Question

Describe the unit cell of sodium chloride with a neat diagram stating:

1. Type of bonding
2. Type of unit cell
3. Number of nearest neighbours around sodium and chloride.

Answer 1

Unit Cell of Sodium Chloride (NaCl)

(a) Type of bonding: Ionic bonding

• Sodium (Na) loses one electron to become Na⁺.
• Chlorine (Cl) gains one electron to become Cl⁻.
• The solid is held together by strong electrostatic forces of attraction between oppositely charged ions.

(b) Type of unit cell: 

• Face-Centred Cubic (FCC) or Cubic Close-Packed (ccp) structure.
• Chloride ions (Cl⁻) occupy the corners and the face centers of the cube.
• Sodium ions (Na⁺) occupy all octahedral voids, i.e., the edge centers and the body center.
• This is also referred to as a rock salt structure.

(c) Number of nearest neighbours around sodium and chloride.

• Each Na⁺ ion is surrounded by 6 Cl⁻ions.
• Each Cl⁻ ion is surrounded by 6 Na⁺ ions.
• Hence, Coordination Number = 6 : 6