Why the third period of Modern periodic table contains `8` elements and not `18`?

Explanation

To answer the question of why the third period of the Modern Periodic Table contains 8 elements and not 18, we can break down the explanation into a step-by-step solution:

### Step-by-Step Solution:

1. **Understanding Periods in the Periodic Table**:
- The Modern Periodic Table is organized into periods, which are horizontal rows. Each period corresponds to the filling of a new principal energy level (n).

2. **Identifying the Third Period**:
- The third period corresponds to the principal quantum number \( n = 3 \). This means that the electrons being added are filling the 3s and 3p subshells.

3. **Subshells in the Third Period**:
- In the third period, the relevant subshells are:
- 3s subshell: This subshell has 1 orbital.
- 3p subshell: This subshell has 3 orbitals.

4. **Calculating the Number of Electrons**:
- Each orbital can hold a maximum of 2 electrons (according to the Pauli Exclusion Principle).
- Therefore, the total number of electrons that can be accommodated in the third period is calculated as follows:
- For the 3s subshell: \( 1 \text{ orbital} \times 2 \text{ electrons/orbital} = 2 \text{ electrons} \)
- For the 3p subshell: \( 3 \text{ orbitals} \times 2 \text{ electrons/orbital} = 6 \text{ electrons} \)
- Adding these together gives: \( 2 + 6 = 8 \text{ electrons} \).

5. **Conclusion**:
- Since the third period can accommodate only 8 electrons, it contains only 8 elements. These elements are sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar).

### Summary:
The third period of the Modern Periodic Table contains 8 elements because it corresponds to the filling of the 3s and 3p subshells, which can collectively hold a maximum of 8 electrons.

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