The rotation group in four dimensions 2? 4 is applied to the study of the 2s m 2p n configurations of atoms. This Lie group is used in both the mathematical sense, describing the transformation properties of the angular parts of the 2s and 2p electrons, and in the more physical sense of an approximate symmetry group for the first-row atoms. The various states of each configuration are classified by means of i? 4 . By expressing the Coulomb interaction in terms of tensors, transforming according to irreducible representations of i? 4 , Coulomb and exchange integrals are evaluated group theoretically. The method is extended to 3s m Sp n 3d r configurations. The states are classified by means of i? 4 and Coulomb and exchange integrals are approximately evaluated.The theory of groups has long played an important role in the quantum theory of atomic structure. The indistinguishability of electrons and the Pauli exclusion principle lead naturally to the introduction of the permutation group. The use of the three-dimensional rotation group in classifying states of definite angular momentum is of course well known. The group R 3 is not sufficiently large to explain the n 2 -fold degeneracy of the nth level of hydrogen-like atoms, and in 1935 Fock 1 realized that the group R 4 would. He showed that, iif the Fourier-transformed Hamiltonian is stereographically projected from three-dimensional p-space onto a four-dimensional sphere, the Hamiltonian exhibits four-dimensional rotation symmetry. It is this additional symmetry beyond R 3 which is responsible for the degeneracy of orbitals of the same principal quantum number. Each set of orbitals of a given principal quantum number when projected onto the four-dimensional sphere transforms according to one of the irreducible representations of R 4 .In his study of atomic spectra, Racah 2 found that in order to classify states of d? 1 and of f n it is useful to introduce Lie groups larger than the R 3 describing the orbital angular momentum, and smaller than the SU n describing the permutation symmetry and spin. These groups describe in more detail than does R 3 the transformation properties of the angular parts of the orbitals under consideration. Consequently such groups are able to provide the additional quantum numbers that can distinguish identical terms, e. g., the two 2 D states of d 3 are labeled by their seniority, a quantum number arising from R 5 . These groups deal with the mathematical problems of the classification of states and will be called here "mathematical groups" as opposed to the "physical groups" such as the R 4 introduced by Fock.In this paper we consider both descriptions of groups in an investigation of the application of R 4 to the study of the 2s m 2p n configurations of atoms. We discuss the relationship between the two descriptions of groups. For the four 2s and 2p orbitals, the appropriate group for both the mathematical and physical descriptions is R 4 , and the representations used in the analysis of atomic configurations are the same. The four 2...