Polyatomic ionic naming and writing

Polyatomic Ions

Poly- more than one

Atomic- atoms

Ions- has a charge (could be positive or negative)

So…. polyatomic ions are compounds consisting of more than one atom that has a charge

We will have a reference sheet with the Polyatomic ions that we’re going to use (link)

Naming Compounds with Polyatomic Ions

First, because these are IONS, they follow the rules of IONIC bonding

The hardest part of this lesson is that you need to be able to recognize when something is a polyatomic ion

You do this by practicing with them and reading over the reference sheet

For instance, when you see NO₃; you need to recognize that's Nitrate

I will try to put them in parenthesis most times, so if you see something in parenthesis that’s going to be a polyatomic ion

Examples

A very common mistake that students make with the above example is that they don’t recognise that NO₃ is nitrate, so they name it Aluminum Nitrogen trioxide

Another common mistake that students make is that they think that subscript two affects the name. Polyatomic ions follow ionic rules, which is don’t use prefixes

Al(NO₃)₃

Ca(OH)₂

Aluminum Nitrate

Calcium Hydroxide

Writing Compounds with Polyatomic Ions

First, because these are IONS, they follow the rules of IONIC bonding

Again the hardest part of this lesson is that you need to be able to recognize when something is a polyatomic ion

Most polyatomics end with -ate or -ite (one exception is Hydroxide)

-ide is typically the element and -ate or -ite are polyatomics

Balance charges

Never use prefixes

Examples

If the charges are not equal, you must criss-cross to balance

These charges are already balanced, so no criss-cross

Notice both polys here end in -ate

Ca(NO₃)₂

Ca(CO₃)

Calcium Nitrate

Calcium Carbonate

Ca⁺² (NO₃)^-1

Ca⁺² (CO₃)^-2

Most Common Mistake

The above is the correct answer

The below is the most common error in this section. You must put parentheses when putting a subscript with polyatomic ions

Ca(OH)₂

Calcium Hydroxide

CaOH₂

Calcium Hydroxide

Common Mistakes

-ate is a polyatomic ion

Sulfate is SO₄

-ide is the element

Sulfide comes from Sulphur

Ca(SO₄)

Calcium Sulfate

CaS

Calcium Sulfide