POLYATOMIC IONS

• I CAN write a formula containing a polyatomic ion.

BINARY vs POLYATOMIC COMPOUNDS

• Up to this point, we’ve only looked at BINARY IONIC COMPOUNDS….compounds containing only TWO different elements.

• However, not all compounds are binary!• Compounds of THREE OR MORE DIFFERENT

ELEMENTS often contain POLYATOMIC IONS.– The prefix POLY means “more than two” or “many”.

• POLYATOMIC ION – A group of bonded atoms that have an overall “+” or “-” charge and act as if there were a single atom ion.

• There are MANY polyatomic ions.• The following list is a PARTIAL listing of the

more common polyatomic ions, sorted by charge.

Common Polyatomic Ions

Polyatomic Endings

• Just like –IDE generally means you have a binary compound, the endings of polyatomic ions mean something too:– If you see an –ite or –ate ending on a compound’s name,

that indicates the presence of a polyatomic ion.– Both –ite and –ate mean the polyatomic ion contains

OXYGEN along with other elements.– If there are more than one polyatomic ion containing the

same group of elements, the one with the FEWEST oxygen atoms has the –ite ending while the one with the MOST oxygen atoms has the –ate ending.

• EXAMPLE

• Nitrite and Nitrate are two polyatomic ions consisting of NITROGEN and OXYGEN.

• Nitrite = NO2 while Nitrate = NO3 • has less oxygen has more oxygen

• EXCEPTION

• Note that HYDROXIDE and CYANIDE both end in –IDE. Their names are old and they may or may not have oxygen in them, so the older names are still used even though –IDE generally indicates a binary compound.

Working With Polyatomic Ions

• Writing formulas for compounds containing Polyatomic Ions is very similar to writing a Binary Ionic formula.– Write the positive ion first, writing it’s charge above.– Write the negative ion second, writings it charge

above.– If the charges DO NOT CANCEL, use the CRISS-

CROSS method to balance the charges.– Write the final formula.

Using a Polyatomic Ion

• Writing a polyatomic formula often requires one more step.– Since most polyatomic ions already contain

subscript(s), you cannot simply add a subscript on.– For example:• Suppose you needed to add a subscript of 3 on to the

polyatomic ion Carbonate: CO3

• Why can’t you just do this? CO33 or CO6

• YOU CANNOT ALTER THE FORMULA OF A POLYATOMIC ION!!

• So how do you add a subscript to a polyatomic ion?

• You first PLACE THE POLYATOMIC ION INSIDE A PARENTHESIS.

• Then add the SUBSCRIPT AFTER the parenthesis.

• REMEMBER that a SUBSCRIPT MULTIPLIES EVERYTHING INSIDE THE PARENTHESIS!

• So the proper way to add a subscript of 3 to the Carbonate polyatomic ion would be:

• (CO3)3

• Polyatomic Ion inside parenthesis Subscript added OUTSIDE

Practice Problem• Write the formula for a compound of

Aluminum and Sulfate.

• (+3) + (-2 ) = 0• Al SO4

• Since a subscript of 3 must be added to sulfate when writing the formula, first place the sulfate inside a PARENTHESIS before adding the subscript like this:

• (SO4)3

2 3

• So the formula becomes:

• Al2(SO4)3

More Practice

• Write the formula for Ammonium Phosphate.• (on marker board)

• (NH4)3PO4