Question

Explain each of the following observations. Explanations such as “Ca is larger than Mg

because atoms get larger as you down a column of the periodic table” are not acceptable; you

must tell me why this trend occurs.

a) The atomic radius of Ca is larger than the atomic radius of Mg.

b) The atomic radius of K is larger than the atomic radius of Ca.

c) The ionic radius of S2– is larger than the ionic radius of Cl–

.

d) The ionic radius of Zr3+ is larger than the ionic radius of Zr4+

Answer 1

a) Atomic radius of Ca is larger than the atomic radius of Mg because as we move down the group, number of shells increases, screening effect increases and thus, the distance between nucleus and outer electron increases.

Screening effect is the screening of outer electrons by inner electrons due to which outer electrons are less attracted. The number of protons and hence positive charges also increases but the effect of first two is more than the latter. Hence size increases.

b)Atomic size Of K is larger than the atomic radius of Ca.In this, number of electrons are added to the same shell from K to Ca. But number of protons are increasing. Thus, the attraction between outer electrons and nucleus increases and hence size increases.

c) atomic number of S = 16 = number of protons = number of electrons

In S^2-, number of electrons = 18 but number of protons = 16

In Cl^-, number of protons = 17 and number of electrons = 18

Thus, in both S^2- and Cl^- ,number of electrons are same = 18

but number of protons in Cl^- are more than the number of protons in S^2-

Thus, attraction is more in case of Cl^- than in S^2-

Hence size of Cl^- is less than the size of S^2-

or size of S^2- is less than the size of Cl^-

d) Zr³⁺, number of protons = 40, number of electrons = 37

In Zr⁴⁺, number of protons = 40,number of electrons = 36

Thus, number of elctrons in case of Zr⁴⁺ is less than the number of electrons in Zr³⁺

So, attraction in case of Zr⁴⁺ is more than in Zr³⁺

Hence, ionic radius of Zr3+ is larger than the ionic radius of Zr4+