Question

The number of atoms touching a particular atom in a crystal is called its:

• A. valency
• B. co-ordination number
  
• C. lattice parameter
  
• D. atomic packing factor

Answer 1

The correct option is B co-ordination number

Co-ordination number of a central atom in a molecule or crystal is the number of its near neighbours or the number of atoms touching a particular atom in a crystal.
Example: Co-ordination number of Na atom in NaCl is $6$.