In a metal, the atoms give up their valence electrons to become positive ions, and these electrons move freely through the whole crystal as free electrons. The bonding in which free electrons hold the positive ions together is called metallic bonding.
When a voltage is applied to a metal, the free electrons flow in one direction, so metals conduct electricity well. Heat is also carried by the free electrons, so metals conduct heat well too.
An arrangement with atoms at the eight corners and at the centre of a cube is called body-centred cubic. Sodium, potassium and iron at room temperature have this arrangement.
Each atom touches 8 others, so the coordination number is 8, and the atoms fill about 68 % of the volume of the crystal. A unit cell contains 2 atoms.
An arrangement with atoms at the eight corners of a cube and at the centres of its six faces is called face-centred cubic. Copper, silver, gold and aluminium have this arrangement.
The coordination number is 12, and the atoms fill about 74 % of the volume, which makes it one of the most closely packed arrangements. A unit cell contains 4 atoms.
An arrangement in which flat layers of atoms, each packed as closely as possible, are stacked so that every other layer lies in the same position is called hexagonal close-packed. Magnesium, zinc and titanium have this arrangement.
The coordination number is 12, and the atoms fill about 74 % of the volume, so it is just as closely packed as face-centred cubic. Face-centred cubic is also built from the same kind of layers, but there every third layer lies in the same position.
When a force is applied to a metal, the layers of atoms slide over one another, but the free electrons keep holding the atoms together, so the metal changes shape instead of breaking. This is why metals spread into thin sheets when hammered (malleability) and stretch into thin wires when pulled (ductility).
In an ionic crystal, when the layers slide, ions with the same charge come face to face and repel each other, so the crystal breaks.