Formulas Ionicas

Writing Ionic Formulas

1) Write the symbols of the elements. List the metal first.

Mg Br

2) Find the oxidation number for each element.

2+ 1-

Mg Br

3) Drop the signs from the oxidation numbers.

2 1

Mg Br

4) Switch the numbers.

2 1

Mg Br

5) The number 1 is not used.

Mg1Br2 MgBr2

Incorrect Correct

6) If the numbers are the same they are not used.

Be2O2 BeO

Incorrect Correct

Writing Ionic Formulas with TRANSITION METALS

Transition metals can sometimes have different oxidation numbers.

In formulas containing transition metals the oxidation number is often part of the compound name.

In Copper(I) oxide the oxidation number of copper is 1+.

In Copper(II) oxide the oxidation number of copper is 2+.

Writing the formula for copper(I) oxide.

1) Write the symbols of the elements. List the metal first.

Cu O

2) Find the oxidation number for each element.

1+ 2-

Cu O

3) Drop the signs from the oxidation numbers.

1 2

Cu O

4) Switch the numbers.

1 2

Cu O

5) The number 1 is not used.

Cu2O1 Cu2O

Incorrect Correct

6) If the numbers are the same they are not used.

Be2O2 BeO

Incorrect Correct

Writing Ionic Formulas with POLYATOMIC IONS

Polyatomic ions are groups of atoms that act as a single unit.

In formulas they are treated as a single element.

Writing the formula for magnesium phosphate.

1) Write the symbols of the elements. List the metal first.

Mg PO4

2) Find the oxidation number for each element.

2+ 3-

Mg PO4

3) Drop the signs from the oxidation numbers.

2 3

Mg PO4

4) Switch the numbers.

2 3

Mg PO4

6) Parentheses are used if there is more than one polyatomic ion group represented.

Mg3(PO4)2

7) The number 1 is not used.

Mg1(NO3)2 Mg(NO3)2

Incorrect Correct

8) If the numbers are the same they are not used.

Mg2(CO3)2 MgCO3

Incorrect Correct

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