Chapter 6, Problem 74A

Interpretation Introduction

Interpretation:

Between ${\text{Mg}}^{2+}$ and ${\text{Na}}^{+}$ , the ion having a smaller size is to be identified.

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 74A

  ${\text{Mg}}^{2+}$

Explanation of Solution

In ${\text{Mg}}^{2+}$ no. of electrons is 10 but the no. of protons is 12. While in the case of ${\text{Na}}^{+}$ , no. of electrons is 10 and no. of protons is 11. So, no. of electrons is the same but the nuclear charge is more in ${\text{Mg}}^{2+}$ . As nuclear charge increases, electrons in the outermost shell are drawn to it with increasing force, which causes the size of atoms to gradually shrink over time.

Thus, density increases across the period from potassium and chromium.