THREE EASY STEPS…

Writing Chemical Formulas

1. Write the symbol of the cation with its charge.

2. Write the symbol of the anion with its charge.

3. Write in subscript numbers so that the overall charge of the compound is zero. (Use the “criss-cross method”)

4. Reduce to lowest common multiple if necessary.

Example #1—sulfur and aluminum

1. Write the symbol of the cation with its charge.

Al+3 2. Write the symbol of the anion with its

charge.

Al+3S-2

3. Write in subscript #’s so that the overall charge of the compound is zero. (Use criss-cross method)

Al2S3

Example #2—rubidium and bromine

1. Write the symbol of the cation with its charge.

Rb+1

2. Write the symbol of the anion with its charge.

Rb+1Br-1

3. Write in subscript #’s so that the overall charge of the compound is zero. (Use criss-cross method)

Rb1 Br1

RbBr

Example #3—hydroxide ion and calcium

1. Write the symbol of the cation with its charge.

Ca+2

2. Write the symbol of the anion with its charge.

Ca+2OH-1

3. Write in subscript #’s so that the overall charge of the compound is zero. (Use criss-cross method)

Ca1OH2

Put parentheses around polyatomic ions when you need to indicate more than one.

Ca(OH)2

Example #4—carbonate ion and lead (IV)

1. Write the symbol of the cation with its charge.

Pb+4

2. Write the symbol of the anion with its charge.

Pb+4CO3-2

3. Write in subscript #’s so that the overall charge of the compound is zero. (Use the criss-cross method).

Pb2(CO3)4

Now you finish it off!