CBSE Class 10 Chemistry Periodic Classification of Elements Worksheet Set C

Question. Where would you locate the element with electronic configuration 2, 8 in the Modern Periodic Table? 
(A) Group 8
(B) Group 2
(C) Group 18
(D) Group 10

Answer : C

Question. In Mendeleev’s periodic table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the periodic table later ? 
(A) Germanium
(B) Chlorine
(C) Oxygen
(D) Silicon

Answer : A

Question. Which of the following set of elements is written in order of their increasing metallic character? 
(A) Be, Mg, Ca
(B) Na, Li, K
(C) Mg, Al, Si
(D) C, O, N

Answer : A

Question. What type of oxide would Eka– aluminium form?
(A) EO₃
(B) E₂O₂
(C) E₂O₃
(D) EO

Answer : C

Question. Upto which element, the Law of Octaves was found to be applicable ? 
(A) Oxygen
(B) Calcium
(C) Cobalt
(D) Potassium

Answer : B

Question. Which of these does not represent Dobereiner’s triad? C
(A) Li, Na, K
(B) Cl, Br, I
(C) Be, Mg, Ca
(D) N, P, As

Answer : D

Question. Which of the following are the characteristics of isotopes of an element? 
(i) Isotopes of an element have same atomic masses.
(ii) Isotopes of an element have same atomic number.
(iii) Isotopes of an element show same physical properties.
(iv) Isotopes of an element show same chemical properties.
(A) (i), (iii) and (iv)
(B) (ii), (iii) and (iv)
(C) (ii) and (iii)
(D) (ii) and (iv)

Answer : D

Question. The positions of four elements A, B, C and D in the modern periodic table are shown below. Which element is most likely to form an acidic oxide?

(A) A
(B) B
(C) C
(D) D

Answer : C

Question. According to Mendeleev’s Periodic Law, the elements were arranged in the periodic table in the order of
(A) increasing atomic number
(B) decreasing atomic number
(C) increasing atomic masses
(D) decreasing atomic masses 

Answer : C

Question. Arrange the following elements in the order of their decreasing metallic character: Na, Si, Cl, Mg, Al. 
(A) Cl > Si >Al > Mg >Na
(B) Na >Mg >Al >Si > Cl
(C) Na > Al > Mg > Cl > Si
(D) Al > Na> Si > Ca> Mg

Answer : B

Question. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic table? 
(A) The elements become less metallic in nature
(B) The number of valence electrons increases
(C) The atoms lose their electrons more easily
(D) The oxides become more acidic.

Answer : C

Question. An element ‘X’ is forming an acidic oxide. Its position in modern periodic table will be: 
(A) Group 1 and Period 3
(B) Group 2 and Period 3
(C) Group 13 and Period 3
(D) Group 16 and Period 3

Answer : D

Question. Elements P, Q, R and S have atomic numbers 11, 15, 17 and 18 respectively. Which of them are reactive non-metals? 
(A) P and Q
(B) P and R
(C) Q and R
(D) R and S

Answer : C

Question. Which of the following is the outermost shell for elements of period 2? 
(A) K shell
(B) L shell
(C) M shell
(D) N shell

Answer : B

Question. Consider the following statements about an element ‘X’ with number of protons 13.
(I) It forms amphoteric oxide.
(II) Its valency is three.
(III) The formula of its chloride is XCl.
(A) only (I)
(B) only (II)
(C) (I) and (III)
(D) (I) and (II)

Answer : D

Question. Which of the following elements will form an acidic oxide? 
(A) An element with atomic number 7
(B) An element with atomic number 3
(C) An element with atomic number 12
(D) An element with atomic number 19

Answer : A

Assertion and Reason Based MCQs :

Directions : In the following questions, A statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as.
(A) Both A and R are true and R is the correct explanation of A.
(B) Both A and R are true but R is NOT the correct explanation of A.
(C) A is true but R is false.
(D) A is false and R is true.

Question. Assertion (A): Sixth and seventh periods in the periodic table contains 14 elements.
Reason (R): In the periodic table, 14 elements of sixth and seventh periods are known as lanthanoids and actinoids respectively.

Answer : D

Question. Assertion (A): Smaller the size of an atom, greater is the electronegativity.
Reason (R): Electronegativity refers to the tendency of atom to share electrons with other atom.

Answer : C

Question. Assertion (A): Mendeleev left the gaps under aluminium and silicon and called these Eka-aluminium and Eka-silicon, respectively.
Reason (R): Dobereiner arranged elements on the basis of increasing atomic number.

Answer : C

Question. Assertion (A): The atomic and ionic radii generally decreases towards right in a period.
Reason (R): The ionisation enthalpy increases on moving towards left in a period.

Answer : C

Question. Assertion (A): Mendeleev arranged element in horizontal rows and vertical columns.
Reason (R): Mendeleev ignored the order of atomic weight thinking that the atomic measurements might be incorrect.

