What are Ions?

• An ion is an atom with a charge

• An ion can be either positively or negatively charged

• Ions are formed when atoms either gain or lose electrons

• All ions have a full valence shell (outershell) of electrons.

Why do Ions Form?

• Atoms are most stable when their valence shells are completely filled.

• To have a full valence shell an atom can either lose or gain electrons.

• When atoms gain or lose electrons, they are no longer neutral.

• Protons = Electrons

• Charge is neutral

• Less stable than an ion

​​Atoms

• ​Protons ≠ electrons

• Charge is either positive or negative

• More stable than an atom

• Has a full valence shell

Ions

Positively Charged Ions

• Positive ions are known as cations.

• Cations are formed when an atom loses its valence electrons to another atom.

• Cations have more protons than electrons.

• Elements in groups 1-3 form cations.

​Atom

​Cation

Negatively Charged Ions

• Negative ions are known as anions.

• Anions are formed when one atom gains valence electrons from another atom.

• Anions have more electrons than protons.

• Elements in groups 5-7 form anions.

​Atom

​Anion

• Sodium is in group 1, so it loses valence electrons.

• Chlorine is in group 7, so it gains valence electrons.

• The sodium 'donates' its electron to chlorine so they both have full valence shells.

• The cation and anion formed are attracted to each other because they have opposite charges. This is known as ionic bonding.

​Formation of Ions

Group 8 Elements

• All the elements in group 8 already have a full outer shell of electrons.

• This means they don't have to gain or lose electrons like the rest of the elements on the Periodic Table.

• Elements in group 8 don't form ions.