# How to find the valence of transition elements?

Let's take Iron for example

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.

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.
• 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: $\ce{2Fe + 6H+ -> 2Fe^3+ + 3H2}$ – bon Dec 16 '14 at 18:52