L M N
2 8 8 2
The element is calcium.

Question. Justify the given statements :
(a) Most of the space in an atom is empty.
(b) The elements are identified by their atomic numbers and not by their mass numbers.
Answer. (a) This can be justified by Rutherford’s a-scattering experiment. Since most of the alpha particles could pass through the sheet, made up of atoms of gold, undeflected,
this means that they did not come across any obstruction.
Thus, most of the space in an atom is empty or hollow.
(b) In the study of the atomic structure, we have seen that the isotopes of an element have different mass numbers. In isobars, the atoms of different elements have same mass
numbers. However, the atomic numbers of no two elements can be the same. Therefore, the elements are identified by their atomic numbers and not by their mass numbers.

Short Answer Type Question

Question. (a) Electronic configuration of a neutral atom ‘X’ is 2, 8, 6. What is the electronic configuration of X^2– ?
(b) What is a valence shell? How many electrons can be present in valence shell?
Answer. (a) X = 2, 8, 6
No. of electrons in neutral atom = 2 + 8 + 6 = 16
X + 2e^– → X^2–
No. of electrons in X^2– = 16 + 2 = 18
Electronic configuration of X²– = 2, 8, 8
(b) The outermost shell of an atom is called valence shell.
The number of electrons that can be present in valence shell is 1 - 8.

Question. Atoms of elements R, S and T have 8, 9 and 11 protons respectively. Neon has 10 protons.
(a) What is the chemical formula of the compound formed between?
(i) R and T (ii) S and T
(b) What is the formula of a molecule of R?
Answer. R(p = 8), Electronic configuration = 2, 6
S(p = 9), Electronic configuration = 2, 7
T(p = 11), Electronic configuration = 2, 8, 1
(a) (i) RT₂ (ii) ST
(b) R₂

Question. (a) Which fact is proved by the following observation in Rutherford’s scattering experiment ‘very few alpha particles are deflected back’?
(b) How will you find the valency of nitrogen,oxygen and fluorine?
Answer. (a) This shows that in the centre of atom a very small positively charged body called nucleus is present.
(b) (i) Nitrogen has 5 electrons in valence shell, hence its valency is 8 – 5 = 3.
(ii) Oxygen has 6 electrons in valence shell, hence its valency is 8 – 6 = 2.
(iii) Fluorine has 7 electrons in valence shell, hence its valency is 8 – 7 = 1.

Question. (a) Explain, why ³₂He and ³₁H are not considered isotopes.
(b) What are octet and duplet rules? How do elements attain octet?
Answer. 
(a) 3/2He and 3/1H are not considered as isotopes because they have different atomic numbers and are different elements.
(b) Octet rule was proposed by G.N.Lewis. According to this rule “The atom of an element combines with another atom to have eight electrons in its outermost shell”. An atom having 8 electrons in its outermost shell is least reactive or most stable. If there is only one shell, then stability is attained by having 2 electrons in the shell and this is called duplet rule.
Element attains octet in the following ways :
(i) by losing or gaining electrons.
(ii) by sharing electrons with other atoms.

Question. (a) On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
(b) What are canal rays?
Answer. (a) According to Thomson’s model of an atom :
(i) An atom consists of a positively charged sphere and the electrons are embedded like the seeds in a watermelon.
(ii) The negative and positive charges are equal in magnitude.
So, the atom as a whole is electrically neutral.
(b) The beam of rays which travel in a direction away from anode towards cathode when a gas taken in a discharge tube is subjected to the action of high voltage under low pressure are known as canal rays. It is also called anode rays. It was discovered by E. Goldstein in 1886.

Question. For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer. (a) F : Because it was not proposed by J.J. Thomson.
(b) F : Because neutron is an independent sub-atomic particle.
(c) T : Because it is a fact known from experiments.
(d) F : Because tincture iodine is a solution of ordinary iodine in alcohol.

