Excellent question, Nicole.
First, let's start with a few basics to make sure we're all on the
same page. An ion is an atom or molecule that has non zero electric
charge. That is, an ion has either more protons (+) than electrons (-)
or more electrons than protons.
Ions can be formed from one atom having more or less electrons. Some common examples are H+, Cl-, Ca++.
Sometimes ions can be formed from a molecule (a group of atoms
stuck together) having a net electric charge. Some fairly common
examples are HO- and NH4+.
So polyatomic ions are pretty much like regular ions except that they are made from groups of atoms instead of just one.
I hope this answers your question.
(published on 10/22/2007)