Chemistry Reference
In-Depth Information
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Chemical Formulas
Lesson 5-1: Oxidation Numbers
Two common salts, which you are probably familiar with, are sodium
chloride (NaCl), which we sometimes call “table salt,” and calcium chlo-
ride (CaCl
2
), which many people use to melt the ice on driveways. Why
does the chlorine have a subscript of “2” in one chemical formula but not
in the other? The ratio by which elements combine in order to form com-
pounds can often be predicted by using the oxidation numbers of the ele-
ments in the compound. The oxidation number represents the charge, or
the apparent charge, of an element in the compound.
Atoms can often gain or lose electrons to form ions. When an atom loses
electrons, it will have more protons than electrons and show a positive charge.
If an atom of aluminum loses three electrons, it will be left with a net charge
of +3, and we indicate that with the symbol Al
3+
. When an atom gains elec-
trons, it will have more electrons than protons and will have a net negative
charge. For example, if an atom of sulfur gains two additional electrons, it
will have a net charge of -2. We represent such an ion with the symbol S
2-
.
When you are dealing with elements that only show one oxidation state,
determining the ratio by which they form can be quite easy. The idea is that
the elements will combine in such a ratio that there will be a net charge of
zero, for a neutral compound. For example, how would we find the chemi-
cal formula for the compound of sodium and chlorine? If I told you that
sodium (Na) has an oxidation number of +1 and chlorine (Cl) has an oxida-
tion number of -1, they must combine in a 1:1 ratio because (+1) + (-1) = 0.
The chemical formula for the compound formed by these elements, in a
1:1 ratio, would be NaCl. No subscripts are needed or used, because they
are each understood to be 1.
