Question

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Why do electrons enter the 4s orbital before entering the 3d orbital?


because the 3d orbital is at a lower energy level


because both the 4s and 3d orbitals are at the same energy level


because the 4s orbital is at a lower energy level


because the 4s orbital is at a higher energy level

Answer 1

Because the 4s orbital is at a lower energy level.

The 4s orbital has a lower energy because it is more penetrating than a 3d orbital. A 4s electron can get closer the nucleus than a 3d electron, so it has a lower potential energy.

The other options are incorrect because they contradict the correct one.