Class X – Periodic Classification of Elements

Ans. The Newlands’ law of octaves states that every eighth element has properties similar to that of the first when the elements are arranged in the increasing order of their atomic masses.

Ans. Atomic mass as a physical property and nature and formulae of oxide and hydride formed, and chemical property was used by Mendeleev.

Following are the two observations which posed a challenge to Mendeleev’s Periodic Law.

(i) Increasing order of atomic weights could not be maintained while matching chemical properties. Chemical properties do not depend upon atomic mass.

(ii) Isotopes have different atomic mass but same chemical properties.

Ans. Mendeleev’s Periodic Law: Properties of elements are the periodic function of their atomic masses.

Achievements:

It could classify all the elements discovered at that time.

It helped in discovery of new elements.

It helped in correction of atomic mass of some of the elements.

Gaps were left for undiscovered elements in the Mendeleev’s Periodic Table

(b) Any three limitations of Mendeleev’s classification.

Three limitations of Mendeleev’s classification are:

(i) Position of hydrogen was not justified.

(ii) Increasing order of atomic mass could not be maintained.

(iii) Isotopes have similar chemical properties but different atomic masses, they cannot be given separate places.

Ans. Properties of elements are the periodic function of their atomic number.

(a) There are 18 groups and

(b) 7 periods in the Modern Periodic Table.

(a) ₆C, ₈O, ₁₀Ne are in same period i.e., 2^nd period as they have two shells, ₁₁Na and ₁₄Si are in the same period, i.e., 3^rd period as they have three shells.

(b) ₆C, ₁₄Si are in the same group because they have same number of valence electrons, i.e., four.

Ans. Elements with atomic number 12, 38 resemble calcium in physical and chemical properties because they have same number of valence electrons and belong to the same group 2.

Mg (12): 2, 8, 2

Ca (20): 2, 8, 8, 2

Sr (38): 2, 8, 18, 8, 2

Ans.

K,        L,        M

M =     2,         8,         3

Atomic number = 13

Valency = 3

Ans. Calcium and Strontium would show chemical reactions similar to magnesium. They have the same number of electrons in the outermost shell

Be, Mg, Ca, Sr, Ba

Same period

Group 1

Same period

Noble gas

Elements of same group have similar properties due to same number of valence electrons, therefore, they have same valency.

Elements of same period have different properties as they differ in number of valence electrons.

The valency of elements remains the same in going down a group

On moving from left to right in a period, the valency of elements increases from 1 to 4 and then decreases to 0 (zero)

Li        Ba       B         C         N         O         F          Ne

2,1       2,2       2,3       2,4       2,5       2,6       2,7       2,8

Na(2,8,1)

K(2,8,8,1)

Rb(2,8,18,8,1)

Cr

Fr

Sulphur (2,8,6)

Chlorine (2,8,7)

Chlorine (Z=17) is placed after sulphur (Z = 16) in the same period i.e., third period. The size of chlorine is smaller than that of sulphur and its atom needs only one electron to have noble gas electronic configuration while sulphur atom needs two electrons. Therefore, chlorine has greater attraction for electrons than sulphur. It is more electronegative than sulphur.

Li        Be        B         C         N         O         F

+3        +4        +5        +6        +7        +8        +9

Nuclear charge increases à

ßSize decreases

Li (more metallic)

F (More non-metallic)

(a) It is because nuclear charge increases due to increase in atomic number, therefore, force of attraction between nucleus and valence electrons increases, i.e., effective nuclear charge increases, hence atomic radii decrease from Li to F

(b) (i) Most metallic element is Li as it can lose electrons easily due to larger atomic size

(ii) Most non-metallic element is ‘F’ because it can gain electrons easily due to smallest atomic size.

Na                                           Mg                                          Al

2,8,1                                        2,8,2                                        2,8,3

More reactive                         Less reactive                          Least reactive

Size maximum

(a) Na has the largest atomic radius because it has 11 protons and 11 electrons, therefore least effective nuclear charge.

(b) Al is least reactive because it has smallest atomic size due to 13 protons and 13 electrons, it has greater effective nuclear charge, therefore, cannot lost electrons easily, hence it is least reactive.