How do atomic sizes vary in a group and in a period? Give reason for the variations.

Explanation

### Step-by-Step Solution:

1. **Understanding Atomic Size**: Atomic size refers to the distance from the nucleus of an atom to the outermost shell of electrons. It is typically measured in picometers or angstroms.

2. **Variation in a Group**:
- When moving **down a group** in the periodic table (for example, from lithium to sodium to potassium), the atomic size **increases**.
- **Reason**: As we move down a group, the number of electron shells increases. For instance:
- Lithium (Li) has 1 shell.
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