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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
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.
Click on the image for a
3D Chime model.
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:
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.
|Group 1A (Alkali metals)
||LiH, NaCl, KBr
|Group 2A (Alkaline earth metals)
||CaCl2, MgF2, BaBr2
|Group 3A (B, Al, ...)
||AlCl3, BF3, GaBr3
|Group 4A (C, Si, ...)
||CCl4, CH4, SiCl4
|Group 5A (N, P, ...)
||NH3, PCl3, NCl3
|Group 6A (O, S, ...)
||H2S, H2O, Cl2O
|Group 7A (halogens)
||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