Space Lattice and Unit Cell
Crystal is a homogeneous portion of a crystalline substance, composed of a regular pattern of structural units (ions, atoms or molecules) by plane surfaces making definite angles with each other giving a regular geometric form.
A regular array of points (showing atoms/ions) in three dimensions is commonly called as a space lattice, or lattice.
Each point in a space lattice represents an atom or a group of atoms.
Each point in a space lattice has identical surroundings throughout.
A three dimensional group of lattice points which when repeated in space generates the crystal called unit cell.
The unit cell is described by the lengths of its edges, a, b, c (which are related to the spacing between layers) and the angles between the edges,