State the order of filling atomic orbitals following Auf- bau principle.

The Aufbau principle describes the order in which atomic orbitals are filled with electrons. This filling follows a specific sequence based on energy levels. Here’s how it works:

Order of Filling Atomic Orbitals

Electrons occupy orbitals starting from the lowest energy level to the highest. The general order can be summarized as follows:

• 1s
• 2s
• 2p
• 3s
• 3p
• 4s
• 3d
• 4p
• 5s
• 4d
• 5p
• 6s
• 4f
• 5d
• 6p
• 7s
• 5f
• 6d
• 7p

Key Points to Remember

1. **Energy Levels**: Orbitals are filled based on increasing energy, not just by their principal quantum number.

2. **Hund's Rule**: Within a subshell, electrons will fill degenerate orbitals singly before pairing up.

3. **Pauli Exclusion Principle**: No two electrons can have the same set of quantum numbers, meaning each orbital can hold a maximum of two electrons with opposite spins.

This systematic approach helps in predicting the electron configuration of elements, which is crucial for understanding their chemical behavior.