Chapter 7, Problem 1KSP

Often we can compare properties of two elements based solely on periodic trends. For which pair of elements is the periodic trend not sufficient to determine which has the higher first ionization energy?

$\begin{array}{l}\left(\text{a}\right)\hfill \\ \text{C and Si}\hfill \\ \left(\text{b}\right)\hfill \\ \text{Al and Ga}\hfill \\ \left(\text{c}\right)\hfill \\ \text{Ga and Si}\hfill \\ \left(\text{d}\right)\hfill \\ \text{Tl and Sn}\hfill \\ \left(\text{e}\right)\hfill \\ \text{B and Si}\hfill \end{array}$

Interpretation Introduction

Interpretation:

A pair of elements is to be identified, which cannot be compared on the basis of ionization enthalpy by using the periodic trend.

Concept introduction:

Effective nuclear charge increases across a period and $IE$ also increases across the period.

It is more difficult to remove a paired core electron (electron from the inner shells) than an unpaired valence electron.

Ionization energy tends to increase across a period and decreases down the group.

Answer to Problem 1KSP

Correct answer: Option (b).

Explanation of Solution

Reason for correct option:

For the elements, $\text{Al and Ga}$, $\text{Ga}$ is below $\text{Al}$ in the same group, indicating that $\text{Al}$ has a higher ionization enthalpy $\left(IE\right)$ than $\text{Ga}$. But $\text{Ga}$ has a higher $IE$ than $\text{Al}$ because the effective nuclear charge of gallium is slightly higher than that of aluminum.

Hence, option (b) is the correct answer.

Reason for incorrect options:

Option (a) is incorrect because $\text{Si}$ is below $\text{C}$ in the same group, clearly indicating that $\text{C}$ has a higher $IE$ than $\text{Si}$.

Option (c) is incorrect because $\text{Ga}$ is below $\text{Si}$ but to its left, indicating that $\text{Si}$ has a higher $IE$ than $\text{Ga}$.

Option (d) is incorrect because $\text{Tl}$ is below $\text{Sn}$ but to its left, indicating that $\text{Sn}$ has a higher $IE$ than $\text{Tl}$.

Option (e) is incorrect because $\text{Si}$ is below $\text{B}$ but to its right, indicating that $\text{Si}$ has a higher $IE$ than $\text{B}$.

Hence, options(a), (c), (d), and (e) are incorrect.