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Since hydrogen has a valence of one, you can measure the valence of an element by the number of atoms of hydrogen that one atom of the element can combine with, or replace. Look at these formulas: HCl, H2O, NH3, and CH4. What are the valences of chlorine, oxygen, nitrogen, and carbon in these compounds? Answer
The common valences of the elements follow a simple periodic trend:
|Group 1A (Alkali metals)||1||LiH, NaCl, KBr|
|Group 2A (Alkaline earth metals)||2||CaCl2, MgF2, BaBr2|
|Group 3A (B, Al, ...)||3||AlCl3, BF3, GaBr3|
|Group 4A (C, Si, ...)||4||CCl4, CH4, SiCl4|
|Group 5A (N, P, ...)||3||NH3, PCl3, NCl3|
|Group 6A (O, S, ...)||2||H2S, H2O, Cl2O|
|Group 7A (halogens)||1||HCl, HF, F2O|
Many elements have several possible valences. This is especially true of elements in the third period and below. Sulfur, for example, usually has a valence of 2 (as in H2S). But in some compounds, it has a valence of four (as in the highly reactive compound SF4) or even six (as in SF6, which is one of the most inert chemical compounds known). So the formulas of compounds guessed using the most common valences of elements are not always correct. Again, more advanced bonding theories can explain these "hypervalent" structures.
So common valences alone are an oversimplification. More sophisticated treatments of chemical bonding are necessary to explain most structures.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/compounds/faq/print-predicting-binary-covalent-formulas.shtml