Answer : A

Question. Assertion (A): According to Mendeleev, periodic properties of elements is a function of their atomic number.
Reason (R): Atomic number is equal to the number of protons.

Answer : D

Question. Assertion (A): Noble gases are highly reactive. 
Reason (R): Noble gases have stable closed shell electronic configuration.

Answer : D

Question. Assertion (A): In a triad, the three elements have same gaps between their atomic masses.
Reason (R): Elements in a triad have similar properties.

Answer : D

Question. Assertion (A): Be and Al show similar properties.
Reason (R): The metallic radius of Be is less than the metallic radius of Al.

Answer : B

Case-based MCQs :

Read the passage and answer the questions given below :

Mendeleev was a Russian chemist, who contributed the most for the development of periodic table of elements wherein the elements were arranged on the basis of their fundamental property, the atomic mass and also on the similarity of chemical properties. Only 63 elements were known at his time. He arranged the 63 elements in the increasing order of their atomic masses and found that there was a periodic recurrence of elements with similar physical and chemical properties. He observed that elements with similar properties fall in the same vertical column. These vertical columns are called groups and horizontal rows of elements are called periods. Mendeleev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium

Question. The elements eka aluminium and eka silicon discovered by Mendeleev later found place in periodic table. Both of these elements belong to: 
(A) Period 2
(B) Group 13
(C) Group 14
(D) Period 4

Answer : D

Question. How do we classify these newly discovered elements (eka-aluminium and eka-silicon)? 
(A) Metals
(B) Non metals
(C) Metalloids
(D) Inert gases

Answer : C

Question. Mendeleev arranged the periodic table on the basis of which fundamental property? 
(A) Atomic mass
(B) Atomic number
(C) Number of neutrons
(D) Valence electrons

Answer : A

Question. Eka aluminium and eka silicon were later replaced respectively as : 
(A) Germanium and gallium
(B) Gallium and scandium
(C) Gallium and germanium
(D) Germanium and scandium

Answer : C

Read the passage and answer the questions given below :

Using the given part of the periodic table, answer the questions given below :

Question. Which of these elements have smallest atomic size? 
(A) B
(B) C
(C) D
(D) E

Answer : D

Question. Which of them will have largest atomic radii? 
(A) E
(B) X
(C) C
(D) D

Answer : B

Question. What is the valency of element E? 
(A) 1
(B) 3
(C) 2
(D) 0

Answer : C

Question. Identify the elements which have similar chemical properties as the element X. 
(A) Y and Z.
(B) Y and B
(C) All Y, Z and B
(D) None of these

Answer : A

Read the passage and answer the questions given below :

Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells as K, L and M. But are different in their outermost shells. It was found that elements A and G combine to form an ionic compound which can also be extracted from sea water. Oxides of the elements A and B are basic in nature while those of E and F are acidic. The oxide of elements D is almost neutral.

Question. Which two of these elements could definitely be metals ? 
(A) E and F
(B) A and B
(C) D
(D) G

Answer : B

Question. Which two elements amongst these are likely to be the non–metals ? 
(A) A and G
(B) D and F
(C) E and F
(D) A and B

Answer : C

Question. To which period the listed elements belong? 
(A) 2nd
(B) 7th
(C) 8th
(D) 3rd

Answer : D

Question. Which one of the following is most likely to be found in gaseous state at room temperature ? 
(A) A
(B) H
(C) D
(D) B

Answer : B

Read the passage and answer the questions given below :

From the following table of the periodic table, answer the questions given:

Question. Which is the most reactive metal ? 
(A) Lithium
(B) X
(C) Y
(D) Z

Answer : D

Question. Which of the following element belongs to group 2? 
(A) Sodium
(B) Magnesium
(C) Aluminium
(D) Carbon

Answer : B

Question. Which other element is likely to present in the group in which fluorine is present? 
(A) Neon
(B) Aluminium
(C) Chlorine
(D) None of the above

Answer : C

Question. Name the family of fluorine, Q, R, T: A
(A) Alkali metals
(B) Noble gas
(C) Halogens
(D) Alkaline metals

Answer : C

Very Short Answer Type Questions :

Question. If the atomic number of three element X, Y and Z are 3, 11 and 17 respectively. Which two elements will show similar chemical properties. Justify.
Answer : X and Y will show similar chemical properties as these have same valence electrons.
X = 2, (1)
Y = 2, 8, (1) 

Question. Write the total number of periods in modern periodic table.
Answer : The total number of periods in modern periodic table is seven. 

Question. Define electropositivity. 
Answer : Electropositivity is the tendency of an element to lose electrons and form positive ions in a chemical reactions. 

Question. How many elements are in 2^nd and 5^th period of Modern Periodic table. 
Answer : 2^nd period has 8 elements, 5^th period has 18 elements.

