You find the ground state configuration of negative ions by continuing the filling order as for atoms and (with one exception) by reversing the order for positive ions.

The of an atom or ion is the state in which all its electrons are in the lowest energy orbitals possible.

Negative Ions

You form negative ions by adding electrons until you reach a configuration. For example,

##”O” = “[He]2s”^2 “2p”^4##; ##”O”^”2-” = “[He]2s”^2 “2p”^6 = “[Ne]”##

##”Cl” = “[Ne]3s”^2 “3p”^5##; ##”Cl”^ “- ” = “[Ne]3s”^2 “3p”^6 = “[Ar]”##

##”P” = “[Ne]3s”^2 “3p”^3##; ##”P”^”3-” = “[Ne]3s”^2 “3p”^6= “[Ar]”##

Positive Ions

You form positive ions by removing electrons until you reach a noble gas configuration.

The order of removal is ##”p”## before ##”s”## before ##”d”##.

For example,

##”Na” = “1s”^2 “2s”^2″2p”^6 “3s”##; ##”Na”^”+” = “1s”^2 “2s”^2 “2p”^6 = “[Ne]”##

##”Ca” = “1s”^2 “2s”^2 “2p”^6 “3s”^2 “3p”^6 “4s”^2##; ##”Ca”^”2+” = “1s”^2 “2s”^2 “2p”^6 “3s”^2 “3p”^6 = “[Ar]”##

Transition metal ions are hard to predict, because they can lose ##”d”## electrons to form more than one cation and do not achieve noble gas configurations.

##”Fe” = “[Ar]4s”^2 “3d”^6##; ##”Fe”^”2+” = “[Ar]3d”^6##; ##”Fe”^”3+” = “[Ar]3d”^5##

Usually, you are not required to predict the ground states for transition metal ions, but you must be able to write the electron configurations when given the symbols for the ions.