Classification of elements :

(1) Inert gases : These atom have been full fill outer most s and p subshell and having the configuration ns²np⁶(Exception helium having 1s²) due to stable configuration these element do not show chemical activity under normal condition and thus termed as inert gases. Under special conditions, the higher members do form some compounds with other elements and hence, the name has been changed to Noble gases. Six element He, Ne, Ar, Kr, Xeand Rnbelongs to this group.

(2) Representative or normal elements : Element in which atom have all shells complete except outermost shell which is incomplete. The number of electrons in the outermost shell varies from 1 to 7 i.e. the configuration of the outermost shell varies from ns¹to ns²np⁵. These consist of some metals, all non metals and metalloid. These are the elements which are found in nature in abundance and active in nature. On account of this, these elements are called representative elements. All the three types of valencies are observed in the case of these elements. These elements have one of the following configuration in their outermost shell.

The number of electrons present in the outermost shell signify the group to which these element belong.

(3) Transition elements : In the atoms of these elements the outermost shell and the penultimate shell (Next to the outermost) are incomplete. These elements have the general configuration (n–1)d^1–9 ns^{0 or 1 or 2} either in the ground state or in excited state. These elements are present inIV^th, V^th, VI^th and VII^thperiod of periodic table and called transition elements. There are four transition series every series consists 9 elements each.

(4) Inner transition elements : Atoms of these elements have three outermost shell incomplete. The general configuration is (n–2)f^1–14(n–1)d^{0 or 1}ns². There are two series of elements.

(i) Lanthanides or rare earths fromCe(58) toLu(71).

(ii) Actinides from Th(90)to Lr(103)

Each series consists of 14 elements i.e. in the lanthanides, 4fis gradually filled up while in actinides, 5fis gradually filled up.

The properties of these elements are similar to transition elements. The members of actinide series are radioactive and majority of them are not found in nature. The elements from atomic number of 93 onwards are called transuranic elements and have been discovered artificial means.

In this classification the element, Zn, Cd and Hg have not been included in any of the four groups of elements. The elements Lu and Lr of the inner transition group have (n–2)fshell complete consisting 14 electron hence their inclusion in this group is not justified.