Question. (a) Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
(b) Write the distribution of electrons in carbon and sodium atoms.
Answer. 
(a) Mass number of helium = 4
Number of protons = 2
Number of neutrons (n)
= Mass number (A) – No. of protons (p)
= 4 – 2 = 2
Thus, no. of neutrons = 2
(b) Atomic number of carbon = 6
Hence first shell (K-shell) have 2 electrons and the remaining 4 electrons will be present in the second shell, i.e. L-shell.
Thus the distribution will be
K L
2 4
Atomic number of sodium = 11. Hence, first shell (K-shell) will have 2 electrons and second shell (L-shell) will have 8 electrons and third shell (M-shell) will have 1 electron. Thus, the distribution will be
K L M
2 8 1

Question. Summarise the rules for writing the distribution of electrons in various shells for the first eighteen elements.
Answer. 
The distribution of elements in different orbits is governed by a scheme called Bohr-Bury scheme. There are following rules :
(i) The maximum number of electrons present in any shell is given by the formula 2n2. Where n = no. of orbit.
(ii) The maximum number of electrons that can be accommodated in the outermost shell is 8.
(iii) Electrons in an atom do not occupy a new shell unless all the inner shells are completely filled.

Question. (a) What is the number of valence electrons in the atom of an element A having atomic number 20? Name the valence shell of this atom.
(b) The atom of an element has 9 protons, 9 electrons and 10 neutrons.
(i) What is the atomic number of the element?
(ii) What is the mass number of the element?
(iii) Name the element and give its electronic configuration.
(iv) Predict the valency of the element.
Answer. 
(a) The electronic configuration of element A is
K L M N
2 8 8 2
Therefore, the N shell is the outermost shell or the valence shell. Number of valence electrons
= number of electrons in the outermost shell
= 2.
(b) (i) The atomic no. of element = No. of protons = 9
(ii) The mass no. of element
= No. of protons + No. of neutrons
= 9 + 10 = 19
(iii) The element with Z = 9 is fluorine (F).
Its electronic configuration : 2, 7.
(iv) The valency of fluorine is 1 and is calculated as 8 – 7 = 1.

Question. Use the information to answer the following questions :
Element P Q R S T U V
Proton number 7 8 10 12 15 18 19
(a) Which of these elements have only four filled electron shells?
(b) Which of these elements have a complete outermost shell?
(c) Which of these elements have 5 valence electrons?
(d) Which of these elements have 6 valence electrons?
(e) Which of these elements have 2 valence electrons?
(f) Write the valencies of each of the elements.
Answer. 
(a) V(2, 8, 8, 1) (b) R(2, 8) and U(2, 8, 8)
(c) P(2, 5) and T(2, 8, 5) (d) Q(2, 6)
(e) S(2, 8, 2)
(f) P(3), Q(2), R(0), S(2), T(3), U(0), V(1)

Question. (a) Explain the formation of a cation. Give its main characteristics.
(b) What is electronic configuration of Al³⁺?
Answer. 
(a)A cation is formed when an atom loses one or more than one electrons from valence shell.
M → M⁺ + e^–
For example,
Na → Na⁺ + 1e^–
Mg → Mg²⁺ + 2e^–
Characteristics of cations :
(i) Cations are positively charged.
(ii) Cations are formed when an atom loses electrons from its valence shell to attain octet.
(iii) Cations are smaller in size than parent atom.
(iv) The charge acquired by a cation is equal to the number of electrons lost by the valence shell.
(b)Atomic number of aluminium is 13 and its electronic configuration is 2, 8, 3. Al³⁺ is formed by removing 3 electrons from aluminium atom.
Al – 3e^– → Al³⁺
Hence, electronic configuration of Al³⁺ is 2, 8.

