Oxidation number question

[akimhaneul]

I don't understand how the nitrogen in this molecule will have a oxidation number of -3. It does get 3 electrons from the 3 hydrogens but the overall charge of the nh3 is +1. How does this work?

Thanks!

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[aldol16]

N is more electronegative than C so it gets the electron from that bond. The count is now -1. It also gets one electron each from the three hydrogens, giving it a formal oxidation of -4. But it has a positive charge, so we have to take an electron, or one of those negative charges, away to leave it with -3.

 

[akimhaneul]

N is more electronegative than C so it gets the electron from that bond. The count is now -1. It also gets one electron each from the three hydrogens, giving it a formal oxidation of -4. But it has a positive charge, so we have to take an electron, or one of those negative charges, away to leave it with -3.

This is kinda dumb question but how do you just take away an electron?

 

[desperadoflakes]

Here is the way I calculate oxidation states when looking at a structure.


Oxidation State = (# of valence electrons in the free unbound atom) - (# of electrons bound to atoms LESS electronegative + # of electrons in lone pairs)

Think of each bond (each line) as two electrons - one electron comes from one atom, and the other comes from the other. That's a covalent bond.

Now, when thinking of oxidation states we PRETEND that that covalent bond is actually an ionic bond - the less electronegative atom is GIVING their electron to the more electronegative one. So in H-F, let's think of F actually possessing both electrons in that bond between H and F.

Let's calculate the oxidation states in HF.

A free hydrogen has one valence electron. A hydrogen "ionically" bound to Fluorine (which is more electronegative) has NO valence electrons. 1-0 = + 1

Free fluorine has 7 valence electrons (3 lone pairs and one radical). Fluorine bound to HF has 8 valence electrons (2 in the bond and 3 lone pairs). 7-8 = -1

Now let's look at this nitrogen. Nitrogen normally has 5 valence electrons. You know this because it's in the 5th group on the periodic table (so it has 2 2s electrons and 3 2p electrons).

It's bound to 3 hydrogens and one carbon. It's more electronegative than hydrogen and carbon, so it gets all of their electrons - it has 8 "valence" electrons now.

5-8 = -3

A note: If the nitrogen were bound to another nitrogen, then we would say each nitrogen gets 1 electron from the bound. It works like this for any 2 same bound atoms.

Hope this helps!