Question

When electrons are filling energy levels, the lowest energy sublevels are occupied first. This is ____________.

Answer 1

It is called Aufbau Principle

Answer 2

When electrons are filling energy levels, the lowest energy sublevels are occupied first. This is Hund's rule.

Hund's rules state that:

Every orbital in a sublevel has to be singularly occupied before any other orbital is able to be doubly occupied.

All of the electrons in single occupied orbitals have to have the same spin to maximize the total spin.