Question. How will you find the valency of chlorine, sulphur and magnesium?
Answer. Valency of an atom is the number of electrons gained,lost or shared so as to complete the octet of electrons in the valence shell.
Valency of chlorine: It has electronic configuration = 2, 8, 7
Thus, one electron is gained to complete its octet and so its valency is 1.
Valency of sulphur: It has electronic configuration = 2, 8, 6
Thus, two electrons are gained to complete its octet and hence its valency = 2
Valency of magnesium : It has electronic configuration = 2, 8, 2
Thus, it can lose two electrons to attain octet and hence its valency = 2

Long Answer Type Question

Question. Observe the given figure and answer the questions that follow :

(a) Which experiment is represented by the given figure?
(b) List three observations of this experiment.
(c) State conclusions drawn from each observation of this experiment.
(d) Write the features of the nuclear model of an atom.
(e) What were the drawbacks of this model of an atom?
Answer. (a) Rutherford’s a-particle scattering experiment
(b) Rutherford observed that :
(i) Most of the a-particles (nearly 99%) passed through the gold foil undeflected.
(ii) Some of the a-particles were deflected by small angles.
(iii) A very few a-particles (1 in 12,000) were either deflected by very large angles or were actually reflected back along their path.
(c) Rutherford explained his observation as follows :
(i) Since most of the a-particles passed through the foil undeflected, it indicates that the most of the space in an atom is empty.
(ii) a-Particles being positively charged and having considerable mass, could be deflected only by some heavy, positively charged centre. The small angle of deflection of a-particles indicated the presence of a heavy positive centre in the atom. Rutherford named this positive centre as nucleus.
(iii) a-Particles which make head-on collision with heavy positive centre are deflected through large angles. Since

Question. Give reasons for the following :
(a) Isotopes of an element are chemically similar.
(b) An atom is electrically neutral.
(c) Noble gases show least reactivity.
(d) Nucleus of an atom is heavy and positively charged.
(e) Ions are more stable than atoms.
Answer. (a) Isotopes of an element have same atomic number as well as electronic configuration. Since the chemical properties of elements are related to their electronic configuration, i.e., the elements with similar configuration will have similar properties. Thus, the isotopes of an element are chemically similar.
(b) In an atom, the number of protons in the nucleus is equal to the number of electrons in the extra-nuclear portion. Since each proton and each electron has the same charge but with opposite magnitude, the atom is electrically neutral.
(c) The atoms of noble gas elements have complete outermost shells. Hence, they are least reactive.
(d) Nucleus of an atom is made up of protons which are positively charged and neutrons that are neutral. Hence net charge on nucleus is positive. The total mass of neutron and
proton makes it heavy.
(e) When an atom changes into an ion (cation or anion) the valence shell of the ion has a complete octet or duplet which makes ions more stable than atoms.

Question. The table shows the numbers of electrons, neutrons and protons in some atoms and ions of elements.
(The letters used in the table are not the chemical symbols of the elements.)
Answer. 
(a) V
(b) U
(c) T and W
(d) S
(e) S

Question. What kind of elements have a tendency to gain electrons? What are they commonly called?
Ans : The elements having 5, 6 or 7 valence electrons have the tendency to gain electrons. They are commonly called non-metals.

Question. In what way the Rutherford proposed atomic model?
Ans :Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits.
There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus.

Question. Give reasons :
(i) Mass number of an atom excludes the mass of an electron.
(ii) Nucleus of an atom is charged.
(iii) Alpha-particle scattering experiment was possible by using gold foil only and not by foil of any other metal.
Ans :
(i) Mass number of an atom excludes the mass of an electron because electrons have negligible mass in comparison to protons and neutrons.
(ii) Nucleus of an atom consists of protons and neutrons. Protons are positively charged particles. So, the nucleus of an atom is charged.
(iii) Because an extremely thin film was required for the experiment and it was only possible by using gold, as gold is a highly malleable metal.

Question. Is it possible for the atom of an element to have one electron, one proton and no neutron? If so, name the element.
Ans : Yes, it is true for hydrogen atom which is represented as 11 H.

Question. Which particles were discovered by Chadwick, Thomson and Goldstein?
Ans :
(i) Chadwick – Neutron
(ii) Thomson – Electron
(iii) Goldstein – Proton

Question. Is an atom electrically neutral?
Ans : Yes, because number of protons (+ve charge) inside the nucleus are equal to the number of electrons (–ve charge) outside the nucleus.

