Atoms with small electronegativities, which have strong tendencies to lose one, two, or three electrons and become positive ions, can associate in a special kind of solid without the presence of negative ions. These are the metals.

A metal is an orderly crystalline packing of positive ions, each one having given up electrons from its outermost, incomplete electron shell (top of next page). These electrons are mobile, and are free to wander from one end of the metal to the other.

The electrons surround the positive ions and hold the metal together. Without them, the full force of electrostatic repulsion between positive ions would blow the metal apart. With them, the positive ions can be packed together like marbles in a box.

The structures of most metals are really just as simple as that: they represent efficient ways of packing spheres in a limited space.

Lithium metal is held together by one electron per positive ion, and beryllium by twice as many electrons. The additional electrons help to hold the positive ions together more strongly and make beryllium a harder metal than lithium.

These electrons, instead of being confined to the vicinity of one atom as in a gas, are completely delocalized.

As we already have seen for benzene and for carbonic and nitric acids, delocalization enhances stability. Part of the forces that hold a metal together comes from this delocalization.