Chromium is a metal with many oxidation states, which of the following compounds has chromium in its lowest oxidation state? Cr2O3 Cr2O7^2- MgCr2O3

Oxidation numbers are sometimes difficult, but they will get easier with time. For the general solutions, you will need to consult the general rules to oxidations numbers (should be in any textbook, and in this library).

Use the rules you know to find the unknown oxidation state...

The general idea is that you set up an algebraic expression equalling to the total charge on the complex, where the sum of the oxidation states of all elements in the complex (multiplied by their coefficients) will equal the overall charge on the complex.

For example in Cr2O3

The rule that we will know is that oxygen is normally in the -2 oxidation state.

Our algebraic expression will be

2Cr + 3O = 0

subsituting for the oxygen rule...

2Cr + 3(-2) = 0

now solve for Cr and you are complete....(should be +3)

In dichromate (Cr2O7^2-) we set it up like

2Cr + 7(-2) = -2

Solve (Cr = +6)

In MgCrO3 we have a group 2 metal in the complex.... the rules state that Mg will most likely be in a +2 oxidation state...

Mg + Cr + 3O = 0

+2 + Cr + 3(-2) = 0

You can solve this one on your own for practice

Cr = +4