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

Formulas for Compound with Polyatomic Ions

Example 3

We’re asked to write the formula for iron(III) hydroxide

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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.

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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.

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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,

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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,

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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.

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

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

Fe3+ OH–

+3 –1Total Positive Charge

Total Negative Charge

OH–

OH–

Write the formula for iron(III) hydroxide.

And these do not add up to zero.

Fe3+ OH–

+3 –1Total Positive Charge

Total Negative Charge

OH–

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

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

OH–

Write the formula for iron(III) hydroxide.

The total negative charge is now negative 3

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

OH–

Write the formula for iron(III) hydroxide.

Now the positive and negative charges are balanced, they do add up to zero.

Fe3+ OH–

+3 –3Total Positive Charge

Total Negative Charge

OH–

OH–

Write the formula for iron(III) hydroxide.

These DO add up to zero

So in the formula, we have one iron ion,

Fe3+ OH–

+3 –3

OH–

OH–

Write the formula for iron(III) hydroxide.

Fe(OH)3

And 3 hydroxide ions

Fe3+ OH–

+3 –3

OH–

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,

Fe3+ OH–

+3 –3

OH–

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.

Fe3+ OH–

+3 –3

OH–

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