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Let's take Iron for example

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As we can see there are 4 desolate electrons in normal state. Can you tell me which electron moves to make the valence 2 and then 3. We have learnt to find the valence only using the configuration formula.

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for 2nd and 3rd rows for 4b-7b groups the highest oxidation state equal to the group number is preferable over all else and for the 1st row for 4b-7b groups the highest oxidation state is equal the group number.

Unfortunately, for all other information you'll need case-by-case approach (i.e. plainly remember it).

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  • $\begingroup$ Indeed, in some graduate inorganic classes, a common question is to give the oxidation states, common oxides, and a few other items for every transition metal. $\endgroup$ – Geoff Hutchison Dec 15 '14 at 5:14
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The two 4s electrons are lost to form the $\ce{Fe^2+}$ ion and then the spin paired 3d electron is lost to form the $\ce{Fe^3+}$ ion. As a general rule the 4s electrons are lost before the 3d (at least for period 4 elements) and spin paired electrons are lost before unpaired ones.

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  • $\begingroup$ where are these electrons gone? $\endgroup$ – Annalian Loverre Dec 16 '14 at 17:48
  • $\begingroup$ The electrons are transferred to another atom or ion to form either another (usually negative) ion, or a neutral atom. E.g. The reaction of iron with an acid such as hydrochloric acid: $\ce{2Fe + 6H+ -> 2Fe^3+ + 3H2}$ $\endgroup$ – bon Dec 16 '14 at 18:52

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