Q.(i) What is meant by the term 'coordination number'?

$(i)$ Coordination number is the number of nearest neighbouring atoms/ions in immediate contact with a given atom/ion in the crystal. …

Step 1 – Definition.

The coordination number of an atom (or ion) in a crystal is the number of its nearest neighbours — the particles in direct contact with it.

Step 2 – ccp (cubic close-packed).

Each sphere in ccp touches $6$ in its own layer, $3$ in the layer above and $3$ in the layer below:

Step 3 – bcc (body-centred cubic).

The central atom touches the $8$ corner atoms of the cube: …

Define coordination number as nearest-neighbour count, then recall/derive the standard values: …

• Quoting 8 for ccp or 12 for bcc (values swapped).
• Counting next-nearest neighbours as well. …