NCERT Exemplar Solutions Class 9 Science Structure of the Atom

Multiple Choice Questions (MCQs).......................

Question 1:  Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8,1                  (b) 2, 8, 2                   (c) 1,8, 3                     (d) 8, 2, 2

Solution 1:   (b) 2, 8, 2. 

The atomic number of magnesium is 12 and the number of electrons is 12. As a consequence, the electronic configuration is 2, 8, 2 (due to the fact that 12=2 + 8 + 2).

Question 2:  Rutherford’s alpha(α) particles scattering experiment resulted into discovery of
(a) electron                                     

(b) proton

(c) nucleus in the atom              

(d) atomic mass

Solution 2:   (c) nucleus in the atom.

Rutherford's a-particle scattering experiment led to the discovery of the atom's nucleus. As a result of this experiment, the nucleus of the atom was discovered.

Question 3:  The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

Solution 3:  (a)

Given that, number of electrons in element X = 15 and number of neutrons = 16 Atomic number of element X = number of protons = number of electrons in neutral atom = 15 and,

Atomic Mass number = number of protons + number of neutrons = 15+16=31

Thus, the element is represented as (a)

Question 4:  Dalton’s atomic theory successfully explained.

(i)   law of conservation of mass                     

(ii) law of constant composition

(iii) law of radioactivity       

(iv) law of multiple proportion

(a) (i), (ii) and (iii)                                      (b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)                                    (d) (i), (ii) and (iv)

Solution 4:   (d) (i), (ii) and (iv).

Dalton's atomic theory successfully explained the laws of chemical combination, While he made no mention of radioactivity in his theory.

Question 5:  Which of the following statements about Rutherford’s model of atom are correct?
(i)   Considered the nucleus as positively charged.
(ii) Established that the a-particles are four times as heavy as a hydrogen atom.
(iii) Can be compared to solar system.
(iv) Was in agreement with Thomson’s model.
(a) (i) and (iii)       (b) (ii) and (iii)               (c) (i) and (iv)                 (d) Only (i)

Solution 5:   (a) (i) and (iii).

According to the Rutherford model, an atom has a central positively charged nucleus around which electrons revolve. This model is called a planetary model because it is similar to the solar system.

Question 6:  Which of the following are true for an element?
(i)   Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic number = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and  (ii)             (b) (i) and  (iii)                 (c) (ii) and (iii)                 (d) (ii) and (iv)

Solution 6:   (d) (ii) and (iv).

Options (ii) and (iv) are correct.

Question 7:  In the Thomson’s model of atom, which of the following statements are correct?
(i)  The mass of the atom is assumed to be uniformly distributed over the atom.
(ii)  The positive charge is assumed to be uniformly distributed over the atom.
(iii) The electrons are uniformly distributed in the positively charged sphere.
(iv) The electrons attract each other to stabilize the atom.
(a) (i), (ii) and (iii)                                     (b) (i) and (iii)
(c) (i) and (iv)                                             (d) (i), (iii) and (iv)

Solution 7:   (a) (i), (ii) and (iii). 

The first three options (i) (ii), and (iii) are right. An atom, according to Thomson's model, is made up of a positively charged sphere with negatively charged electrons contained within it. Since the magnitudes of the negative and positive charges in an atom are equal, the atom is electrically neutral.

Question 8:  Rutherford’s a-particle scattering experiment showed that
(i)   electrons have negative charge.
(ii)  the mass and positive charge of the atom is concentrated in the nucleus.
(iii) neutron exists in the nucleus.
(iv) most of the space in atom is empty.
Which of the above statements are correct?
(a) (i) and (iii)          (b) (ii) and (iv)          (c) (i) and (iv)           (d) (iii) and (iv)

Solution 8:   (b) (ii) and (iv). 

The second and fourth points are right. A positively charged, compact, and very small nucleus comprises all of the protons and neutrons in an atom. Protons have a positive charge, while neutrons have none. Since most alpha particles move straight through the gold foil with no deflection, most of the space is empty.
Thomson clarified that electrons have a negative charge. Chadwick was the first to discover the presence of neutrons.

Question 9:  The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?

(a) 13                  

(b) 10                

(c) 14                   

(d) 16

Solution 9:   (b) 10.                

Given that,  charge = + 3,

mass number = 27 and

number of neutrons = 14

So, Number of protons = atomic number
Atomic number = mass number – number of neutrons = 27-14 = 13

This is aluminum's atomic number. As a consequence, this material is aluminum (Al).
Number of electrons in the Al atom = 13

Number of electrons in the AI^{3 +} ion = 13 – 3 = 10.
It is created by the loss of three electrons from a neutral atom.

