Typically, two atoms or molecules bond together because they have equal, opposing charges. Sodium chloride, for example, forms when positively charged sodium ions come into contact with negatively charged chloride ions: Na(+) + Cl(-) ---> NaCl. For compounds such as these, the resulting product is negative. Sometimes, however, a positive metal ion (such as sodium, copper or cobalt) will bond with a neutral compound, resulting in the formation of a metal "complex" which bears part (or, in some cases, all) of the metal ion's original charge. Determining the charge, or oxidation number, of such complexes is simple.
Instructions
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1
Determine the charge of the metal ion from the complex's formation equation. Give the equation CoCl2 + 4 H2O = Co(H2O)4 + 2Cl-, you know that since each chloride ion bears a minus one charge, that cobalt must counteract it with a positive two charge, or Co(2+).
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2
Identify what charge, if any, the nonmetal compound contributes. In the complex of cobalt and water, for example, you know that water is always neutral.
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3
Add together the charge of your metal ion and the nonmetal compound with which it complexes to get the oxidation number of your metal complex. For Co(H2O)4, this is: (2+) + 0 = 2+, or Co(H2O)4(2+).