[Epsilon]

Hey guys.  Im currently studying organic chemistry as a means to pass the gamsat and have hit a hurdle.  Being obsessive compulsive I cant continue studying until I figure this out.  Anyway Im in the section about assigning formal charges and am stuck when trying to rationalise NH2.  It says the NH2 ion has 2 singly bonded H atoms to a nitrogen atom....fair enough.  But it then says that there are 2 pairs of unbonded electrons leftover.  How is that possible?  I mean, doesnt N only have 5 valence electrons?  So if this is the case, how is it that there are TWO pairs (four electrons) leftover...when my mathematics says that there would be only three....


If someone could help Id really appreciate it.

[Telamond]

What's the formal charge of NH₂?
Have you heard of the octet rule? Do you know how to count electrons?

[Epsilon]

The formal charge is -1

My qstn is basically, how can there be two hydrogen bonds and two pairs of unpaired electrons when nitrogen only has 5 valence electrons to begin with

[Telamond]

Nitrogen has 5 valence electrons.
1 electron is shared with each hydrogen to form a covalent bond. Nitrogen has 7 electrons now, but since formal charge is -1, it has 8 electrons.
The 4 remaining electrons which are located on nitrogen form two electron pairs.

[fledarmus]

It says the NH2 ion has 2 singly bonded H atoms to a nitrogen atom....

The NH₂ ion is NH₂^- - it has an extra electron to give it a -1 charge. So you are getting five electrons from the nitrogen, 1 from each hydrogen, and 1 extra to give a negative charge - total of eight electrons (octet around nitrogen). Two are shared with each hydrogen, (total of 4 shared) and there are 4 left over to form the two lone pairs.