Question

What is the common oxidation state of manganese metal?

Answer 1

Good question. How do you determine the most stable oxidation state?

Explanation:

Manganese metal has a very large redox manifold, and can form compounds with metal oxidation states from `0` to `VII+`. Subvalent oxidation states of `-II` and `-I` are known. Manganese has 7 valence electrons, and thus many possible oxidation states.

In water, in common with most of the transition metals, manganese commonly forms `Mn^(2+)` ions. So `Mn(II+)` compounds and complexes are common. This has a `d^5` ground state, and as a consequence, `Mn(II+)` are VERY palely coloured, i.e. almost colourless.

Answer 2

You can determine the most stable oxidation state by identifying the ground-state electron configuration.

Since an oxidation state is defined as the hypothetical charge of a pure ion, the ground-state configuration for the corresponding `"Mn"` ion should define the most stable oxidation state.

You can write the valence configuration as (`Z = 25`)`bb(""^"[*]")`:

`ul(" "" ")`
`" 4s"` (`-"7.84 eV"`)

`ul(uarr color(white)(darr)) " " ul(uarr color(white)(darr)) " " ul(uarr color(white)(darr)) " " ul(uarr color(white)(darr)) " " ul(uarr color(white)(darr))`
`underbrace(" "" "" "" "" "" "" "" "" "" "" "" "" "" ")`
`" "" "" "" "" ""3d orbitals"` (`-"11.4 eV"`)

This is said to be most stable because

• as many electrons are unpaired as possible (coulombic repulsions from paired electrons are minimized), being distributed in accordance with Hund's rule.
• they all occupy the non-highest-energy valence orbitals.

(We don't worry about the core orbitals because they are already low in energy and fully-occupied, thus not participating in bonding.)

In this state, manganese has lost its two valence `4s` electrons, going from an electron configuration of `[Ar] 3d^5 4s^2` for `stackrel(color(blue)(0))"Mn"` to `[Ar] 3d^5` for `stackrel(color(blue)(+2))"Mn"`.

So, the `+2` state is most stable.

`bb(""^"[*]")`: http://media.pearsoncmg.com/bc/bc_0media_chem/adv_chem/pdf/11054_appB_ts.pdf