In accordance with oxygen’s position in Mendeleev’s periodic system of elements, the electrons of the oxygen atom are arranged in two shells: two in the inner shell and six in the outer shell (configuration Is 2 2s 2 2p 4 ). Since the outer shell is not filled and the ionization potential and the electron affinity are and eV, respectively, the oxygen atom usually acquires electrons in the course of formation of chemical compounds and has a negative effective charge. Conversely, rare are the compounds in which electrons are torn away (or more precisely, pulled away) from the oxygen atom (as in F 2 O and F 2 O 2 ). In the past, proceeding solely from the position of oxygen in the periodic system, a negative charge ( — 2) was ascribed to the oxygen atom. However, experimental data have indicated that the ion O 2- does not exist either in the free state or in compounds and that the effective negative charge of the oxygen atom practically never exceeds unity.