Question

why the last electron enters a(n-1) orbital only after the na subshell is completely filled​

Answer 1

Answer:

The Aufbau section discussed how electrons fill the lowest energy orbitals first, and then move up to higher energy orbitals only after the lower energy orbitals are full. Certainly, 1s orbitals should be filled before 2s orbitals, because the 1s orbitals have a lower value of n, and thus a lower energy.