JEE Main & Advanced Chemistry The Solid State / ठोस प्रावस्था 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, \[\alpha ,\,\beta ,\gamma .\]

 Types of units cells         

A units cell is obtained by joining the lattice points. The choice of lattice points to draw a unit cell is made on the basis of the external geometry of the crystal, and symmetry of the lattice. There are four different types of unit cells. These are,              

(1) Primitive or simple cubic (sc) : Atoms are arranged only at the corners of the unit cell.

(2) Body centred cubic (bcc) : Atoms are arranged at the corners and at the centre of the unit cell.

(3) Face centred cubic (fcc) : Atoms are arranged at the corners and at the centre of each faces of the unit cell.

(4) Side centered : Atoms are arranged at the centre of only one set of faces in addition to the atoms at the corner  of the unit cell.