We’ll go over another example of writing the formula for a compound with polyatomic ions.

We’re asked to write the formula for iron(III) hydroxide Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide.

Iron(III) is the ion of iron with a positive 3 charge Write the formula for iron(III) hydroxide.

And hydroxide is the polyatomic ion O H minus Write the formula for iron(III) hydroxide.

The iron(III) ion is found on the periodic table and on the ion table (click) it is Fe with a positive 3 charge. Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. From Periodic Table From Ion Table

Hydroxide is a polyatomic ion found on the ion table.(click) It is OH with a charge of negative 1. Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. From Ion Table

Even though the formula for the hydroxide ion has no little numbers, or subscripts in it like many of the polyatomic ions do, Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. Even though OH – does not have any subscripts in it, It is still a polyatomic ion, Because is has two different elements (oxygen and hydrogen)

It is still a polyatomic ion Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. Even though OH – does not have any subscripts in it, It is still a polyatomic ion, Because is has two different elements (oxygen and hydrogen)

Because is contains two different elements, Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. Even though OH – does not have any subscripts in it, It is still a polyatomic ion, Because is has two different elements (oxygen and hydrogen)

oxygen Fe 3+ OH–OH– +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. Even though OH – does not have any subscripts in it, It is still a polyatomic ion, Because is has two different elements (oxygen and hydrogen)

And hydrogen. There is more than one capital letter in the formula, which makes it a polyatomic ion. Fe 3+ OH–OH– +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. Even though OH – does not have any subscripts in it, It is still a polyatomic ion, Because is has two different elements (oxygen and hydrogen)

You can see that if we have one Fe 3 plus ion and one OH minus ion, the total positive charge is positive 3 and the total negative charge is negative 1 Fe 3+ OH – +3 –1 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide.

And these do not add up to zero. Fe 3+ OH – +3 –1 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. These do not add up to zero

To make the charges balance, (click) we add two more hydroxide ions Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide.

The total negative charge is now negative 3 Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide.

Now the positive and negative charges are balanced, they do add up to zero. Fe 3+ OH – +3 –3 Total Positive Charge Total Negative Charge OH – Write the formula for iron(III) hydroxide. These DO add up to zero

So in the formula, we have one iron ion, Fe 3+ OH – +3 –3 OH – Write the formula for iron(III) hydroxide. Fe(OH) 3

And 3 hydroxide ions Fe 3+ OH – +3 –3 OH – Write the formula for iron(III) hydroxide. Fe(OH) 3

Now the 3 here is a subscript to the right of the polyatomic ion hydroxide, indicating this formula has 3 hydroxides. Because OH is a polyatomic ion and there is a subscript to the right of it, Fe 3+ OH – +3 –3 OH – Write the formula for iron(III) hydroxide. Fe(OH) 3 A subscript to the right of the polyatomic ion

Brackets must be written around the OH in the formula. Fe 3+ OH – +3 –3 OH – Write the formula for iron(III) hydroxide. Fe(OH) 3 A subscript to the right of the polyatomic ion

So the final formula for iron(III) hydroxide is (click) Fe (OH in brackets), 3. Write the formula for iron(III) hydroxide. Fe(OH) 3