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

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


Formula:Name: nitrate

3NO

The name of this polyatomic ion (click) is nitrate.

ON

O

–

O


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.


polyatomic ions are easy to recognize because the formula always has more than one capital letter.


More than one Capital Letter in the

Formula


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.



Formula

The only positive polyatomic ion we’ll be using is ammonium, with the formula NH4 plus.


Positive Polyatomic

Ion

All the negative polyatomic ions are shaded in pink. You can see, there are quite a few of these.


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


End in “ate”

A few of the names end in “ite”, like chlorite, hypochlorite, and nitrite


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.


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.


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–




CrO3–

We see that it forms the Ba 2 plus ion

Ba2+ CrO3–




ClO3–

Chlorate is a polyatomic ion on the ion table.

Ba2+ CrO3–




ClO3–

It’s formula is ClO3 with a negative 1 charge.

Ba2+ ClO3–




ClO3–

With one barium and one chlorate ion, the total positive charge is positive 2 and the total negative charge is negative 1.


Ba2+ ClO3–



ClO3–

The charges are not balanced, they do not add up to zero.


Ba2+ ClO3–



These do not add up to zero

ClO3–

So we add one more chlorate ion


Ba2+ ClO3–



ClO3–

And the total negative charge is now negative 2


Ba2+ ClO3–



ClO3–

So now the charges are balanced.


Ba2+ ClO3–



NOW these add up to zero

ClO3–

There is one Ba ion


Ba2+ ClO3–

Ba(ClO3)2

ClO3–

To two chlorate ions.


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…


Ba2+ ClO3–

Ba(ClO3)2

A subscript referring to a

polyatomic ion

ClO3–

Brackets must be drawn around the polyatomic ion.


Ba2+ ClO3–

Ba(ClO3)2

A subscript referring to a

polyatomic ion

ClO3–

And now we have the final formula for barium chlorate: Ba,


Ba2+ ClO3–

Ba(ClO3)2Formula:

ClO3–

ClO3 in brackets,


Ba2+ ClO3–

Ba(ClO3)2Formula:

ClO3–

2


Ba2+ ClO3–

Ba(ClO3)2Formula:

ClO3–

The formula tells us that the compound barium chlorate has one barium ion


Ba2+ ClO3–

Ba(ClO3)2Formula:1 barium ion

ClO3–

For every 2 chlorate ions


Ba2+ ClO3–

Ba(ClO3)2Formula:1 barium ion 2 chlorate ions

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

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