Question

Electrons enter the 4s sub-level before the 3d sub-level because...

A) there is only one 4s orbital and there are 5 3d orbitals
B) the 4s orbital is spherical
C) the 4s orbital has a lower energy
D) the 3d orbitals have a lower energy

Answer 1

Electrons fill the 4s sub-level before the 3d sub-level because the 4s orbital has a slightly lower energy, which makes it the energetically favorable position for electrons according to the Aufbau principle.

Step-by-step explanation:

Electrons enter the 4s sub-level before the 3d sub-level because the 4s orbital has a lower energy than the 3d orbitals. This is due to the energy levels of the orbitals, where the 4s orbital is slightly lower in energy than the 3d sublevel, according to observed experimental results and theoretical calculations. Following the Aufbau principle, electrons occupy the orbitals with the lowest energy first. As the principal quantum number increases, the attraction to the nucleus becomes weaker, and the 4s orbital even though it is at a higher principal quantum level, this orbital is lower in energy due to less electron shielding compared to the 3d orbitals.