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As I was doing my Ap Chem homework, sometimes the question would use Iron(III) and sometimes it would use Ferric. What's the difference in usage? Also, what's the difference between Iron(II) and Ferrous?

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  • $\begingroup$ Hi Just, While this is an important question (and you've gotten some answers that nailed it right on the head), I think that's it's found quite readily by searching online. $\endgroup$
    – jonsca
    Aug 12 '12 at 20:05
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There is no difference, in both the examples you cite. The forms Iron(II) and Iron(III) are preferred today, versus the older forms ferric and ferrous, respectively, because they more explicitly give the oxidation states of Fe, +2 and +3 in this example.

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The difference in usage is simply a choice. The usage of the Roman numerals to indicate oxidation state is known as Stock nomenclature, devised by Alfred Stock in 1919.
The other method, using the -ic and -ous suffixes is derived from Latin. The -ic suffix is used for the higher oxidation state of the metal, while the -ous suffix is used for the lower oxidation state of the metal.

However, the use of the Latin suffixes is no longer recommended for indication of oxidation state, but it may be used for metals exhibiting not more than two valences.

From IUPAC:

The following systems are in use but not recommended:

The system of indicating valence by means of the suffixes -ous and -ic added to the root of the name of the cation may be retained for elements exhibiting not more than two valences.

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