What is the efficiency of packing in case of a metal crystal for
(i) simple cubic (ii) body-centred cubic (iii) face-centred cubic (with the assumptions that atoms are touching each other)

Explanation

To determine the packing efficiency of a metal crystal for different types of unit cells, we can follow these steps:

### Step 1: Understand Packing Efficiency
Packing efficiency is defined as the fraction of volume occupied by the atoms in a unit cell compared to the total volume of the unit cell. It is usually expressed as a percentage.

### Step 2: Simple Cubic Packing Efficiency
In a simple cubic unit cell:
- There is 1 atom per unit cell (1/8 of an atom at each of the 8 corners).
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