is non-directional because an ion has the same attraction from all directions for an ion of opposite charge.

In the same way, a magnet attracts iron filings from all directions.

This means that packing efficiency in the crystal lattice determines the arrangement of ions in a crystal.

There is no preferred direction for the bonds.

Thus, in a crystal of ##”NaCl”##, a sodium ion has 6 chloride ions as nearest neighbours.

The ##”Na-Cl”## “bonds” are all at angles of 90° to each other.

In a crystal of cesium chloride, a cesium ion has eight chloride atoms as nearest neighbours.

The ##”Cs-Cl”## “bonds” are at different angles.

The differences arise because ##”Cs”^”+”## and ##”Cl”^”-“## have almost the same ionic radii (about 170 pm) and pack in one arrangement.

A ##”Na”^”+”## ion is smaller (116 pm) than ##”Cl”^”-“##, so the ions pack in a different arrangement.