Could you please explain the basics of valences. Thanks.
Chemical valence, also referred to as oxidation number, is an empirical
assignment of one [or sometimes several] positive or negative numbers to a
chemical element. The most likely formula for a chemical compound is when
the number of atoms X the valence number [assume it to be positive] of
that element, equals the number of atoms X the valence number [assume it
to be negative] sum to zero, or the net charge on the compound, for
example, (SO4)^ -2. Although the concept of valence is firmly based on the
quantum mechanics of atoms, in practice it is an empirical counting game
that a student basically has to memorize the common valences associated
with an element. So Ca++ has a valence of +2, Cl- has a valence of -1, and
they combine to form CaCl2, in which one positive valence of
+2 and one negative valence of -1 sum to give a net charge of zero.
The problem is that this is only a rough guideline and it has many
exceptions and hand-waving qualifications. Here are a few examples: What
is the valence of oxygen in ozone, O3? Sulfur, being below oxygen would be
expected to have a valence of -2, which it often does, but SF6 is a
perfectly good stable chemical. The inert gases: He, Ne, Ar, Kr, and Xe
are assigned a valence of zero because the don't react to form any
chemical compounds -- well almost none. Under the proper conditions Kr and
Xe will form a variety of compounds with fluorine and/or oxygen. The list
of exceptions and qulaifications is very long.
If you search the website search engine http://www.about.com for the topic
"valence" you will find discussions of varying degrees of sophistication,
so you can learn as much, or as little, as you choose.
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Update: June 2012