Ionic compounds with 3 or more elements contain polyatomic ions. We’ll show you how to write the formulas for compounds with polyatomic ions.

Polyatomic ion
- A group of atoms that stay together and act as a single unit. The whole ion has a charge.
A polyatomic ion is

Polyatomic ion
- A group of atoms that stay together and act as a single unit. The whole ion has a charge.
O
O 3–
N O
A group of atoms that bond together

Polyatomic ion
- A group of atoms that stays together and acts as a single unit.
The whole ion has a charge.
O
O 3–
N O
And it stays together to act like a single unit

Polyatomic ion
- A group of atoms that stays together and acts as a single unit.
The whole ion has a charge.
O
O
N
–
O
The whole ion has a net charge.

Polyatomic ion
- A group of atoms that stays together and acts as a single unit.
The whole ion has a charge.
NO
3

Name: nitrate
O
O
N
–
O
In the example shown here there is one nitrogen atom and three oxygen atoms and the ion has a charge of negative 1, so the formula is (click) NO3 minus

Polyatomic ion
- A group of atoms that stays together and acts as a single unit.
The whole ion has a charge.
NO
3

The name of this polyatomic ion (click) is nitrate.
O
O
N
–
O

Formulas and Charges of Common Ions
Here is a table that contains the formulas and charges of the ions we will be using in this course.

Formulas and Charges of Common Ions
The polyatomic ions are the ones with more than one element. In this table, they are shaded. The other ions on here are not polyatomic, they are ions of single elements.

Formulas and Charges of Common Ions
More than one Capital
Letter in the Formula
More than one Capital
Letter in the Formula polyatomic ions are easy to recognize because the formula always has more than one capital letter.

Formulas and Charges of Common Ions
More than one Capital
Letter in the Formula
For example, the hydroxide ion is polyatomic, because it has a capital “O” and a capital “H”. You will always be able to use a table like this when you are writing formulas for ionic compounds .

Formulas and Charges of Common Ions
Positive
Polyatomic
Ion
The only positive polyatomic ion we’ll be using is ammonium, with the formula
NH4 plus.

Formulas and Charges of Common Ions
Negative
Polyatomic
Ions
All the negative polyatomic ions are shaded in pink. You can see, there are quite a few of these.

Formulas and Charges of Common Ions
End in
“ate”
If you look at the names of these, you can see that many of them end in “ate”, like acetate, carbonate, and nitrate

Formulas and Charges of Common Ions
End in
“ite”
A few of the names end in “ite”, like chlorite, hypochlorite, and nitrite

Formulas and Charges of Common Ions
End in
“ide”
And three polyatomic ions have names ending in “ide”, cyanide, bisulphide, and hydroxide. The other ions whose names in in “ide” on this table are not polyatomic ions, they are ions of a single element.

Formulas and Charges of Common Ions
2 possible names
Notice that 4 of the polyatomic ions, have two possible names, separated by a comma. Either name can be used. For example HCO3 minus can be called hydrogen carbonate or bicarbonate.

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ CrO
CrO
–
3
3
–
+2 –2
Total
Negative
Charge
We’re asked to write the formula for barium chlorate

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ CrO
CrO
–
3
3
–
+2 –2
Total
Negative
Charge
Looking up barium on the periodic table

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ CrO
CrO
–
3
3
–
+2 –2
Total
Negative
Charge
We see that it forms the Ba 2 plus ion

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ CrO
ClO
–
3
3
–
+2 –2
Total
Negative
Charge
Chlorate is a polyatomic ion on the ion table.

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ ClO
ClO
–
3
3
–
+2 –2
Total
Negative
Charge
It’s formula is ClO3 with a negative 1 charge.

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ ClO
ClO
–
3
3
–
+2 –1
Total
Negative
Charge
With one barium and one chlorate ion, the total positive charge is positive 2 and the total negative charge is negative 1.

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ ClO
ClO
–
3
3
–
+2 –1
These do not add up to zero
Total
Negative
Charge
The charges are not balanced, they do not add up to zero.

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ ClO
ClO
–
3
3
–
+2 –1
Total
Negative
Charge
So we add one more chlorate ion

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ ClO
ClO
–
3
3
–
+2 –2
Total
Negative
Charge
And the total negative charge is now negative 2

Write the formula for barium chlorate
Total
Positive
Charge
Ba 2+ ClO
ClO
–
3
3
–
+2 –2
NOW these add up to zero
Total
Negative
Charge
So now the charges are balanced.

Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Ba ( ClO
3
)
2
There is one Ba ion

Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Ba ( ClO
3
)
2
To two chlorate ions.

Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Ba ( ClO
3
)
2
A subscript referring to a polyatomic ion
The subscript 2 here, to the right of ClO3, means there are 2 chlorate ions in the formula. Whenever a subscript refers to a polyatomic ion…

Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Ba ( ClO
3
)
2
A subscript referring to a polyatomic ion
Brackets must be drawn around the polyatomic ion.

Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Formula: Ba ( ClO
3
)
2
And now we have the final formula for barium chlorate: Ba,

Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Formula: Ba ( ClO
3
)
2
ClO3 in brackets,

2
Write the formula for barium chlorate
Ba 2+ ClO
ClO
–
3
3
–
Formula: Ba ( ClO
3
)
2

Write the formula for barium chlorate
Ba
ClO
ClO
–
3
3
–
Ba (ClO
3
)
2
1 barium ion
The formula tells us that the compound barium chlorate has one barium ion

Write the formula for barium chlorate
Ba 2+ ClO
ClO
Ba ( ClO
3
)
2
1 barium ion 2 chlorate ions
For every 2 chlorate ions