Question. Why do some elements possess fractional atomic mass?
Ans : Some elements possess fractional atomic mass because they occur in nature in different isotopic forms. So, accordingly their average mass is calculated.

Question. How was the neutron discovered?
Ans : Atom was considered to have electrons and protons only till 1920. But electrons have negligible mass. Then entire mass of the atom was considered to be only due to the protons present in it. In 1920, Rutherford found that atomic masses of all elements are higher than the mass of all protons and electrons in their atoms. Chadwick discovered the presence of an electrically neutral particle inside the atom in 1932.

Question. Define isotopes. Why do isotopes have same atomic number but different mass number? Explain with the help of an example.
Ans : Atoms of the same element, having the same atomic number but different mass numbers are called isotopes. Isotopes have same atomic number but different mass number because they contain different number of neutrons.
For example : In nature chlorine occurs in two isotopic forms; 17Cl35 and 17Cl37.
Here atomic number of both the atoms is same but due to the difference in the number of neutrons, their mass number is different.

Question. What type of charge is present on the nucleus of an atom?
Ans : Positive charge.

Question. Explain the essential features of experiment that led to the discovery of isotopes.
Ans : The m e values of the ionized atoms were determined by studying their deflections under the influence of electrical and magnetic fields. Ionised atoms of an element do not give a unique mass and chemical properties of these atoms are identical.

Question. State the major drawback in Rutherford’s model of an atom. Mention two features of Bohr’s model which helped compensate this drawback.
Ans : The major drawback of Rutherford’s model of an atom is that it does not explain the stability of an atom. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. So, revolving electron wouldlose energy and finally fall into the nucleus.
Two features of Bohr’s model which helped to resolve this drawback : Only certain special orbits known as discrete orbits of electrons are allowed inside the atom. While revolving in these discrete orbits, the electrons do not radiate energy.

Question. What ar canal rays? Who discovered them? What is the charge and mass of canal ray?
Ans : New radiations in a gas discharge tube which are positively charged are known as canal rays. They were discovered by E. Goldstein. Charge on canal rays is positive and its mass is one unit.

Question. Define the following terms :
(i) Electronic configuration
(ii) Valence shell
(iii) Valency
Ans :
(i) The distribution of electrons amongst different orbits of an atom is known as electronic configuration.
(ii) The outermost shell of an atom is called its valence shell.
(iii) The combining capacity of an atom is called its valency or the number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom? What is the valency of this element? Name the element.
Ans : The maximum numbers of electrons that can occupy K and L shells of an atom are 2 and 8 respectively. Therefore, if K and L shells of an atom are full then the total number of electrons in the atom would be 2 + 8 = 10 electrons. So, the valency of this element is zero. The element is neon (Ne).

Question. State the charge and mass of a neutron.
Ans : Neutron has no charge and its mass is equal to that of a proton.
Ans :
(i) Most of the space inside the atom is empty because most of the a -particles passed through the gold foil without getting deflected.
(ii) Very few particles are deflected from their path, indicating that positive charge of the atom occupies very little space.
(iii) A very small fraction of particles was deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a small volume within the atom.

Question. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? And (ii) what is the charge on the atom?
Ans : (i) Atomic number = Number of protons = 8
(ii) The charge of the atom is zero, as total numbern of positive charge is equal to total number of negative charge. Number of protons = Number of electrons 8 = 8

Question. What is the limitation of J.J. Thomson’s model of an atom?
Ans : The major limitation of J.J. Thomson’s model is that it does not explain how positively charged particles are shielded from negatively charged particles, without getting neutralized.

Question. Name an element which has one electron, one proton and no neutron.
Ans : Hydrogen atom (1H1) has one electron, one proton and no neutron.

Question. Write drawbacks of Thomson’s model.
Ans : Drawbacks of Thomson’s Model : It could not explain about the stability of an atom, i.e., how both positive and negative charges could remain so close together. It could not explain the results of experiments (such as alpha ray scattering experiment) carried out by other scientists.

