We’ll go over another example of writing the formula for a compound with
polyatomic ions.
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
–3
We’re asked to write the formula for iron(III) hydroxide
Total
Negative
Charge
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.
3+
Fe
–
OH
–
OH
–
OH
From
Total
Periodic
Positive
Table
Charge
+3
From Ion
Table
–3
Total
Negative
Charge
The iron(III) ion is found on the periodic table and on the ion table (click) it is Fe
with a positive 3 charge.
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
–3
From Ion
Table
Total
Negative
Charge
Hydroxide is a polyatomic ion found on the ion table.(click) It is OH with a charge
of negative 1.
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
Even though OH does not have
any subscripts in–
it,
–
OH
Total
Positive
Charge
+3
It is still a polyatomic ion,
Because is has two different
Total
elements (oxygen and hydrogen)
–3
Negative
Charge
Even though the formula for the hydroxide ion has no little numbers, or subscripts
in it like many of the polyatomic ions do,
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
Even though OH does not have
any subscripts in–
it,
–
OH
Total
Positive
Charge
It is still a polyatomic ion
+3
It is still a polyatomic ion,
Because is has two different
Total
elements (oxygen and hydrogen)
–3
Negative
Charge
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
Even though OH does not have
any subscripts in–
it,
–
OH
Total
Positive
Charge
+3
It is still a polyatomic ion,
Because is has two different
Total
elements (oxygen and hydrogen)
Because is contains two different elements,
–3
Negative
Charge
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
Even though OH does not have
any subscripts in–
it,
–
OH
Total
Positive
Charge
oxygen
+3
It is still a polyatomic ion,
Because is has two different
Total
elements (oxygen and hydrogen)
–3
Negative
Charge
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
Even though OH does not have
any subscripts in–
it,
–
OH
Total
Positive
Charge
+3
It is still a polyatomic ion,
Because is has two different
Total
elements (oxygen and hydrogen)
–3
Negative
Charge
And hydrogen. There is more than one capital letter in the formula, which makes it
a polyatomic ion.
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
–1
Total
Negative
Charge
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
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
And these do not add up to zero.
–1
These do not
add up to zero
Total
Negative
Charge
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
–3
Total
Negative
Charge
To make the charges balance, (click) we add two more hydroxide ions
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
The total negative charge is now negative 3
–3
Total
Negative
Charge
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
Total
Positive
Charge
+3
–3
These DO add up
to zero
Total
Negative
Charge
Now the positive and negative charges are balanced, they do add up to zero.
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
+3
–3
Fe(OH)3
So in the formula, we have one iron ion,
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
+3
–3
Fe(OH)3
And 3 hydroxide ions
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
+3
–3
Fe(OH)3
A subscript to
the right of the
polyatomic ion
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,
Write the formula for iron(III) hydroxide.
3+
Fe
–
OH
–
OH
–
OH
+3
–3
Fe(OH)3
A subscript to
the right of the
polyatomic ion
Brackets must be written around the OH in the formula.
Write the formula for iron(III) hydroxide.
Fe(OH)3
So the final formula for iron(III) hydroxide is (click) Fe (OH in brackets) , 3.