NITROGEN

Student : sir,why does nitrogen has negative charge in NH3,but positive charge in NO2???

Teacher : very simple actually.Based on the Pauling scale of electronegativity nitrogen N = 3.04, means nitrogen was put on the periodic table around the electropositive like and electronegative like.

If N was bounded to the more likely electropositive element,it has negative oxidation number,vice versa.

The oxidation numbers of nitrogen are -3,-2,-1,0,+1,+2,+3,+4 and +5.

If we compare between nitrogen in NH3 and NO2...

The electronegativity of O= 3.44 is more electronegative than N and H=2.20 is more electropositive than N.

The oxidation number of N in NH3 is -3.

Based in the electron configuration of N : 1s2 2s2 2p3,so...

If the nitrogen was bounded to electropositive element,the element is able to share the electron to the empty orbital (in this case orbital p) and makes the nitrogen's oxidation number negative.Hidrogen shares 3 electrons and fulfills the empty orbital p of nitrogen so the nitrogen's oxidation number would be -3.

While in NO2 is +4,nitrogen shares 4 electrons valence to oxygen,so the oxidation number of nitrogen is +4. 

I hope it answers your question..