Chapter 12.2, Problem 5RQ

Interpretation Introduction

Interpretation:

The reason for cations to be smaller and anions to be larger than their parent atoms needs to be explained.

Concept introduction:

Cations are formed by loss of electrons.

Anions are formed by gain of electrons.

Explanation of Solution

Cations are smaller than their parent atoms because after losing the electrons, ion has less number of electrons but number of protons remains same. Thus same nuclear charge has to hold less number of electrons. Therefore, effective nuclear charge increases and the size decrease.

Anions are larger than their parent atoms because after gaining the electrons, ion has more number of electrons but number of protons remains same. Thus same nuclear charge has to hold more number of electrons. Therefore, effective nuclear charge decreases and the size increase.

For example,

  $N{a}^{+}$ is smaller than $Na$

       $N{a}^{+}$    $Na$

Protons		11		11
Electrons	10		11

Thus, in $N{a}^{+}$ 11 protons are to hold 10 protons. This increases effective nuclear charge. So size decreases.

  $C{l}^{-}$ is larger than $Cl$

       $C{l}^{-}$    $Cl$

Protons		17		17
Electrons	18		17

Thus, in $C{l}^{-}$ 17 protons are to hold 18 protons. This decreases effective nuclear charge. So size increases.

Conclusion

Thus, cations are always smaller and anions are always larger than their parent atoms.