While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?

Explanation

To explain why the 4s orbital is filled before the 3d orbital but the reverse occurs during ionization, we can break down the explanation into a series of steps:

### Step-by-Step Solution:

1. **Understanding the Aufbau Principle**:
- The Aufbau principle states that electrons fill atomic orbitals starting from the lowest energy level to the highest.

2. **Applying the n + l Rule**:
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