Question. Where is neutron located in an atom?
Ans : It is located in the nucleus of an atom.

Question. What is discharge tube?
Ans : A discharge tube is a glass tube about 70 cm long and 5 cm of diameter. Two metal electrodes are sealed at the two ends, one is connected to negative terminal of battery and other to the positive terminal. A side tube is fused at the centre of the glass tube which serves to pump out air from it, using a suction pump.

Question. Who discovered neutron?
Ans : James Chadwick.

Question. Write three main features of Rutherford’s nucleus model of an atom.
Ans : On the basis of a -particle scattering experiment, Rutherford proposed a model of atom. According to him :
There is positively charged centre in an atom called the nucleus which contains the whole mass of the atom.
The electrons revolve around the nucleus.
The size of the nucleus is very small as compared to the size of the atom.

Question. What do you think would be the observation if the a -particle scattering experiment is carried out using afoil of a metal other than gold?
Ans :  If experiment is done by using a foil of some other metal, then results would not have been such as most of the particles will be deflected back. Gold can be beaten to an extremely thin sheet as it is extremely malleable metal. Rutherford could predict with the use of extremely thin gold foil that, “Most of the space inside the atom is empty and nucleus is positively charged occupying a small volume within the atom.”

Question. Helium atom has two electrons in its valence shell but its valency is not two. Why?
Ans : Helium atom has two electrons in its valence shell but its valency is not two as its outermost shell is completely filled, its combining capacity or valency is zero.

Question. Write the symbols of two isotopes of uranium.
Ans : Uranium 238 (92U238) and Uranium 235 (92U235).

Question. What kind of elements have a tendency to lose electrons? What are they commonly called?
Ans : The elements having 1, 2 or 3 valence electrons have the tendency to loose electrons. They are commonly called metals.

Question. What is the relation between physical and chemical properties of isobars?
Ans : Isobars have different atomic numbers, so different electronic configuration and have different chemical properties. They are atoms of different elements, hence they have different physical properties.

Question. Is there any relationship between atomic number, mass number, isotopes, isobars and valency of an atom? Explain.
Ans : Atomic number : It tells the number of protons (Z). Atomic Mass : Total number of proton and neutron (A) is called atomic mass.
Isotopes : When atoms of same element have same number of protons but different number of neutrons is called isotopes.
Isobars : When atoms of different element have same atomic mass but different atomic number, such atoms are called isobars.
Valency : It is the combining capacity of an atom or it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. State the properties of isotopes.
Ans : Isotopes have the following uses : Treatment of disease : Radioactive isotopes are used for the treatment of dreadful diseases like cancer.
Cobalt-60 is used to kill malignant cells in patients suffering from cancer.
(i) It is used in chemical analysis.
(ii) It is used to detect disease in plants.
(iii) Dating of plants/animals being obtained by using carbon-14 after excavation.
(iv) Thyroid disorders can be treated by I-131.
(v) Leukaemia can be treated by using P-32.
(vi) Uranium-235 can produce electricity.

Question. Name the isotope used for treatment of cancer.
Ans : Isotope of cobalt : Co-60.

Question. Give the mass and charge of an electron.
Ans : (i) The mass of electron is about 9.0 × 10–31 kg.
(ii) An electron is negatively charged particle and has a charge of 1.6 × 10–19 coulomb.

Question. Find the number of neutrons in 31X15.
Ans : 31X15 indicate that no. of proton
                             = 15 and mass number = 31
         Mass number = No. of protons
   + No. of neutrons = 31
Number of neutrons = 31 – number of protons
                              = 31 – 15 = 16

Question. Is C1-35 and C1-37 have different valencies?
Ans : No, it is because these are isotopes of chlorine that have same atomic number but different mass number.

Question. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans : The positively charged particle in the nucleus of an atom is called ‘proton’.

Question. Where is the mass of an atom concentrated?
Ans : Mass of an atom is concentrated in nucleus.