Question. State one reason that explains the position of Hydrogen in group I. 
Answer : Hydrogen should be placed in group I, since it has only one electron in its outermost shell. 

Question. How many vertical columns are present in modern periodic table? What are they called? 
Answer : There are 18 vertical columns in the modern periodic table and these are known as groups. 

Question. The atomic radii of first group elements are given below:
Group I element     Atomic radii (pm)
  Na                           86
  K                             231
  Rb                           244
  Cs                           282
State the reason behind the observed trend in the above elements. 
Answer : In a group, as we move from top to bottom, the number of shells increases. Hence, the atomic radius increases.

Question. State the formula to fill up the maximum number of electrons in a shell. 
Answer : 2n2, where n is the number of shell. 

Question. Why noble gases are placed in a separate group in modern periodic table? 
Answer : It is because they resemble with each other but do not resemble with other group elements. 

Question. Name any two elements that have two electrons in their valence shell.
Answer : (i) Magnesium: 2, 8, 2 
(ii) Calcium : 2, 8, 8, 2.

Question. How does valency of an element vary across a period? 
Answer : The valency of an element first increases and then decreases across a period.

Question. How many horizontal rows are present in modern periodic table? What are they called? 
Answer : There are seven horizontal rows in the modern periodic table. These rows are called periods.

Question. State one reason for placing Mg and Ca in the same group of the periodic table.
Answer : Due to the presence of 2 electrons in the valence shell and similar chemical properties.

Short Answer Type Questions :

Question. The atomic number of an element is 20.
(a) Write its electronic configuration and determine its valency.
(b) Is it a metal or a non-metal ?
(c) Write the formula of its chloride.
(d) Is it more reactive or less reactive than Mg (atomic number 12) ? Give reason for your answer.
Answer : (a) Electronic Configuration, X₍₂₀₎ – 2, 8, 8, 2 
Valence electrons- 2
Hence valency is 2 
(b) It is a metal 
(c) XCl₂ 
(d) It is more reactive than Mg as reactivity increases down the group. Mg- III Period and X₂₀ (Ca)- IV Period. 

Question. What is the location of metals and non-metals in the Modern Periodic Table?
Answer : Metals are placed on the left side and non-metals are placed on the right side of the Periodic Table.

Question. How many elements are present in first, second, third and fourth period?
Answer : 

Question. An element X belongs to group 17 and element Y belongs to group 1. What type of bond will they form?
Answer : 

Question. Given below are 3 elements W, X, Y and Z the atomic numbers are 9, 10, 16, 17. Predict the following:
(a) Two elements lying in same group.
(b) Element in second period.
Answer : 

(a) Two elements in same group — W and Z
(b) Element in second period — W and X

Question. State the difference between Modern Periodic Table and Mendeleev’s Periodic Table.
Answer : 

Question. For the following given elements predict the;
(a) Valency
(b) Period number
(c) Group number
Na (11), Al(13), Cl(17), K(19)
Answer : 

Question. Elements of group 1 are given below with their atomic number:
Li (3) Na (11) K (19)
(a) Give their atomic size.
(b) Reactivity.
Answer : 

(a) The atomic size goes on increasing as new shell is added in each element as we move from top to bottom. So it is Li < Na < K.
(b) The reactivity increases as it is easy to lose electrons if the size of atom is big, the nuclear force decreases K is more reactive than Na and Li.

Question. The position of three elements A, B and C in the Periodic Table are shown below:

Giving reasons, explain the following:
(a) Element A is a non-metal.
(b) Element B has a larger atomic size than element C
(c) Element C has a valency of 1. (CBSE 2008 F)
Answer : (a) A is a non-metal because it can gain electrons easily, it has 7 valence electrons and form negative ions to become stable.
(b) The atomic number of B is less than C, it has less nuclear charge, less force of attraction between protons in the nucleus and valence electrons, hence its size is bigger than C.
(c) Element ‘C’ has 7 valence electrons, it can gain 1 electron to become stable so its valency is 1.

Question. The position of three elements A, B and C in the Periodic Table are shown below:

Giving reason, explain the following:
(a) Element A is a metal.
(b) Element C has a larger size than B.
(c) Element B has a valency of 3.
Answer : (a) ‘A’ is a metal because its valence electron is 1, it can readily loose electron to became stable.
(b) Element ‘C’ belongs to 3rd period it has 3 shells whereas ‘B’ has only 2 shells, it belongs to 2nd period, distance between nucleus and valence electrons is more in C, hence its size is bigger than B.
(c) ‘B’ belongs to III group, has 3 valence electrons, it can loose 3 electrons to become stable hence its valency is 3.

