What is unit cell ? Write formula to calculate the density of a unit cell.

Explanation

### Step-by-Step Solution

**Step 1: Definition of a Unit Cell**
A unit cell is defined as the smallest repeating unit of a crystal lattice. It is the basic building block of the crystal structure and contains the arrangement of atoms, ions, or molecules in a three-dimensional space.

**Step 2: Characteristics of a Unit Cell**
- The unit cell must contain at least one atom.
- It can be visualized as a three-dimensional box that, when repeated in space, forms the entire crystal lattice.
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