The Aufbau Principle states that in the ground state of an atom, an electron enters the orbital with lowest energy first and subsequent electrons are fed in the order of increasing energies. The word 'aufbau' in German means 'building up'. Here, it refers to the filling up of orbitals with electrons.

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he Aufbau principle, simply put, means electrons are added to orbitals as protons are added to an atom.

The Aufbau principle outlines the rules used to determine how electrons organize into shells and subshells around the atomic nucleus.
Electrons go into the sub-shell having the lowest possible energy.