How would you justify the presence of `18` elements in the `5th` period of the periodic table?

Explanation

To justify the presence of 18 elements in the 5th period of the periodic table, we can follow these steps:

### Step 1: Understand the Electron Configuration
The 5th period corresponds to the principal quantum number \( n = 5 \). The order of filling of electrons in atomic orbitals follows the Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level to the highest.

### Step 2: Identify the Orbitals in the 5th Period
In the 5th period, the orbitals that are filled with electrons are:
- \( 5s \)
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