Normally, when a molecule is made up of different atoms, the atoms can be in the form of ions. An ion is like a molecule, but, it has a charge. Molecules do not carry an electric charge, they are electrically neutral.

An ion with a positive charge is called a cation, whereas, an ion with a negative charge is called an anion. Because ions have charges, they are not stable. They try and pair up with ions having the opposite charge.