There are `2, 8` and `8` elements in the first, second and third periods of periodic table respectively. Explain.

Explanation

To explain why there are 2, 8, and 8 elements in the first, second, and third periods of the periodic table respectively, we need to understand the structure of electron shells and subshells.

### Step-by-Step Solution:

1. **Understanding Periods**:
- The periodic table is arranged in periods (horizontal rows) that represent the energy levels of electrons in an atom. Each period corresponds to the filling of a new electron shell.

2. **First Period**:
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