Aufbau principle states, that, electrons shall fill up the available orbitals of an atom, starting from low energy to high energy, so that, electrons and the atom have minimum energy and be stable.
So, electrons starts filling from the first orbit (n =1) and proceed filling in the increasing order of ‘n’ value.
If ‘n’ is the principal quantum number of any orbital,
Number of orbitals = n2 and Number of electrons in any shell of ‘n’ = 2n2
Number of sub-orbitals in orbitals, ‘s’ is 1′. ‘p’ is 3 and ‘d’ is 5.
Each orbital can have a maximum of two electrons.
The maximum filling up of electron in the orbitals, as per Aufbau principle is-
1s2 2s2 2p6 3s2 3p6 42s 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6