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Ionic compounds with 3 or more elements contain polyatomic ions. We’ll show you how to write the formulas for compounds with polyatomic ions.
Formulas for Compound with Polyatomic Ions
Example 1
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.
A group of atoms that bond together
ON O
O
3–Polyatomic ion- A group of atoms that stay together and act as a single unit. The whole ion has a charge.
And it stays together to act like a single unit
ON O
O
3–Polyatomic ion- A group of atoms that stays together and acts as a single unit. The whole ion has a charge.
The whole ion has a net charge.
ON
O
–
O
Polyatomic ion- A group of atoms that stays together and acts as a single unit. The whole ion has a charge.
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
ON
O
–
O
Polyatomic ion- A group of atoms that stays together and acts as a single unit. The whole ion has a charge.
Formula:Name: nitrate
3NO
The name of this polyatomic ion (click) is nitrate.
ON
O
–
O
Polyatomic ion- A group of atoms that stays together and acts as a single unit. The whole ion has a charge.
Formula:Name: nitrate
3NO
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
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
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
More than one Capital Letter in the
Formula
The only positive polyatomic ion we’ll be using is ammonium, with the formula NH4 plus.
Formulas and Charges of Common Ions
Positive Polyatomic
Ion
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
Negative Polyatomic
Ions
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 “ate”
A few of the names end in “ite”, like chlorite, hypochlorite, and nitrite
Formulas and Charges of Common Ions
End in “ite”
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
End in “ide”
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.
Formulas and Charges of Common Ions
2 possible names
CrO3–
We’re asked to write the formula for barium chlorate
Write the formula for barium chlorate
Ba2+ CrO3–
+2 –2Total Positive Charge
Total Negative Charge
CrO3–
Looking up barium on the periodic table
Ba2+ CrO3–
+2 –2Total Positive Charge
Total Negative Charge
Write the formula for barium chlorate
CrO3–
We see that it forms the Ba 2 plus ion
Ba2+ CrO3–
+2 –2Total Positive Charge
Total Negative Charge
Write the formula for barium chlorate
ClO3–
Chlorate is a polyatomic ion on the ion table.
Ba2+ CrO3–
+2 –2Total Positive Charge
Total Negative Charge
Write the formula for barium chlorate
ClO3–
It’s formula is ClO3 with a negative 1 charge.
Ba2+ ClO3–
+2 –2Total Positive Charge
Total Negative Charge
Write the formula for barium chlorate
ClO3–
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
Ba2+ ClO3–
+2 –1Total Positive Charge
Total Negative Charge
ClO3–
The charges are not balanced, they do not add up to zero.
Write the formula for barium chlorate
Ba2+ ClO3–
+2 –1Total Positive Charge
Total Negative Charge
These do not add up to zero
ClO3–
So we add one more chlorate ion
Write the formula for barium chlorate
Ba2+ ClO3–
+2 –1Total Positive Charge
Total Negative Charge
ClO3–
And the total negative charge is now negative 2
Write the formula for barium chlorate
Ba2+ ClO3–
+2 –2Total Positive Charge
Total Negative Charge
ClO3–
So now the charges are balanced.
Write the formula for barium chlorate
Ba2+ ClO3–
+2 –2Total Positive Charge
Total Negative Charge
NOW these add up to zero
ClO3–
There is one Ba ion
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2
ClO3–
To two chlorate ions.
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2
ClO3–
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
Ba2+ ClO3–
Ba(ClO3)2
A subscript referring to a
polyatomic ion
ClO3–
Brackets must be drawn around the polyatomic ion.
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2
A subscript referring to a
polyatomic ion
ClO3–
And now we have the final formula for barium chlorate: Ba,
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2Formula:
ClO3–
ClO3 in brackets,
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2Formula:
ClO3–
2
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2Formula:
ClO3–
The formula tells us that the compound barium chlorate has one barium ion
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2Formula:1 barium ion
ClO3–
For every 2 chlorate ions
Write the formula for barium chlorate
Ba2+ ClO3–
Ba(ClO3)2Formula:1 barium ion 2 chlorate ions