In particular, in organic chemistry, the electronegativity of functional groups are taken into consideration significantly to predict reaction mechanism and to explain inductive effects of functional groups. In the present study, considering the relationship with charge of the electronic energy for atoms, Sanderson's electronegativity equalization principle and Density Functional Theory (DFT), we have obtained a new equation by which group electronegativity can be calculated from ionization energies (I) and electron affinities (A) of atoms that constitute the group, where the results obtained were compared with equation of Sanderson who proposed that group electronegativity is the geometric mean of atomic electronegativities. For a large number of groups, it was found to be a very close agreement with a standard deviation of 0.12. The advantage of the present equation is that it can be used for ionic groups. In addition, the derived equation can be used to calculate Mulliken and Pauling electronegativities of molecules. (C) 2014 Elsevier B.V. All rights reserved.