Metallic Solids

Metallic solids, such as copper, aluminum, and gold, are solids that have metal atoms as their constituent particles, which typically are held together by strong metallic bonds.

In a metallic bond, the electron clouds of neighboring metal atoms overlap, such that these valence electrons are delocalized throughout the solid. Metallic solids are sometimes described as a lattice of stationary metal cations in a sea of delocalized electrons.

This results in a more flexible internal structure than that of molecular or ionic solids. Metallic solids generally can bend and deform without breaking to a greater extent than other solids.

As exhibited by gold and mercury, their melting points vary widely depending on the strength of the metallic bonds. The mobile electrons also make these solids excellent conductors of heat and electricity.

The atoms of pure metallic solids are typically arranged, or ‘packed’, in one of the two most efficient possible configurations for spheres of a uniform size: hexagonal close-packing or cubic close-packing.

In hexagonal close-packing, the metal atoms are arranged hexagonally in two repeating layers, where the atoms in the second layer occupy the depressions of the first layer. The third layer is a repetition of the first layer, resulting in a characteristic ABAB arrangement.

Zinc metal exhibits a hexagonal close-packed, or hcp, lattice. Its unit cell contains two atoms that are not on the lattice points.

In cubic close-packing, the atoms are arranged hexagonally in three repeating layers. The atoms in every fourth layer align with the atoms in the first layer, making a characteristic ABCABC arrangement.

Copper metal has a cubic close-packed, or ccp, lattice. This lattice is also called face-centered cubic, or fcc, owing to the arrangement being that of a face-centered cubic lattice with an identical atom on every lattice point.

In both types of packing, each atom has a coordination number of twelve. In hcp stacking, each atom is in direct contact with six atoms from the same layer, three from the layer above, and three from the layer below.