Question. The position of 3 elements A, B and C in the Periodic Table is shown below:

Giving reasons, explain the following:
(a) Element A is a non-metal.
(b) Atom of element C has larger size than A.
(c) Element B has a valency of 1.
Answer : (a) A belongs to 7th group, has 7 valence electron, it can gain 1 electron to become stable. So it is a non-metal as it forms negative ion.
(b) ‘C’ has 3 shells ‘A’ has 2 shells so C is bigger than A.
(c) ‘B’ has one valence electron, it can loose this electron to become stable. So its valency is 1.

Question. The electronic configuration of an element is 2, 8, 4. State its:
(a) group and period in the Modern Periodic Table.
(b) name and write its one physical property. 
Answer : (a) Group – 14, Period – 3 
(b) Silicon 
Metalloid / poor conductor of electricity (or any other property)

Question. Can the following groups of elements be classified as Dobereiner’s triad ?
(a) Na, Si, Cl
(b) Be, Mg, Ca
Atomic mass of Be-9; Na-23, Mg-24, Si-28, Cl-35, Ca-40.
Justify your answer in each case. 
Answer : (a) Na, Si, Cl – Average of atomic masses of Na and Cl is not equal to the atomic mass of Si. 
(b) Be, Mg, Ca – The average of atomic masses of Be and Ca is equal to the atomic mass of Mg. 
Atomic mass of Mg =
Atomic mass of Be + Atomic mass of Ca/2
= 9 + 40/2 = 49/2 = 24.5

Question. Define the following terms ?
(i) Valency,
(ii) Atomic size.
Answer : (i) Valency: The combining power or the combining capacity of an atom is called its valency.
(ii) Atomic size: Atomic size or atomic radius is the distance between the centre of the nucleus and the outermost shell of an isolated atom.

Question. An element P (atomic number 20) reacts with an element Q (atomic number 17) to form a compound. Answer the following questions by giving reason: Write the position of P and Q in the Modern Periodic Table and the molecular formula of the compound formed when P reacts with Q.
Answer :

Question. The electronic configuration of an element ‘X’ is 2, 8, 6. To which group and period of the modern periodic table does ‘X’ belong. State its valency.
Answer : X: 2, 8, 6
(a) Since ‘X’ has three energy shells and period number of an element is equal to the number of energy shells, X belongs to 3rd period.
(b) X has 6 valence electrons it belongs to group 16.
(c) Valency will be 2. To acquire noble gas configuration it will gain 2 electrons.

Question. Write the names given to the vertical columns and horizontal rows in the Modern Periodic Table.
How does the metallic character of elements vary on moving down a vertical column ? How does the size of atomic radius vary on moving left to right in a horizontal row ? Give reason in support of your answer in the above two cases.
Answer : Vertical Column — Groups 
Horizontal Rows — Periods 
(i) Metallic character increases.
Reason: Ability to lose electrons increases on moving down the group due to increase in distance between the nucleus and the valence electrons/decrease in the attraction between the nucleus and the valence electrons.
(ii) Atomic radius decreases.
Reason: The nuclear charge increases on moving from left to right across a period resulting in increase in the attraction between the nucleus and the valence electrons.

Question. An element ‘M’ with electronic configuration (2, 8, 2) combines separately with (NO₃)^–, (SO₄)^2– and (PO₄)^3– radicals. Write the formula of the three compounds so formed. To which group and period of the Modern Periodic Table does the elements ‘M’ belong ? Will ‘M’ form covalent or ionic compounds? Give reason to justify your answer.
Answer : The electronic configuration (2, 8, 2) of the element ‘M’ suggests that it belongs to group 2 and period 3 of the Modern Periodic Table and its valency is 2. The chemical formula of the compounds are:
M(NO₃)² /Mg(NO₃)²: MSO₄ / MgSO₄: M₃ (PO₄)² Mg₃(PO₄)²
‘M’ will form ionic compounds by losing two electrons 

Question. State the main aim of classifying elements. Which is more fundamental property of elements that is used in the development of Modern Periodic Table ? Name and state the law based on this fundamental property. On which side of the periodic table one can find metals, non-metals and metalloids ?
OR
Write the main aim of classifying elements. Name the basic property of elements used in the development of Modern Periodic Table. State the Modern Periodic Law. On which side (part) of the Modern Periodic Table do you find metals, metalloids and non-metals ?
Answer : (i) Aim of Classification: For systematic and simplified study of elements and their compounds. 
(ii) Basic property: Atomic Number. 
(iii) Modern periodic Law: The properties of elements are a periodic function of their atomic number. 
(iv) Metals are found on the left side and centre of the Modern Periodic Table. 
(v) Metalloids are found in a zig-zag manner between the metals and the non-metals. 
(vi) Non-metals are found on the right side of the Modern Periodic Table.

Question. The electronic configuration of an element ‘X’ is 2, 8, 6. To which group and period of the modern periodic table does ‘X’ belong. State its valency and justify your answer in each case.
Answer : X: 2, 8, 6
(a) Since ‘X’ has three energy shells and period number of an element is equal to the number of energy shells. X belongs to 3rd period. 
(b X has 6 valence electrons it belongs to group 16. 
(c) Valency will be 2. To acquire noble gas configuration it will gain 2 electrons. 