Question. Why do noble gases show low reactivity?
Ans : The outermost shell of the atoms of noble gases are complete. So, they show little chemical reactivity.

Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Ans : Number of electrons in full K-shell = 2
Number of electrons in full L-shell = 8
Total number of electrons in an atom is 2 + 8 = 10 electrons.

Question. Why an atom is electrically neutral?
Ans : An atom is electrically neutral because it contains equal number of positively charged particles and negatively charged particles, i.e. protons and electrons.

Question. If Z = 3, what would be the valency of the element? Also, name the element.
Ans : Atomic number Z = 3
Electronic configuration = 2 (K), 1 (L)
Valence shell has 1 electron in the outermost shell, so
valency of element is 1. The element is lithium.

Question. Why is the valency of Na is 1 and not 7?
Ans : It is easy for Na atom to lose one electron instead of gaining seven electrons. So, valency of Na is one and not seven.

Question. Name the scientist who discovered neutrons.
Ans : J. Chadwick.

Question. What was Chadwick’s experiment?
Ans : In Chadwick’s experiment, he bombarded beryllium atoms with high speed particles.
(i) The rays emitted from beryllium during bombardment had speeded about one-tenth the speed of light.
(ii) Later Chadwick was able to show that these rays consisted of neutral particles called neutron.

Question. Which shell can accommodate a maximum of 32 electrons?
Ans : Fourth shell can accommodate a maximum of 32 electrons.

Question. Define the term “Half life”.
Ans : Radioactive elements are unstable and disintegrate with time emitting alpha and beta particles along with gamma rays. The rate of disintegration depends on the amount of substance. As the amount of the substance decreases, disintegration becomes slower. But it takes a long time for whole of the substance to decay. Thus, the rate of radioactive decay is measured in terms of half time. The time taken by half of the atoms of radioactive element to disintegrate is called its half-time.

Question. What is the mass of a neutron?
Ans : The mass of a neutron is 1.675 × 10–24 g.

Question. What is the charge and mass of a -particles?
Ans : Charge is 2 and mass is 4 amu.

Question. What is the charge and mass of alpha particle?
Ans : Charge = + 2 units
Mass = 4 units

Question. Explain Rutherford’s atomic model.
Ans : Rutherford purposed a model of an atom on the basis of a-particles scattering experiment. This is known as Rutherford’s nuclear model of atom.
(i) An atom consist, a heavy positively charged core called nucleus.
(ii) Nucleus is surrounded by electrons.
(iii) Electrons and nucleus are held together by electrostatic force of attraction.
(iv) Size of nucleus is very small as compared to the size of atom.
(v) Almost the entire mass of the atom is concentrated in the nucleus.

Question. Name the particles which determine the mass of an atom.
Ans : Proton and neutron are the particles which determine the mass of an atom.

Question.Why did Rutherford select a gold foil in his a-ray scattering experiment?
Ans : Rutherford selected a gold foil in his a-ray scattering experiment because gold has high malleability and can be hammered into thin sheet.

Question. Electron attributes negative charge, protons attribute positive charge. An atom has both but why there is no charge?
Ans : The negative and positive charges of electrons and protons respectively are equal in magnitude. So, the atom as a whole is electrically neutral.

Question. Write the charge and mass of an electron.
Ans : Its mass is 2000 1 times that of proton and it is negatively charged.

Question. What are nucleons?
Ans : Protons and neutrons reside in the nucleus of an atom and are thus called nucleons.

Question. An element 12X24 loses two electrons to form a cation which combines with the anion of element 17Y35 formed by gaining an electron.
(i) Write the electronic configuration of element X.
(ii) Write the electronic configuration of the anion of element Y.
(iii) Write the formula for the compound formed by combination of X and Y.
Ans :
(i) X = 2, 8, 2
(ii) Y = 2, 8, 8
(iii) XY2

Question. Why isotopes of an element are chemically similar?
Ans : Isotopes of an element are chemically similar because these have same electronic configuration.

Question. What is an orbit?
Ans : Orbit is the path of electron around the nucleus.

