The interactions of atoms in polyatomic molecules are described qualitatively. Particular attention is paid to atoms of the types of F, O, N, C, where the valences come from $p$ electrons. Directional effects are discussed, namely that the two valences of O, and the three of N, tend to be mutually at right angles, and the four of C have tetrahedral symmetry. Numerous examples are given, from the structure of metals and of organic and inorganic compounds. Mathematical treatment is postponed until a later paper.