The isotropic electron–nuclear spin interactions of the second-row elements in the Periodic Table are interpreted semiquantitatively on the basis of the molecular orbital theory. For an atom Y that is bonded to m atoms, Xi(i = 1, 2, ···, m), the hyperfine constant, aY, has the form aY = QYYpYY0 + ∑ i = 1m QXiYpii0 + ∑ i = 1m RYXiYpYi0, where pYY0, pii0, and pYi0 are elements of the π-electron spin density matrix, and QBA and RABA are the σ–π parameters for the nucleus A and resulting from the π AO spin density matrix elements, pBB0 and pAB0. The spin polarization through the σ–π interaction is shown to be divided into three terms according to their mechanisms; (i) the spin polarization in the AB σ bond, which induces opposite spin densities on atomic orbitals of atoms A and B, (ii) the spin polarization which appears as the difference in the hybrid ratios of the hybrid σ-atomic orbitals for different spins, and (iii) the spin polarization of the inner-shell electrons. An empirical analysis correlating the observed hyperfine coupling constants with the calculated π-atomic-orbital spin density matrix elements by the above equation gives the following results: for a coplanar CHC2 fragment QCC, QC′C, and RCC″C are found to be 46.0 ± 0.1, − 17.3 ± 0.1, and − 1.95 ± 0.25 G, respectively; for a NC2 fragment QNN = 29.0 ± 1.6, QCN = − 4.3 ± 0.8, and RNCN = 0.9 ± 1.5 G and for 17O in a carbonyl fragment QOO = − 22.0 ± 3.0, QCO = 75.4 ± 7.6, and ROCO = 71.8 ± 9.6 G.