Compounds Containing Polyatomic Ions

(Compound ions)

A group of atoms covalently bonded and possessing a charge is called a radical or polyatomic ion. For example the sulphate ion ( SO₄^-2 ) is made up of a single sulphur atom and 4 oxygen atoms bonded together. This group of atoms possess a 2- charge. It has a valence of two and it can form two bonds.

How do we write formulas for compounds containing polyatomic ions?

What is the formula for sodium sulphate? Think of the compound as having two parts, in much the same way as a binary compound, with sodium as one part and sulphate as the other. Write the symbols and valences of each part of the compound, then apply the cross over rule.

Na⁺¹ SO₄^-2 -> Na₂SO₄

Remember to:

omit the charges

drop the subscript if is a 1

reduce the subscripts

In addition, when more than one polyatomic ion is present in a formula, brackets must be placed around the formula of the polyatomic ion.

For example: calcium nitrate

Ca⁺² NO₃^-1 -> Ca(NO₃)₂

Try some for fun!

1. ammonium chloride

2. magnesium chlorate

3. aluminum sulphate

4. sodium chromate

5. sodium permanganate

6. calcium sulphate

7. ammonium phosphate

8. lithium acetate

9. HClO₃

10. Ba₃(PO₄)₂

11. CaCrO₄

12. Al₂(SO₄)₃

Relationships Among Related Polyatomic Ions

Ion
Formula

Perchlorate ClO₄^-1 one more O atom than "-ate"

Chlorate ClO₃^-1

Chlorite ClO₂^-1 one less O atom than "-ate"

Hypochlorite ClO^-1 two less O atoms than "-ate"

Relationships Among Related Polyatomic Ions
Ion	Formula
Perchlorate	ClO₄^-1	one more O atom than "-ate"
Chlorate	ClO₃^-1
Chlorite	ClO₂^-1	one less O atom than "-ate"
Hypochlorite	ClO^-1	two less O atoms than "-ate"

You try!

1. persulphate

2. sulphate

3. sulphite

4. hyposulphite

5. nitrate

6. hyponitrate

7. pernitrate

8. nitrite

You're doing great! Now you're ready for the next page.....

Valency chart

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