Page 1 : ‘Bules for Filling of electrons in various orbitals, The filling of electrons into the orbitals of different atoms takes place according to the 3 rules - aufbau, principle, Pauli’s exclusion principle and the Hund’s rule of maximum multiplicity., , 1. Aufbau principle:, ‘The German word aufbau means ‘build up’. The building up of orbitals means the filling up of orbitals with, electrons. It states that the orbitals are filled in order of their increasing energies. in other words, electrons first, ‘occupy the lowest energy orbital and then to higher energy orbitals., This rule has two sub rules:, a) The various orbitals are filled in the increasing order of their (n+) value., b) If two orbitals have the same (n+l) values, the orbital with the lower n value is filled first., ‘The increasing order of orbitals is as follows:, 1s, 2s, 2p, 35, 3p, 45, 3d, 4p, Ss, 4d, Sp, 65, 4f, Sd, 6p, 75..., , 2. Pauli’s Exclusion Principle, , It states that no two electrons in an atom can have the same set of four quantum numbers. i.e. an, orbital can accommodate @ maximum of only 2 electrons with opposite spin., , 1f2 electrons have same values for n, Land m, they should have different values for s. i.e. if, first electron, it should be -¥% for the second electron., , 3. Hund’s rule of maximum multiplicity, , It states that electron pairing takes place only after partially filling all the degenerate orbitals. Orbitals, having same energies are called degenerate orbitals. For example the electronic configuration of Nis 1s? 2s, 2px'py'pz! and not 1s? 2s? 2px’py"., Electronic Configuration of Atoms, , The distribution of electrons into various orbitals of an atom is called its electronic configuration. The, electronic configuration of different atoms can be represented in two ways., , () pd"... notation, , (i) Orbital diagram, , The electrons in the completely filled shells are known as core electrons and the electrons in the outer, most shell are called valence electrons., Stability of Completely Filled and Half Filled Subshells, , For atoms having half filled or completely filled electronic configurations have extra stability compared to, other atoms. This is due to their symmetrical distribution of electrons and greater exchange energy. For example,, the electronic configuration of Cr is [Ar] 3d°4s* and not 3d“4s”. This is because d° represents a half filled, configuration and has extra stability. Similarly for Cu the electronic configuration is [Ar] 3d°°4s" and not 3d°4s”., , % for the