Chapter 7, Problem 71A

Interpretation Introduction

Interpretation: Whether the coordination number of an ion can be predicted from the formula of an ionic compound or not needs to be explained.

Concept Introduction: The coordination number is defined as the number of ions that surround an ion of opposite charge. In an ionic compound, the total charge is zero thus, positive and negative charge ions combine in a ratio such that the overall compound is neutral.

Explanation of Solution

An ionic compound is formed by a chemical combination of positive and negative charge ions. Here, metal forms cation (positive charge ion), and non-metal forms anion (negative charge ion). From an ionic compound, the charge on an ion can be predicted. The coordination number is the number of ions surrounding an oppositely charge ion. It may or may not be equal to the charge of the ion thus, one cannot predict the coordination number from the formula of an ionic compound.

For example, the formula for the ionic compound sodium chloride is NaCl. In dissociated form, it can be represented as ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions. Here, the charge on the Na ion is +1, and the charge on the Cl ion is -1. In the crystal lattice, 1 Na atom is surrounded by 6 Cl atoms, and 1 Cl atom is surrounded by 6 Na atom; thus, the coordination number of both Na and Cl is 6, not 1.