Valency of oxygen

Consider now the bonds to each O2- ion in the perovskite structure. First, there are two bonds to Ti4+ ions that have a character of 4/6 each, which gives a total of 4/3. However, there are four Ca2+ ions on the corners of the face of the cube where an oxide ion resides. These four bonds must add up to a valence of 2/3 so that the total valence of 2 for oxygen is satisfied. If each Ca-O bond amounts to a bond character of 1/6, four such bonds would give the required 2/3 bond to complete the valence of oxygen. From this it follows that each Ca2+ must be surrounded by 12 oxide ions so that 12(1/6) = 2, the valence of calcium. It should be apparent that the concept of electrostatic bond character is a very important tool for understanding crystal structures. 

This article will be concerned with the chemistry of inorganic solid compounds or composites with interesting and useful properties in which the valence of oxygen is —2. Readers interested in molecular oxides, oxyhahdes, molecular oxygenyl complexes, peroxides, superoxides, or ozonides should consult established publications " and the index of this encyclopedia. 

In silica and the silicates, each atom of silicon is surrounded tetrahedrally by four atoms of oxygen. This structure is in agreement with the tetra-valency of silicon and with the directional character of the four sp hybrid bonds. In such an SiO group, only one of the two valencies of oxygen... 

Ideas about the octet were really intrinsic to the periodic table and the law of octaves, the periodic repetition of chemical properties every eighth element (noble gases were unknown in the 1860s). Mendeleev observed that the highest known valence of any element was 8 [for example, osmium tetroxide (OSO4), the valence of oxygen is 2]. He found that if one summed the (periodic table) column number and multiplied by the number of equivalents, the total was commonly 8 HCl (1 + 7) H2O ([1 times 2] + 6) ASH3 (5 + [1 times 3]). 

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