Question. (i) An ion X2+ contains 10 electrons and 12 neutrons. What is the atomic number and mass number of the element X?
(ii) Is it possible in an atom to have 12 protons and 13 electrons?
(iii) Why helium gas is inert?
Ans : (i) Atomic number = 12, Mass number = 24.
(ii) No, it is not possible. An atom is electrically neutral. The number of positively charged particles (protons) is always equal to the number of negatively charged particles (electrons).
(iii) Helium atom has completely filled outermost shell. Thus, it is inert.

Question. What were the drawbacks of Rutherford’s model of an atom?
Ans : The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration and the charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so,
the atom should be highly unstable and hence matter would not exist in the form that we know.

CHAPTER 2: STRUCTURE OF ATOM:

1. Energy of electron in H-atom, E_n = -2.18 x 10 ^-18 / n2 Jatom^-1

2. Energy of electron in H-like species atom, E_n = -2.18 x 10 ^-18 z² / n² Jatom-1

3. Angular momentum of electron, mvr = nh/2π

4. de- Broglies wavelength, λ = h / mv or λ = h / p

5. Plank’s equation E = hϑ

6. Einstein equation , E = mc2

7. Wave number( )=1/ λ = R_H[1/n₁² – 1/n₂²]

8. Isotopes: These are the atoms of the same element having the same atomicnumber but different mass number.e g ₁H¹,₁H²,₁H³

9. Isobars: Isobars are the atoms of different elements having the same massnumber but different atomic number.

e g 18Ar^{40 ,}₂₀Ca⁴⁰ 

10. Isoelectronic species: These are those species which have the same numberof electrons.

11. Electromagnetic spectrum:-It is a continuous spectrum. It consists of a range of electromagnetic radiations arranged in the order of increasing wavelengths or decreasing frequencies.

12. Spectroscopy:-The study of emission or absorption spectra is referred as spectroscopy.

13. o Emission spectrum: The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum.

14. o Absorption spectrum : The spectrum obtained when radiation is passed through a sample of material. The sample absorbs radiation of certain wavelengths. The wavelengths which are absorbed are missing and come as dark lines.

15. Bohr’s model for hydrogen atom:

a. An electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits orenergy levels. These orbits are arranged concentrically around the nucleus.

b. As long as an electron remains in a particular orbit, it does not lose or gain energy and its energy remains constant.

c. When transition occurs between two stationary states that differ in energy, the frequency of the radiation absorbed or emitted can be calculated

d. An electron can move only in those orbits for which its angular momentum is an integral multiple of h/2π The radius of the nth orbit is given by r_n =52.9 pm x n² /Z energy of electron in nth orbit is ;

16. Black body: An ideal body, which emits and absorbs all frequencies, is called a black body. The radiation emitted by such a body is called black body radiation.

17. Photoelectric effect: The phenomenon of ejection of electrons from the surface of metal when light of suitable frequency strikes it is called photoelectric effect. The ejected electrons are called photoelectrons.

18. Threshold frequency (vo): For each metal there is a characteristic minimum frequency below which photoelectric effect is not observed. This is called threshold frequency.

19. Heisenberg uncertainty principle: It is impossible to determine simultaneously the exact position and exact momentum of an electron. Mathematically, ∆x x ∆p ≥ h/ 4 π\

20 .Quantum number’s: These are the numbers obtained by solution of Schrodinger equation used to define the energy of electron its position, orientation and spinning.

21. There are four quantum numbers namely principal, azimuthal, magnetic and spin quantum no.

22. Bohr burry’s rule:- The lower the value of (n + l) for an orbital, the lower is its energy. If two orbitals have the same value of (n + l), the orbital with lower value of n will have the lower energy.

23. Degenerate orbitals- The orbitals having the same energy are called degenerate orbitals.

24. Aufbau principle: electron filled in various subshell in increasing order of their energies.

25. Pauli’s exclusion principle: No two electrons in an atom can have same set of four quantum numbers.

26. Hunds rule of maximum multiplicity: Paring of electrons in the orbitals belonging to the same sub-shell does not take place until each orbital belonging to that sub-shell has got one electron each.

