The preferred valences are shown in green, those less common in orange, and those rarely displayed in red. The periodic nature can clearly be seen where the preferred valency gradually climbs to a maximum value, before falling again on to the next period. The enormous variety of available valences for the transition elements can be seen, as can the lack of choice for the rare earths. The actinide rare earths about the element Uranium depart from the usual valency of three. Switch the TABLE display to '29' to see the Electronic valence Shell arrangements of the Periodic Table, and the DATA display to '10' to see example compounds of a selected element, or to '9' to see the Electronic Configurations of a selected element.
The valency of an element is basically the number of bonds it may form, and specifically, if negative, is the number of atoms of hydrogen (or its equivalent) which that element can hold in chemical combination, (or which it can displace ,if positive). The valency of an ion is equal to its charge. Valences range from -4 to +8. Thus, oxygen is divalent, as in carbon dioxide, O:C:O, whereas carbon is tetravalent. The rule of thumb that atoms form bonds until they have attained a noble gas electron configuration is still useful, despite the many exceptions of hypervalent compounds and electron-deficient compounds like BöHÿ. Those exhibiting valences above +4 are said to be hypervalent or 'octet expanded' because they do not obey the octet completion rule, examples being SFÿ or OsOû. Bonds may be single, as in molecular hydrogen, H-H, double as in molecular oxygen, O=O, triple as in molecular nitrogen, N╬N, or rarely quadruple as in [ReöClÜ]çé where there is a quadruple bond between the two rhenium atoms.