Question. Write the electronic configuration of two elements P (atomic number 17) and Q (atomic number 19) and determine their group numbers and period numbers in the Modern Periodic Table.
Answer : Electronic configuration of ‘P’ — 2, 8, 7
Group number — 17
Period number — 3^rd
Electronic configuration of ‘Q‘ — 2, 8, 8, 1
Group number — 1
Period number — 4^th

Question. Two elements ‘A’ and ‘B’ belong to the 3rd period of Modern periodic table and are in group 2 and 13 respectively. Compare their following characteristics in tabular form:
(i) Number of electrons in their atoms
(ii) Size of their atoms
(iii) Their tendencies to lose electrons
(iv) The formula of their oxides
(v) Their metallic character
(vi) The formula of their chlorides.
Answer : 

Question. An element ‘M’ has atomic number 12.
(i) Write its electronic configuration and valency.
(ii) Is ‘M’ a metal or a non-metal ? Give reason in support of your answer. 
Answer : (i) Electronic Configuration — 2, 8, 2
Valency — 2.
(ii) Metal
There are two electrons in its outermost shell and it easily loses them to form a positive ion.

Question. An element ‘X’ with electronic configuration (2, 8, 2) combines separately with two radicals, (NO₃)^– and (SO₄)^2–.
(i) Is ‘X’ a metal or a non-metal ? Write the nature of its oxide.
(ii) Write the formula of the compounds of ’X’ formed by the combination of these radicals. Are these compounds covalent or electrovalent ?
Answer : (i) X is a metal. Nature of its oxide is basic.
(ii) X(NO₃)₂, XSO₄
These compounds are ionic/electrovalent. 

Question. How does the atomic radius of the elements change on going
(i) from left to right in a period, and
(ii) down a group
in the Modern Periodic Table ? Give reason in support of your answer.
Answer : (i) Atomic radius decreases. 
Reason: Nuclear charge increases which tends to pull the electrons closer to the nucleus. 
(ii) Atomic radius increases. 
Reason: Number of shells increases on going down the group. 

Question. An element ‘X’ belong to 3rd period and group 13 of the Modern Periodic Table.
(a) Determine the valence electrons and the valency of ‘X’.
(b) Molecular formula of the compound formed when ‘X’ reacts with an element ‘Y’(atomic number = 8).
(c) Write the name and formula of the compound formed when ‘X’ combines with chlorine.
Answer : (a) Group 13 means valence electrons are 3 and valency is 3.
(b) Y (8) – 2, 6 X = 2, 8, 3
 Valency – 2 Valency – 3
Compound formed – X₂Y₃ /Al₂O₃ 
(c) X Chlorine Cl
Valency -3 Valency -1
Compound formed – XCl₃ /AlCl₃ 

Question. Consider the following elements (atomic numbers are given in parenthesis)
Ca(20); K(19); F(9); Be(4) 
(a) Select:
(i) The elements having one electron in the outermost shell.
(ii) Two elements of the same group. Write the number of this group.
(b) Write the formula of the compound formed by the union of Ca(20) and the element X(2, 8, 7).
Answer : (a) (i) K (Potassium — 2, 8, 8, 1)
(ii) Be and Ca in same group because both have same number of valence electrons in their outermost shell. The number of this group is 2.
(b) Ca X
Valency 2 1
Thus, the formula of the compound is Ca₁X₂ = CaX₂ 

Question. Write the electronic configuration of two elements X and Y whose atomic numbers are 20 and 17 respectively. Write the molecular formula of the compound formed when element X reacts with element Y. Draw electron-dot structure of the product and also state the nature of the bond formed between both the elements.
Answer : X₍₂₀₎ – 2, 8, 8, 2 
Y(17) – 2, 8, 7 
XY₂ 
X + 2 Y = X²⁺ [ Y ]^2– 
Ionic/eletrovalent bond. 

Question. Write the name, symbol and electronic configuration of an element X whose atomic number is 11. 
Answer : Name – Sodium 
Symbol – Na 
Electronic configuration – 2, 8, 1 

Question. The following table shows the position of five elements A, B, C, D and E in the modern periodic table.

Answer the following giving reasons:
(i) Which element is a metal with valency two? (ii) Which element is least reactive?
(iii) Out of D and E which element has a smaller atomic radius? 
Answer : (i) D, as it is on the left side of the table in group 2. (ii) C, as it is in the group 18/ Noble gas.
(iii) E, as we move from left to right across a period, atomic radius decreases. 1+1+1

Question. The position of eight elements in the modern periodic table is given below where atomic numbers of elements are given in the parenthesis.

