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:
(a) 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.
(b) 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.
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.
Lattice Parameters of a Unit Cell
The edges of unit cell (i.e., length, breadth and height) a, b, c are called primitives and the three angles α, β, γ are known as interfacial angles of a unit cell
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.