Can you predict the formulas for binary covalent compounds?

The common valences of the elements in the compound can be used to guess the formulas of some binary compounds. But the "tinkertoy" picture of atoms implied by valence numbers is too simple to rationalize many binary formulas.

	Methanol, CH3OH Click on the image for a 3D Chime model.

Valence is the number of chemical bonds an atom forms. If you look at a tinkertoy model of a methanol (CH₃OH) molecule at right, the hydrogens (in white) have a valence of one, the carbon (in green) has a valence of 4, and the oxygen (in red) has a valence of two.

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	Common valence	Examples
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

For example, a compound between oxygen and fluorine would be expected to have the formula F₂O, because fluorine has a valence of one, and oxygen has a valence of two.

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 senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/compounds/faq/predicting-binary-covalent-formulas.shtml