(i) State electronic configuration of Ca.
(ii) Predict the number of valence electrons in Rb.
(iii) What is the number of shells in Sr ?
(iv) Predict whether K is a metal or a non-metal.
(v) Which one of these elements has the largest atom in size ?
(vi) Arrange Be, Ca, Mg and Rb in the increasing order of the size of their respective atoms.
Answer : (i) Ca = 2, 8, 8, 2 
(ii) Valence electrons in Rb = 1
(iii) Five 
(iv) Metal 
(v) Rb is biggest in size. 
(vi) Be < Mg < Ca < Rb. 

Question. Three elements ‘X’, ‘Y’ and ‘Z’ have atomic numbers 7, 8 and 9 respectively.
(a) State their positions (Group number and period number both) in the Modern Periodic Table.
(b) Arrange these elements in the decreasing order of their atomic radii.
(c) Write the formula of the compound formed when ‘X’ combines with ‘Z’.
Answer : (a) X (7) - 2,5 Group 15; Period 2
Y(8 ) - 2,6 Group 16; Period 2
Z(9) - 2,7 Group 17; Period 2 
(b) X > Y > Z 
(c) XZ₃ 

Question. Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why is it so?
Answer : Atomic Size: The distance from centre of nucleus to outermost shell of an atom is atomic radius.
Atomic size is measured in Angstroms, (A°).
where 1 Angstroms = 10-10 metres. 
Along the period from left to right atomic radius decreases.
Reason: Nuclear charge increases which tends to pull the electrons closer to the nucleus.
Down the group: Atomic radius increases
Reason: Number of shells increases on going down the group.

Question. The elements ₄Be, ₁₂Mg and ₂₀Ca each having two valence electrons in their valence shells are in periods 2, 3 and 4 respectively of the modern periodic table. Answer the following questions associated with these elements, giving reason in each case,
(i) In which group should they be?
(ii) Which one of them is least reactive ?
(iii) Which one of them has the largest atomic size ?
Answer : (i) They all belong to group 2 because all three have 2 electrons in their outermost shell.
(ii) Be is least reactive because it has 2 shells and due to more nuclear change it is not easy to take electrons from it.
(iii) Ca is the element having largest atomic radius because it has 4 shells. {Hence, it has the largest atomic size.

Question. What is periodicity in properties of elements with reference to the Modern Periodic Table? Why do all the elements of the same group have similar properties? How does the tendency of elements to gain electrons change as we move from left to right in a period? State the reason of this change.
Answer : • Repetition of similar properties of elements after regular intervals. 
• Because of the same number of valence electrons.
• It increases due to increase in effective nuclear charge which pulls the electrons towards it. 

Question. Study the data of the following three categories A. B and C.

(i) From the given three categories A, B and C, pick the one which forms Dobereiner’s Triads.
(ii) Why did Mendeleev placed elements of category A, B and C in three different groups ?
(iii) Is Newland law of octaves applicable to all the three categories?
Give reason to justify your answer.
Answer : (i) Category A (Li, Na, K) shows Dobereiner’s triads.
(ii) Mendeleev arranged the elements in increasing order of atomic mass and grouped them as per similar chemical property. The elements in A, B and C are similar among themselves but have different chemical properties.
(iii) No, Newland law of Octaves is not applicable because in the three categories every eighth element will not show same property as first. Also, Newland Octaves law is applicable only upto calcium.

Question. How it can be proved that the basic structure of the Modern Periodic Table is based on the electronic configuration of atoms of different elements ?
Answer : Modern periodic table consists of groups and periods, where number of valence electrons determines the group and number of shells determines the period.

Question. Name any two elements of group one and write their electronic configurations. What similarity do you observe in their electronic configurations ?
Write the formula of oxide for any of the above said element.
Answer : (i) Two elements of group 1 are Na, K / Sodium, potassium. 
Electronic configurations Na = 2,8,1; K = 2,8,8,1
(ii) Similarity: Both have one valence electron / One electron in outermost shell. 
(iii) Oxide – Na₂O / K₂O. 

Question. Define groups in the Modern Periodic Table. How do valency, atomic size and metallic character vary in a group.
Answer : Group : Vertical columns in the modern periodic table are known as ‘groups’. There are 18 groups.
(i) Valency remain same in a group.
(ii) Atomic size increases from top to bottom in a group.
(iii) Metallic character also increases from top to bottom in a group. 

Question. In the following table, the positions of six elements A, B, C, D, E and F are given as they are in the Modern Periodic Table:

On the basis of the above table, answer the following questions:
(i) Name the element which forms only covalent compounds.
(ii) Name the element which is a non-metal with valency three.
(iii) Name the element which is a non-metal with valency three.
(iv) Out of B and C, whose atomic radius is bigger and why ?
(v) Write the common name for the family to which the elements D and F belong.
Answer : (i) E 
(ii) B 
(iii) C 
(iv) B, because atomic radius decreases from left to right due to increase in the nuclear charge.
(v) Noble gases.

