(a) The chemical formula of nitride ion is N3−. It has -3 charge on it, thus its valency is 3.
[1 Mark]
(b) The chemical formula of ammonium ion is NH+4. It has +1 charge on it, thus its valency is 1.
[1 Mark]
(c) The chemical formula of hydrogen carbonate ion is HCO−3. It has -1 charge on it, thus its valency is 1.
[1 Mark]