In an atom, the order of filling up of the orbitals is governed by the Aufbau principle, Hund`s rule, and Pauli`s exclusion principle.

1. Aufbau principle: This principle states that the orbitals of lower energy are filled before the orbitals of higher energy. Electrons fill the orbitals starting from the lowest energy level and move progressively to higher energy levels.

2. Hund`s rule: According to Hund`s rule, when orbitals of the same energy (degenerate orbitals) are available, electrons will occupy separate orbitals of the same energy with parallel spins before pairing up. This is to maximize the total spin of the electrons in the atom.

3. Pauli`s exclusion principle: This principle states that no two electrons in an atom can have the same set of quantum numbers. Each electron in an atom must have a unique combination of the four quantum numbers, including the spin quantum number.

Together, these principles determine the order in which the orbitals are filled in an atom, following the energy levels and taking into account the restrictions imposed by the Pauli exclusion principle and Hund`s rule.