Chapter 4: Unit 6. Polyatomic Ions

Polyatomic Ions

Polyatomic ions are charged species, consisting of group of atoms.  They are composed on more than one element. In polyatomic ion, one central nonmetal attached to either oxygen or hydrogen. Central nonmetals are mainly from period 2 and 3  like N, S, P, Cl. Some metalloids also form polyatomic ions. The names of polyatomic cations end in the suffix -onium. Many polyatomic anions have names that end in the suffix-ate or ite.

Only one positively charged or polyatomic cation is NH₄⁺.

Only two polyatomic anions that have end name -ide (OH^–)are hydroxide and cyanide (CN^–)

Most polyatomic ions contain oxygen. That is why sometimes they are called oxo anion. Oxo anions have end name either r -ate or -ite. Lower number of oxygen containing polyatomic ion is named as -ite and higher number of oxygen containing polyatmic ions are named as -ate. Example: NO₂^–: nitrite, NO₃^– : nitrate

When a H⁺ is added to a polyatomic ion, the negative charge of the ion is decreased by one unit.

Example: PO₄^3- : phosphate ion, HPO₄^2-: hydrogen phosphate ion

More important  polyatomic ions are marked with asterisk below.

Common Polyatomic Ions

Formula                                           Name

*NH₄⁺Ammonium

*NO₂⁻                                                     Nitrite

*NO₃−Nitrate

*SO₃²⁻                                                     Sulfite

*SO₄²⁻                                  Sulfate

*HSO₄⁻                                                              Hydrogen sulfate (bisulfate)

*OH⁻                                                                    Hydroxide

*CN⁻                                                                    Cyanide

*PO₄³⁻                                                                Phosphate

*CO₃²⁻                                                                Carbonate

*HCO₃⁻                                                              Hydrogen carbonate (bicarbonate)

*ClO₃⁻                                                                  Chlorate

*C2H₃O₂^–                                                             Acetate

(or CH3COO−)

The following video might help to memorize the polyatomic molecules.

The following video explains in detail how to write ionic compounds with polyatomic ions.

When cations combine with polyatomic ions ionic compounds are formed.  When the charges are switched to form neutral compound, parenthesis is used to use for polyatomic ions.

For example: Na ⁺ + SO₄^2-→   Na₂SO4

But when Ca²⁺ is combined with PO₄^3-, the formula of the compound is::

Ca²⁺ + PO₄^3-  → Ca₃(PO₄)₂

Here is another video to practice writing ionic compounds with polyatomic ions.

Questions

Name the formula of the ionic compound formed from Fe³⁺and each anion. Then name each compound.

1. OH^–
2. CO₃^2-
3. HPO₄^2-
4. NO₂^_
5. PO₄^3-
6. C₂H₃O₂^–

Ans: a) Fe(OH)₃, Iron(III) Hydroxide
b) Fe₂(CO₃)₃: Iron(III) Carbonate
c) Fe(NO₂)₃: Iron(III) Nitrite
d) FePO4, Iron(III) Phosphate
e) Fe(C₂H₃O₂)₃, iron(III) Acetate