Question

current attempt in progress
why is the last electron in potassium located in the fourth energy level rather than in the third energy level?
potassium is unlike every other group 1a element.
there is no 3d orbital.
the 4s orbital is at a lower energy level than the 3d orbital.
the 4s orbital must have at least one electron before the 3d can be filled.

Explanation:

According to the Aufbau principle, electrons fill orbitals in order of increasing energy. The \(4s\) orbital has a lower energy than the \(3d\) orbital. So, for potassium (\(K\), atomic number \(19\)), after filling the \(3p\) orbitals (\(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}\)), the next electron goes into the \(4s\) orbital (\(4s^{1}\)) rather than the \(3d\) orbital.

Answer:

The \(4s\) orbital is at a lower energy level than the \(3d\) orbital.