What are the rules of atoms?

The rules of the atoms can be majorly explained through three rules and principles;

Aufbau principle Pauli exclusion principle Hund's rule

Hund's rule states that an atomic orbital is firstly singly occupied by electrons before it is doubly occupied until its fully filled. Electrons in a singly occupied orbital have the same spin quantum number.

Aufbau principle states that the lowest atomic orbitals are occupied first, while Pauli exclusion principle states that the electrons in an atomic orbital cannot have the same quantum state.