RULES FOR FILLING ELECTRONS
(a) Aufbau Principle
(b) (n + ℓ) rule
(c) Hund’s maximum multiply principle
(a) Aufbau Principle
1. Aufbau principle gives a sequence in which various subshell are filled up depending on the Energies of various subshell.
2. All the subshells are filled in increasing order of their energies.
3. The sequence in which various subshell are filled are as follows.
(b) (n+1) Rule (For multi electron species)
Principle of (n+ ℓ) ( rule: The subshell with lowest (n+ ℓ) ( value is filled up first, When two or more subshell have same (n+ ℓ) ( value then subshell with lowest value of n is filled up first.
For one electron species like (H)
Energy only depends on principle quantum number
1s < 2s = 2p < 3s = 3p = 3d < 4s = 4p = 4d = 4f < ……..
(c) Hund’s Maximum Multiplicity
1. According to Hund’s rule electrons are distributed among the Orbitals of subshell in such a way as to get maximum number of unpaired electron with parallel spin.
2. Thus the Orbital available in the subshell are first filled singly with parallel spin electron before they get to pair this means that pairing of electrons occurs with the introduction of second electron in ‘s’ & fourth electron in ‘p’ subshell, 6th electron in ‘d’ Subshell & 8th e– in ‘f’ subshell.