# What is the name of the following ion: H2PO4-?

Nov 5, 2015

dihydrogen phosphate

#### Explanation:

dihydrogen (because there are 2 $H$ atoms) plus the phosphate ion ($P {O}_{4}^{\text{3-}}$).

There are a few rules that one must obey in naming polyatomic ions, but it all can get confusing sometimes that most students are reduced to just memorizing the names and chemical symbols for all common cations and anions (both monoatomic and polyatomic) just to cope.

But naming polyatomic ions is basically just knowing the oxidation states of the elements involved.

Here are few of the general rules:

(1) the ion ends with -ate if the ion is in its high oxidation state; otherwise, the ion will end in -ite.

e.g.
phosphate ($P {O}_{4}^{\text{3-}}$) vs. phosphite ($P {O}_{3}^{\text{3-}}$)
nitrate ($N {O}_{3}^{-}$) vs. nitrite ($N {O}_{2}^{-}$)

(2) the ion will have a prefix per- if the ion is in its highest oxidation state; otherwise, the ion will start with hypo-.

e.g.
perchlorate ($C l {O}_{4}^{-}$) vs. chlorate ($C l {O}_{3}^{-}$) vs. chlorite ($C l {O}_{2}^{-}$) vs. hypochlorite ($C l {O}^{-}$)

(3) exceptions to rules (1) and (2) are hydroxide ($O {H}^{-}$), cyanide ($C {N}^{-}$) and peroxide (${O}_{2}^{\text{2-}}$).

(4) for the hydrogen prefixes, it depends on how many they are in the substance (based on the subscript).