A metal can exist in the gaseous, liquid or solid states depending upon the pressure and temperature. Metals have the ability to donate electrons and form a positive ion. Metals have high density, high melting temperature, good electrical and thermal conductivity, metallic lustre and crystalline structure.
Structure of Solids
On the basis of structure, solids are broadly classified as:
- Amorphous Solids. These are solids where the atoms making up the crystal are not arranged in a systematic order. Examples of amorphous solids are wood, plastic, glass, rubber etc.
- Crystalline Solids. These are solids where the atoms making up crystals are arranged in a systematic order. Examples of crystalline solids are: Iron, Copper, Aluminium, Zinc and Nickel, etc.
Space Lattice and Unit Cell
In a crystal, the atoms are arranged in a periodic and regular geometric pattern in space. The arrangement of atoms in a crystal can be described with respect to a three-dimensional net of straight lines, called space lattice. The intersections of lines are points of a space lattice. The important characteristic of a space lattice is that every point has identical surroundings. (Refer Fig 1)
Unit Cell. The number of atoms which constitute a crystal is very large and even the smallest crystals are composed of billions of atoms. The grouping of atoms, whose repetition will produce the crystal is called the unit cell. A unit cell is a building block of the crystal. (Refer Fig 2)
Lattice Parameters of a Unit Cell
The edges of unit cell (i.e., length, breadth and height) a, b, c is called primitives and the three angles α, β, γ are known as interfacial angles of a unit cell. (Refer Fig 3)
These three edges (a, b, c) and three interfacial angles (α, β, γ ) of the unit cell are called lattice parameters or geometrical constant of a crystal system, made up of such unit cells.
Crystal Structures for Metallic Elements
The crystals of most metals have highly symmetrical structure. The most common types of lattices are:
- Body Centred Cubic Structure (BCC)
- Face Centred Cubic Structure (FCC)
- 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. (Refer Fig. 4)
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. (Refer Fig 5)
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. (Refer Fig 6)
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.