The formation mechanism of near-edge-structure features in X-ray absorption spectra of first-row polyatomic systems is discussed. It was found that the molecular π and σ shape resonances in spectra of these systems can be treated as a result of the atomic 1s-2p resonance splitting by a molecular field. This conclusion is in good agreement with data of oscillatior strength measurements for the shape resonances in N₂. It was shown that the energy interval ΔE(π - σ) can be used for determination of the interatomic distance in linear and planar first-row anions in crystals.