Question. (i) Name the element with atomic number 17.
(ii) To which period does it belong ?
(iii) To which group does it belong ?
(iv) Write its electronic configuration.
Answer : (i) Chlorine 
(ii) 3^rd period 
(iii) 17^th group 
(iv) 2, 8, 7. 

Question. Four elements A, B, C and D have atomic numbers 12, 13, 14 and 15 respectively.
Answer the following questions giving reasons:
(i) What is the number of valence electrons and valency of D ?
(ii) Which of them will have largest atomic radii ?
(iii) Which of these elements will form the most basic oxide ? 
Answer : (i) Valence electrons in ‘D’ = 5 and Valency of ‘D’ = 3. 
(ii) ‘A’ will have largest atomic radii because atomic radius decreases across a period from left to right. 
(iii) ‘A’ will form the most basic oxide as it is most metallic. 

Question. The atomic number of an element is 14. Examine if this element will have metallic properties or not. Give reason to justify your answer. 
Answer : Atomic number =14
Electronic configuration = 2, 8, 4
Hence, the element is silicon. This element exhibit the properties of both metals and non-metals. Hence, it is semimetal or metalloid. 

Question. An element ‘X’ has mass number 35 and 18 number of neutrons. Therefore, electronic configuration of X= 2, 8, 7. Write atomic number and electronic configuration of ‘X’. Also write group number, period number and valency of ‘X’.
Answer : Atomic number of X = Mass number of X – Number of neutrons
35 –18 =17
Therefore, Electronic configuration of X= 2, 8, 7
Group number =17
Period number = 3
Valency = 8 – 7 = 1 

Question. How does the tendency of the elements to lose electrons change in the Modern Periodic Table in
(i) a group,
(ii) a period and why ?
Answer : (i) Increases down a group. 
Reason: At each succeeding element down a group, the number of shells increases, so the distance of the valence shell from the nucleus increases, the effective nuclear force of attraction decreases in the last shell, so it becomes easy for the atom to lose electrons. 
(ii) Decreases in a period from left to right. 
Reason: As the effective nuclear charge on the valence electron increases, the attraction between the valence electron and nucleus increases, so it becomes difficult to lose electrons.

Question. An element ‘X’ has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of ‘X’. Also write group number, period number and valency of ‘X’.
Answer : Atomic number of X = Mass number of X – No. of neutrons 
= 35 – 18 = 17 
Therefore, Electronic configuration of
X = 2, 8, 7 
Group number =17 
Period = 3
Valency = 8 – 7 = 1 

Question. Two elements X and Y have atomic numbers 12 and 16 respectively. To which period of the modern periodic table do these two elements belong?
What type of bond will be formed between them and why? Also give the chemical formula of the compound formed. 
Answer : Electronic configuration of X: 2, 8, 2, Y: 2, 8, 6 Both X and Y belongs to 3rd period.
Ionic bond will be formed.
Reason: X will lose 2 electrons and Y will gain 2 electrons to complete their octet and become stable.
Formula is XY.

Question. Write the number of periods and groups in the Modern Periodic Table. How does the metallic character of elements vary on moving (i) from left to right in a period, and (ii) down a group ? Give reason to justify your answer. 
Answer : • Periods – 7, Groups – 18 
• Metallic character decreases along the period because effective nuclear charge increases on the valence electrons hence decrease in tendency to lose electrons. 
• Metallic character increases down a group because effective nuclear charge experienced by valence electrons decrease, hence tendency to lose electron increases. 

Question. An element ‘X’ belongs to 3^rd period and group 16 of the Modern Periodic Table.
(a) Determine the number of valence electrons and the valency of ‘X’.
(b) Molecular formula of the compound when ‘X’ reacts with hydrogen and write its electron dot structure.
(c) Name the element ‘X’ and state whether it is metallic or non-metallic. 
Answer : (a) Electronic Configuration of X -2, 8, 6
Valence electrons = 6 
Valency = 8 - 6 = 2
(b) Formula with hydrogen- H₂X or H₂S 

(c) Sulphur; Non-metal

Long Answer Type Questions :

Question. (a) What was the basis of Mendeleev’s classification of elements ?
(b) List two achievements of Mendeleev’s Periodic table.
(c) List any two observations which posed a challenge to Mendeleev’s periodic law.
Answer : (a) Atomic mass 
(b) (i) He could classify all the 63 elements known at that time.
(ii) He left gaps for the yet to be discovered elements.
(iii) He predicted the properties of such elements.
(c) (i) Position of isotopes
(ii) Irregular increase in atomic masses in going from one element to the next, making the prediction of undiscovered elements difficult.
(iii) Position of Hydrogen. 