Question 10:  Identify the Mg²⁺ ion from the figure where, n and p represent the number of neutrons and protons respectively.

Solution 10:   (d).  K,L,M

Electronic configuration ₁₂Mg atom = 2, 8, 2 and that of Mg²⁺ ion = 2, 8

Number of protons in Mg atom = 2+ 8 + 2 = 12
Number of neutrons in Mg atom = 24 - 12 = 12

As, Mass number of Mg atom = 24 and

Number of neutrons = mass number - number of protons

Number of electrons in Mg²⁺ = 10
hence option (d) is the correct.

Question 11:  In a sample of ethyl ethanoate (CH₃C00C₂H₅) the two oxygen atoms have the same number of electrons but different number of neutrons, which of the following is the correct reason for it?
(a) One of the oxygen atoms have gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars 

Solution 11:   (c) The two oxygen atoms are isotopes.

If the two O-atoms in CH₃C00C₂H₅ are isotopes can, then the two O-atoms have different numbers of neutrons. Since isotopes of the same element have the same number of protons (and electrons), but different numbers of neutrons. 

Question 12:  Elements with valency 1 are

(a) always metals                           

(b) always metalloids

(c) either metals or non-metals             

(d) always non-metals

Solution 12:   (c) either metals or non-metals            

Valency 1 can be present in both metals and non-metals.

Valency 1 refers to metals with one valence electron and non-metals with seven valence electrons. Since metals lose their 1e^- while non-metals gain 1e^- to complete their octet, this is the case.

Question 13:  The first model of an atom was given by
(a) N Bohr               

(b) E Goldstein                 

(c) Rutherford                      

(d) JJ Thomson

Solution 13:   (d) JJ Thomson.

JJ Thomson proposed the first atomic model. He states,  An atom is made up of a positive-charged sphere with negatively charged electrons contained inside it.

Question 14:  An atom with 3 protons and 4 neutrons will have a valency of

(a) 3                   

(b) 7                   

(c) 1                      

(d) 4

Solution 14:   (c) 1.

Given that, number of protons in an atom = 3 and number of neutrons = 4                                                       

 K L
Electronic configuration of ₃Li = 2, 1
As, it has one valence electron, therefore its valency is also 1.

Question 15:  The electron distribution in an aluminum atom is
(a) 2, 8, 3                 

(b) 2, 8, 2                  

(c) 8, 2, 3                  

(d) 2, 3, 8

Solution 15:   (a) 2, 8, 3.                 

Aluminum atom has 13 protons and 13 electrons.

 K L M
Therefore, Electronic configuration of ₁₃AI = 2, 8, 3

Question 16:  Which of the following in figure do not represent Bohr’s model of an atom correctly?

a)      (i) and (ii)

b)      (ii) and (iii)

c)       (ii) and (iv)

d)      (i) and (iv) 

Solution 16:  (c) (ii) and (iv).  

Figures (ii) and (iv) do not accurately reflect Bohr's atom model.

Since the maximum number of electrons in the K (I) shell is two, not four, (ii) is inaccurate, and the maximum capacity of the L (II) shell is eight, not nine. As a consequence, (iv) is also inaccurate.

Question 17:  Which of the following statement is always correct?
(a)An atom has equal number of electrons and protons
(b) An atom has equal number of electrons and neutrons
(c) An atom has equal number of protons and neutrons
(d) An atom has equal number of electrons, protons and neutrons   

Solution 17:  (a)An atom has equal number of electrons and protons. 

The atom is an electrically neutral particle. It is only possible if the number of protons and electrons in an atom is equal.

Question 18:  Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order.

(i) Rutherford’s atomic model               

(ii) Thomson’s atomic model

(iii) Bohr’s atomic model

(a) (i), (ii) and (iii)             (b) (ii), (iii) and (i)               (c) (ii), (i) and (iii)               (d) (iii), (ii) and (i)

Solution 18:   (c) (ii), (i) and (iii).

(i)Thomson's atomic model, (ii) Rutherford's atomic model and (iii) Bohr's atomic model are the right sequence of atomic model improvements.

Short Answer Type Questions...........................

Question 19:  Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.

Solution 19:   Protium [H]  is the element which an isotope of hydrogen and that contain one electron, one proton, and no neutron.

