The longest and shortest periods are

Explanation

To determine the longest and shortest periods in the periodic table, we will analyze the number of elements in each period.

### Step-by-Step Solution:

1. **Identify the Periods**: The periodic table is divided into seven periods. Each period contains a specific number of elements.

2. **Count the Elements in Each Period**:
- **1st Period**: Contains 2 elements (H and He).
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