The crystals of most metals have highly symmetrical structure. The most common types of lattices are
(a) Body Centred Cubic Structure (BCC)
(b) Face Centred Cubic Structure (FCC)
(c) Hexagonal Close Packed Structure (HCP)
Body Centred Cubic Structure (BCC) – In this type of structure, atoms are located at the corners of the cube and one atom at its centre. This type of unit cell is found in metals like lithium, sodium, potassium, barium, vanadium etc.
Face Centred Cubic Structure (FCC) – In this type of structure, atoms are located at the corners of the cube and one atom at the centre of each face. This type of unit cell is found in metals-like copper, silver, gold, aluminium, lead etc.
Hexagon Close Packed Structure (HCP) – In this type of structure, an atom is there at each of the twelve corners of the hexagonal cell, one atom at the centre of each of two hexagonal faces and three atoms in the body of the cell. This type of unit cell is found in metals like zinc, magnesium, lithium, beryllium etc.
Importance of Crystal Structure in Metallic Elements
Crystal structure is important because it contributes to the properties of a material. For example, it is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore, lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. A Face-Centered Cubic (FCC) crystal structure will exhibit more ductility (deform more readily under load before breaking) than a Body-Centered Cubic (BCC) structure. The BCC lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten have the BCC form. The FCC lattice is both cubic and closely packed and forms more ductile materials. Gamma-iron, silver, gold, and lead have FCC structures. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the FCC metals.