Question 20:  Write any two observations which support the fact that atoms are divisible.

Solution 20:   following facts support the idea that atoms are divisible.

i)        The formation of ions, which involves the transfer of electrons, allows for the formation of ionic compounds.

ii)       Because of the difference in the number of neutrons, isotopes of the same element can exist.

The observations above indicate that an atom is made up of various particles including electrons, protons, and neutrons, suggesting that an atom is divisible.

Question 21:  Will ³⁵Cl and ³⁷Cl have different valencies? Justify your answer.

Solution 21:  No, the valences of ³⁵CI and ³⁷CI are the same. The isotopes are ³⁵CI and ³⁷CI. As a consequence, they both have the same number of protons and electrons, as well as the same atomic number, which are 17. As a consequence, their electronic configurations and valencies are similar.

                                               K L M
Electronic configuration = 2, 8, 7 

Valency 8-7 = 1
Therefore, both of them have valency = 1

Question 22:  Why did Rutherford select a gold foil in his a-ray scattering experiment?

Solution 22:  Gold is a heavy metal with a huge number of atoms. A light metal cannot be used since the atom of a light metal would actually be forced forward when struck by a fast moving a-particle, resulting in no scattering. Gold is perhaps the most malleable metal. To obtain the direct observations, a very thin gold foil (=1000 atoms thick) can be produced.

Question 23: Find out the valency of the atoms represented by the Figure (a) and (b).

Solution 23:

a)      It has electronic configuration i.e. K  L  M

                                                             2  8   8

So, its outer shell completed octet.

Hence, its valency is 0.

b)      It has electronic configuration i.e. K  L 

                                                             2  7  

So, it can easily gain one electron to complete its outermost octet.

Hence, its valency is 1.

Question 24:  One electron is present in the outermost shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outermost shell?

Solution 24:  Since one electron, or valence electron, is found in the outermost shell of an element X, it is known as a metal. A cation (positive ion) with a charge of + 1 is produced when this valence electron is removed from the outermost shell.

Question 25:  Write down the electron distribution of chlorine atom. How many electrons are there in the /.-shell?
Atomic number of chlorine is = 17.

Solution 25:  Atomic number of chlorine atom = 17
So, its electronic configuration is-
K L M
2 8 7
L shell of chlorine contains 8 electrons.

Question 26:  In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed? 

Solution 26:   The outermost shell of element X has 6 electrons. Element X requires two electrons to achieve noble gas configuration. As a result, the charge on the shaped anion (X2‘) is -2.

Question 27:  What information do you get from the figure about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form.

Solution 27:

Question 28:  In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

Solution 28:  

The argument made is wrong. According to this argument p > n > e.

But in reality, the number of protons is never greater than the number of neutrons. Since mass number is equal to double the atomic number or greater than double the atomic number, so the number of neutrons can be equal to or greater than the number of protons.

Of course, since the number of protons in a neutral atom equals the number of electrons, so the number of neutrons can be greater than the number of electrons.

Question 29:  Calculate the number of neutrons present in the nucleus of an element X which is represented as ₁₅³¹X. 

Solution 29:   

X element represented as ₁₅³¹X.
where, Atomic number Z = 15
Mass number A = 31
So, Number of neutrons = A – Z = 31 – 15 = 16

Question 31:  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?

Solution 31:
Mass number of calcium is 40 i.e. ₂₀⁴⁰C

Mass number of argon is 40 i.e. ₁₈⁴⁰C

A pair of elements having same mass number but different atomic number is called isobars.

Question 32:  Complete the table on the basis of information available in the symbols given below.

Question 33:  Helium atom has 2 electrons in its valence shell but its valency is not 2. Explain.

Solution 33:   The helium atom only has one shell (the K shell), which can accommodate up to two electrons. As a consequence, it has a duplet shell. It can't lose and gain electrons. As a consequence, it has zero valency. Noble gas, or inert gas, is the name given to it.

Question 34:  Fill in the blanks in the following statements.
(a)Rutherford’s a-particle scattering experiment lead to the discovery of the…………
(b)Isotopes have same…… but different…………
(c)Neon and chlorine have atomic numbers 10 and 17 their valencies will be……… and……….. respectively.
(d)The electronic configuration of silicon is…….. and that of sulphur is…………..

