Q78P - Chemistry: Molecular Nature Of Matter And Change

Question

The sum of ${\mathbf{IE}}_{\mathbf{1}}$ through ${\mathbf{IE}}_{\mathbf{4}}$ for group 4A (14) elements shows a decrease from C to Si,  a slight increase from Si to Ge, a decrease from Ge to Sn and an increase from Sn to Pb.

1. What is the expected trend for IEs down a group?
2. Suggest a reason for the deviations in group 4 A (14).
3. Which group might show even greater deviations?

Step-by-Step Solution

Answer

1. In general, ionization energy decreases down the group
2. The deviations which are there, are due to the presence of the first transition series elements (d block elements), that are present between Si and Ge and of the lanthanides (f block) between Sn and Pb.
3. Group 3 A shows even greater deviations from the general trend of ionization energy. IE from B to Al decreases significantly as one goes down the group.

 

Introduction

Ionization energy

The amount of energy required to remove an electron from an isolated atom or molecule is known as its ionization energy or ionization potential.

Explanation a)

In general, the ionization energy decreases down the group in the periodic table, because the attraction of the nucleus to the valence electrons is decreased with the increased number of shells, while the ionization energy increases as we move in a period from left to right due to increase in nuclear charge, valence electrons are strongly attracted to the nucleus and thus their removal becomes difficult and requires a higher amount of energy.

Explanation b)

The deviations which are there are due to the presence of the first transition series elements (d block elements), that are present between Si and Ge and of the lanthanides (f block) between Sn and Pb. As the presence of d and f electrons provide a poor shielding effect, so the outer electrons become more strongly attracted to the nucleus, which results in higher amount of ionization energy. Therefore, the ionization energy increases from Si to Ge (introduction of d electrons), then again decreases from Ge to Sn as a usual trend, then again the introduction of f electrons makes ionization energy even higher, so IE increases from Sn to Pb.

Table

Explanation c)

Group 3 A shows even greater deviations from general trend of ionization energy. IE from B to Al decreases significantly as one goes down the group. In case of Ga, In and Tl, the d subshell is filled between the   pair and the final   valence electrons. The 4f subshell also intervenes in the building up process of Tl and it is known to us that d and f electrons are poor in screening nuclear charge. Therefore, the valence electrons of Ga, In and Tl are more strongly attracted towards the nucleus, and so have unusually higher ionization enthalpies.

 

Table