Molecular Orbital Theory

✅ Correct answer: 3s < 3p < 4s < 3d

Explanation:
According to the Moeller chart (also known as the Aufbau principle), electrons fill atomic orbitals in the order of increasing energy. The energy order of orbitals is determined using the n + l rule, where:
- n is the principal quantum number
- l is the azimuthal quantum number

If two orbitals have the same n + l value, the orbital with the lower n value has lower energy.

Using this rule:
- 3s (n + l = 3 + 0 = 3)
- 3p (n + l = 3 + 1 = 4)
- 4s (n + l = 4 + 0 = 4)
- 3d (n + l = 3 + 2 = 5)

So the correct order is:
3s < 3p < 4s < 3d