Solution 34:

a)      nucleus.

b)      atomic number, mass number.

c)       zero, one. (Electronic configuration ₁₀Ne – 2, 8. Its outermost shell is completely filled. Hence, its valency is zero. And, Electronic configuration of ₁₇Cl –  2, 8, 7.It can complete its outermost shell by gaining 1 electron. Hence, its valency is one)

d)      2, 8, 4 and 2, 8, 6.

Question 35:  An element X has a mass number 4 and atomic number 2. Write the valency of this element.

Solution 35:
Atomic number = 2
Hence, number of electrons = 2
Thus, the elements has only K shell containing 2 electrons i.e., a complete duplet.

So, the element X is ₂⁴He and it is called noble gas.

 Hence, its valency = 0.

Long Answer Type Questions...............................

Question 36:  Why do helium, neon and argon have a zero valency?

Solution 36: Helium (He), neon (Ne) and argon (Ar) have completely filled outermost shell i.e.

As a consequence, their electronic configuration is stable. They don't lose or gain electrons in any way. As a consequence, they have zero valencies.

Question 37:  The ratio of the radii of hydrogen atom and its nucleus is ~10⁵. Assuming the atom and the nucleus to be spherical.
(a)What will be the ratio of their sizes ?
(b)If atom is represented by planet earth ‘R_e‘ = 6.4 x 10⁶m, estimate the size of the nucleus.

Solution 37:

Question 38:  Enlist the conclusions drawn by Rutherford from his a-ray scattering experiment.

Solution 38:   Conclusion of Rutherford's a-ray scattering experiment 

i)        Since most of the a-particles passed through the gold foil without being deflected, most of the space within the atom is null.

ii)       Only a few particles were deflected from their intended path, meaning that the atom's positive charge takes up very little space.

iii)     A very small fraction of a-particles deflected 180 degrees, meaning that the gold atom's positive charge and mass were concentrated in a very small volume within the atom.

Question 39:  In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?

Solution 39:

According to Thomson’s model, an atom is a positively charged sphere in which electrons were embedded which neutralizes the positive charge so that, the atom as a whole is electrically neutral.
According to Rutherford’s model of atom, the positive charge and mass of the atom are concentrated in the small nucleus in the centre of the atom and negatively charged electrons are distributed (revolving) around the nucleus.

Question 40:  What were the drawbacks of Rutherford’s model of an atom?

Solution 40:   Rutherford's model was unable to describe the atom's stability. This is because, according to Rutherford's model, an atom consists of a small strong positively charged nucleus in the centre, which is surrounded by electrons.

These spinning electrons will lose energy because they are charged particles, and charged particles radiate energy due to acceleration.

As a result, the orbit of the spinning electron will begin to shrink, following a spiral path as shown in figure, and the electron will finally collapse into the nucleus. To put it another way, the atom could disintegrate.

Question 41:  What are the postulates of Bohr’s model of an atom?

Solution 41: The following are the postulates of Bohr's atom model:

i)        In an atom, electrons revolve around the nucleus in definite circular paths called orbits or shells. The letters K, L, M, N.............. or the numbers n = 1,2, 3, 4.............. are used to reflect these.

i)        The formula 2n² gives the maximum number of electrons in a shell, where n is the orbit number or energy level index i.e. 1, 2, 3, ………

           As a consequence, the maximum number of electrons in various shells is:

           The first orbit (K shell) is equal to 2 x 1² = 2,

           The second orbit (or L shell) is equal to 2 x 2 = 8, the third orbit (M shell) is equal to 2 x 32 =18, and so on.

ii)       In the outermost orbit, the maximum number of electrons that can be accommodated is 8.

iii)     Unless the inner shells are filled, electrons cannot fit in a given shell. That is, the shells are filled one at a time.

iv)     Electrons do not emit energy when rotating in distinct orbits.

Question 42:  Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Solution 42:
Atomic number of sodium (Z) = 11
Mass number of sodium (A) = 23
∴ Number of protons in the nucleus = 11
Number of neutrons in the nucleus = 23 -11 = 12
Number of electrons = 11

 electronic configuration of Na atom = K L M

                                                       2 8  1

Na⁺ ion is formed from sodium atom by loss of an electron (present in the outermost shell). Hence, its electronic configuration is 2, 8(K, L). However, number of protons and neutrons remains same.

Question 43:   In the gold foil experiment of Geiger and Marsden, that paved the way for Rutherford’s model of an atom, ~ 1.00% of the a-particles were found to deflect at angles > 50°. If one mole of a-particles were bombarded on the gold foil, compute the number of a-particles that would deflect at angles less than 50°. 

Solution 43: