Question

Hi !
May I know why the 'cyanide ion' is negatively charged, when the carbon is +4 and nitrogen is -3. So shouldn't it be charged with +1 instead...?

Also... how do we distinguish the charge of a compound like NO3- and PO43-

Thanks !!

Answer 1

Answer:

Question 1:

Formal charge=valence electrons-unbonded electrons-(1/2 bonded electrons)

In cyanide, carbon has two unbonded electrons, and there are six bonded electrons. Carbon usually has four valence electrons. Therefore, carbon's formal charge=4-2-(1/2)(6)=-1. Nitrogen also has two unbonded electrons, and there are six bonded electrons. Since nitrogen usually has five valence electrons, its formal charge is 5-2-(1/2)(6)=0. Add the formal charges in a molecule or ion calculate its overall charge. -1+0=-1, so cyanide must have a charge of negative one.

If you are talking about oxidation states, you are correct that nitrogen is -3. Carbon has an oxidation state of +2 in this ion rather than +4 or +1.

Question 2:

In the polyatomic ions $NO_{3} ^ {-}$ and $(PO_{4} )^{3-}$, the charge is in the superscript (above the letters), whereas the numbers of atoms are in the subscript (below the letters). Because $NO_{3} ^{-}$ has - (without a number because the one is implied) in its superscript but 3 in the subscript, it has a charge of -1 and 3 oxygen atoms. On the other hand, $(PO_{4} )^{3-}$ has a 3- in the superscript, so its charge is -3, and it has a 4 in the subscript following the oxygen, so it has 4 oxygen atoms.

Excellent questions!