3d and 4s orbitals

[Nienjou Claire 1E]

Can someone explain in simple terms why 4s orbitals are a lower energy than 3d and thus should be written first?

[Sara Veerman-1H]

When getting to higher energy levels, the energy levels get closer and closer together, so the 4s and 3d levels are really close. At first, the 4s level is lower than 3d (so it will fill up first), but once there are 20+ electrons, the 3d level shifts down slightly and ends up at a lower energy level than 4s (so 3d is written before 4s). I hope this kind of answers your question

[megangeorge-1K]

so the 3d orbital is lower in energy than the 4s orbital?

[OrAmar-1L]

The 3d orbital has n=3, and the 4s orbital has n=4. The principal quantum number (n) determines the energy and size, so the one with the higher n value will have more energy.