Stability of completely filled and half filled subshells:

a. Symmetrical distribution. 

b. Exchange energy

1 Mark Questions:

1.On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole?

2.Where are electrons found in the atom?

3. What are canal rays?

3.Why is the nucleus of an atom positively charged?

4.Why did Rutherford select a gold foil in his α-ray scattering experiment?

5. If Mg²⁺ has 12 protons and 12 neutrons,what is its atomic number and mass number? (CBSE 2010)

6.What are the limitations of Rutherford's model of the atom?

7.Define valency and Give an example of a monovalent element.

8.An atom of an element has 7 electrons in its L shell,name the element and write it's atomic number?

9.Why Mg atom gets 2+ charge when it loses two electrons ?

10.Name the particles which determine the mass of an atom.

11.Which of the following are isotopes and which are isobars? Argon, Protium, Calcium, Deuterium.

2 Mark Questions:

1.What observations in a scattering experiment led Rutherford to make the following observations:
i) Most of the space in an atom is empty.
ii) Nucleus is positively charged.
iii) Whole mass of an atom is concentrated in its centre.

2.Mention any two drawbacks of Rutherford’s model.

3. State the characteristics of nucleus of an atom.

4. Describe Bohr’s model of the atom?

5.The atomic number of chlorine is 17 and mass number is 35.
a. What would be the electronic configuration of a negatively charged chloride ion, Cl- ?
b. What would be the atomic number and mass number of Cl- ?

6.What is electronic configuration and how is the valency of an atom related to it ?

7.Which of the two would be chemically more reactive element,X of atomic number 18 or element Z of atomic number 16 and why?

8.What are isotopes? Why do isotopes show similar chemical properties but they differ in physical properties?

9.Write isotopes of uranium and its uses.

10.Write one pair of Isobars. Why the chemical properties of isobars are not similar?

3 Mark Questions:

1. Explain Rutherford's gold foil experiment with diagram

2. Write three points of difference between isotopes and isobars.

3.The average atomic mass of a sample of an element ‘X’ is 16.2μ. What is the percentage of each isotope 16X and 18X in the sample?( At.No. of X =8)

4.Define valency of an element. Find the valency of chlorine and magnesium.

5.i) What is the similarity in the electronic structure of the following set of atoms?
Lithium, sodium and potassium.
ii) Which of the above element is most reactive and why?

6.Complete the following table.

7.Give reason for the following
a) An atom is electrically neutral
b) Noble gases show least reactivity
c) Nucleus of an atom is heavy and positively charged.
d) Ions are more stable than atoms.

8. An element has an atomic number 12 and mass number 26.Draw a diagram showing the distribution of electrons in the orbits and the nuclear composition of the neutral atom. If this
element X combines with another element Y whose electronic configuration is 2,8,7, what will be the formula of the compound thus formed?

9. Name the Scientists who have contributed the following towards the understanding of the atomic structure.
i) Discovery of electrons
ii) Canal rays
iii) Concept of nucleus
iv) Stationary orbits
v) Indivisibility of atoms
vi) Neutron

Value Based Question :

1.In Kaiga nuclear Power Corporation building,drinking water was found to be contaminated with tritium.
It is highly radioactive.
a) What is tritium? Write its atomic number and mass number.
b) It is an isotope of which element?
c) Name the other two isotopes of the same element.
d) What precautions do you suggest so that this type of incident does not occur again?
e) What is meant by radioactive?

2. Most of the children burst crackers during diwali.It creates a lot of noise pollution and air pollution.
Asthma patients have tough time during diwali.Many children get hurt due to mishandling of fire crackers.Poisonous gases like sulphur dioxide ,phosphorous pentoxide are formed.
a) Name two elements present in gun powder used in crackers.
b) Government has banned bursting of fire crackers after 10pm.Do you agree? Give reason.
c) Why should we not wear synthetic clothes while bursting crackers?