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Name: ian
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2000

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 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.

Vince Calder

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