Question. (a) List any three observations which posed a challenge to Mendeleev’s Periodic Law.
(b) How does the metallic character of elements vary on moving from
(i) left to right in a period,
(ii) From top to bottom in a period of the Modern Periodic Table ? Given reason for your answer.
Answer : (a) (i) No fixed position of H in the periodic table. 
(ii) Position of isotopes not clear. 
(iii) Atomic mass does not increase in a regular manner 
(b) (i) Left to right metallic character decreases. 
Reason: Effective nuclear charge increases / tendency to loose electrons decrease / electropositivity decreases.
(ii) Top to bottom metallic character increases.
Reason: Size of atom increase/tendency to loose electron increases. 

Question. (a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all the three attempts.
(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.
Answer : (a) (i) Dobereiner Periodic Table
Advantage: To predict the atomic mass of middle element in each triad.
Limitation: Dobereiner could identify only three triads.
(ii) Newland Periodic table
Advantage: Every eighth element had properties similar to that of first and co-related the properties of elements with their atomic mass.
Limitation: It was only applicable upto Calcium/only 56 elements and no future element.
(iii) Mendeleev’s Periodic Table
Advantage: Elements with similar properties could be grouped. He predicted the existence of new elements that had not been discovered at that time.
Limitation: No fixed position for hydrogen and isotopes. Atomic masses do not increase in a regular manner.
(b) Henry Moseley: Properties of elements are a periodic function of their atomic number. 

Question. The position of certain elements in the Modem Periodic Table are shown below:

Using the above table answer the following questions giving reasons in each case:
(i) Which element will form only covalent compounds?
(ii) Which element is a non-metal with valency 2 ?
(iii) Which element is a metal with valency 2 ?
(iv) Out of H, C and F which has largest atomic size ?
(v) To which family does H, C and F belong ? 
Answer : (i) Element E is Silicon. It will form covalent bond only as it has four electrons in its outermost orbit and need only four more electrons to become stable.
(ii) Non-metal with valency 2 is B, which is Oxygen.
(iii) Element D is a metal with valency 2. Element D is Magnesium. Due to its low electronegativity, it has a higher tendency to donate electrons.
(iv) Element F has the largest atomic size. Element F is Argon. Argon occupies 3 energy shells compared to elements H and C, which occupies one and two energy shells. Due to this, the atomic radius of Argon is the largest.
(v) Elements H, C and F belong to Group number 18, which means according to their electronic configuration, their octet is complete and thus these elements are stable. They have very low tendency to react with other elements.
Group 18 elements belong to noble gas family. 

Question. Explain giving justification the trends in the following properties of elements, on moving from left to right in a period, in the Modern periodic Table:
(a) Variation of valency.
(b) Change of atomic radius.
(c) Metallic to non-metallic character.
(d) Electronegative character.
(e) Nature of oxides.
Answer : (a) Valency first increases, then decreases
(b) Decreases
(c) Increase
(d) Increases
(e) Change from basic to acidic
(a) Valency is the combining capacity of an element.
Valence electrons are the number of electrons in the outermost shell. Valency and valence electrons are same till the number of outermost electrons is 4, but when it goes beyond 4, then the outermost electrons are subtracted from 8 and valency is determined. Thus, valency first increases along the period and then decreases. 
(b) Along the period, from left to right effective nuclear charge increases as the number of protons increase, due to which force of attraction between nucleus and the valence electron increases therefore, atomic radius decreases. 
(c) Along the period, from left to right effective nuclear charge increases as the number of protons increases, due to which force of attraction between nucleus and the valence electron increases thus, it becomes difficult to lose electron across the period and metallic to non-metallic character increases. 
(d) Along the period, from left to right effective nuclear charge increases as the number of protons increases, due to which force of attraction between nucleus and the valence electron increases. Hence the electron from the outermost orbit is difficult to remove, atomic size decreases therefore electronegativity increases. 
(e) Along the period, from left to right effective nuclear charge increases as the number of protons increases, due to which force of attraction between nucleus and the valence electron increases, hence the electron from the outermost orbit is difficult to remove. Therefore, across the period the metallic to non-metallic character increases. So nature of oxide formation across the period changes from basic to acidic.

Question. (i) Why do we classify elements ?
(ii) What are the two criteria used in the development of Modern Periodic Table ?
(iii) State the position of (a) metals, (b) non-metals and (c) metalloids in the periodic table.
(iv) Would you place two isotopes of chlorine; Cl–35 and Cl–37 in different slots of the periodic table because of their different atomic masses or in the same slot because their chemical properties are same ? 
Answer : (i) To study the properties of elements and to keep the elements with similar properties together. 
(ii) Chemical properties of elements and atomic number. 
(iii) Metals lie on extreme left, metalloids lie in the middle and non-metals lie on the right side. 
(iv) They should be placed in the same slot. Since they have same numbers of electrons/atomic number and Modern Periodic Table is based on atomic number and not on atomic mass.