Chapter 6, Problem 67A

Interpretation Introduction

Interpretation:

Three balls are given. It is to be identified which ball represents $\text{Ca}$ , ${\text{Ca}}^{\text{2+}}$ , and ${\text{Mg}}^{\text{2+}}$ .

Concept Introduction : An ion is formed by gaining or losing electrons by an atom. The number of electrons lost or gained by an atom becomes its charge. If an element loses electrons, it has a positive charge. In the symbol for an ion, the charge is represented as a superscript of the symbol of the atom for that element.

Answer to Problem 67A

Ball a represents $\text{Ca}$ , ball b represents ${\text{Ca}}^{\text{2+}}$ , and ball c represents ${\text{Mg}}^{\text{2+}}$ .

Explanation of Solution

Calcium and magnesium are in the same group. Calcium is placed below magnesium. Atomic size increases down the group. This is because when moving from top to bottom in the group, both the number of shells and nuclear charge increase. The effect of more shells, however, outweighs the effect of a higher nuclear charge which increases atomic radius.

Thus, the size of calcium is larger than magnesium.

Thus, ball a represents $\text{Ca}$ .

Cation size is always smaller than the atomic size due to a decrease in the number of electrons and decreases in nuclear attraction.

Thus, ball b represents ${\text{Ca}}^{\text{2+}}$ , and ball c represents ${\text{Mg}}^{\text{2+}}$ .