State variation in number of shells in Period

Explanation

### Step-by-Step Text Solution:

1. **Understanding Periods in the Periodic Table**:
- Periods are the horizontal rows in the periodic table. Each period corresponds to the number of electron shells that the elements in that period have.

2. **Identifying the Variation in Number of Shells**:
- As we move from left to right across a period, the number of electron shells remains constant. This means that all elements in the same period have the same number of electron shells.

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