Out of bonding and antibonding M.O, which is filled first and why?

Answer

To determine which molecular orbital (M.O.) is filled first between bonding and antibonding, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Molecular Orbitals**:
- Molecular orbitals are formed when atomic orbitals combine. There are two types of molecular orbitals: bonding and antibonding.
- Bonding molecular orbitals (BMO) are lower in energy compared to antibonding molecular orbitals (ABMO).

2. **Energy Levels of Orbitals**:
- The energy of molecular orbitals increases as you move from bonding to antibonding.
- This means that bonding orbitals have a lower energy state, making them more stable than antibonding orbitals.

3. **Filling Order of Electrons**:
- According to the Aufbau principle, electrons fill the lowest energy orbitals first before moving to higher energy orbitals.
- Since bonding molecular orbitals are of lower energy, they will be filled first.

4. **Conclusion**:
- Therefore, bonding molecular orbitals are filled before antibonding molecular orbitals because they have lower energy and are more stable.

### Final Answer:
Bonding molecular orbitals are filled first because they have lower energy compared to antibonding molecular orbitals.

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