The 3-dimensional network of imaginary lines connecting atoms is called space lattice. The metallic crystals can be considered as consisting of tiny blocks which are represented in three dimensional patterns. The tiny block formed by the arrangement of a small group of atoms is called the unit cell. It is that volume of solid from which the entire crystal can be constructed by translational repetition in three dimensions.
If each atom in a lattice is replaced by a point; then each point is called a lattice point and the arrangement of the points is referred to as the (three dimensional) lattice array. Thus a space lattice is. defined as an array of points in three dimensions in which every point has surroundings identical to that every point in the array. If all the atoms, molecules or ions at the lattice points are identical, the lattice is called a Bravias lattice.
The distance between the atoms-points is called inter atomic or lattice spacing. A space lattice is a conventional geometrical basis by which crystal structure can be described. Every point of a space lattice has identical surroundings.
The length of the side of a unit cell is the distance between the atoms of the same kind. In the case of pure metals whose crystals have simple cubic structure, it is equal to the basic distance 'a' only.
There are 14 types of possible lattices and they fall into 7 crystal systems.
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