Hi I am having some trouble with one particular question.

The question is: What is the oxidation number for the atom N is KNO₂?

This is the explanation I got.

It looks like we have 2 unknown oxidation numbers (K & N), but the truth is that there is only one unknown (N). This is because KNO₂ is a polyatomic (because it is called potassium nitrite, & nitrite is a polyatomic ion). Since polyatomic molecules are ionic, this means we know that the oxidation number of K must be +1 by the uncriss-crossing rule. We then look at O & its oxidation number is -2. The molecule is neutral, so the algebraic expression equals 0. We then solve as usual.

I understand every line of that explanation except how we find the oxidation number of K.

How do we use the uncriss-crossing rule to find that K has a charge of +1?

Thank you in advance!

Edited September 6, 201411 yr by vcki29

Potassium is a first row metal, all of which have a +1 oxidation state in ionic form. The reason for this is that they have 1 valence electron and it is much easier to lose that electron to obtain a full outer valence than it is to gain 7. Electrons being negatively charged means that the loss of one electron gives an atom such as potassium a +1 charge / oxidation state. Similarly, the group 2 metals (Ca, Mg, etc.) are all of a +2 oxidation state when ionised.