Page 1 : Hund’s rule of maximum multiplicity, , Hund’s rule states that “The number of unpaired (parallel) electrons in orbitals of equal energy is, maximum., Electron pairing in and orbitals cannot occur until each orbitals of a given subshell contains one electron each, or is singly occupied”. p, d, f, This is due to the fact that electrons being identical in charge, repel each other when present in the same, orbital. This repulsion can however be minimised if two electrons move as far apart as possible by occupying, different degenerate orbitals. All the unpaired electrons in a degenerate set of orbitals will have same spin., Let three electrons enter a p- subshell. Three are three equivalent p orbitals, namely px, py, pz, according to, Hund’s rule, one electron enters each p orbital and it is represented px!, py!, pz!, singly occupied orbitals should have electrons with parallel spins i.e in the same direction either-clockwise or, anticlockwise., , , , , , , , , , , , , , , , , , , , tit Lt, , Px Py Ps, or, , bel Ud, , Px Py Ps, , “Therefore, Hund’s rule can be stated as follows, ‘Pairing of electrons beings only after all the available orbitals of equal energy are singly, occupied., , Aufbau (building up) Principle (n+1) rule, , ‘Electrons enter various orbitals in the order of increasing energy. It means electrons occupy orbitals, of lower energy first before the filling of the orbitals of lower energy first before the filling of the orbitals with, a higher energy commences, , As working rule, the new electron enters an orbital having minimum value of (n+l). If the value of, (n+l) is same for two or orbitals, the new electron enters the orbitals having lower value for n, , For example, consider 2p and 3s orbitals, , For 2p, (n+l) = 2+1 =3, , For 3s, (n+l) = 2+1 =3, , Then the new electron enters 2p orbitals having lowest value of ‘n’ and not 3s orbitals, , The increasing order of energy of various orbitals is 1s < 2s < 2p < 3s < 3p < 4s < 3d< 4 p< 5s<4d, < 5p < 6s < 4f < 5d < 6p< 7s < 5f< 6d< 7p....., , (n+]) rule, , In neutral isolated atom, the lower the value of (n + 1) for an orbital, lower is its energy. However, if, the two different types of orbitals have the same value of (n + 1), the orbitals with lower value of n has lower, energy.