Roman Numerals and Latin Names

When a metal ion has more than one possible charge then the charge must be indicated in some way.

Roman Numerals

The standard way to do express charge is to use a roman numeral. Thus Cu⁺² would be labeled copper II and Cu⁺¹ would be labeled copper I.
In case you've forgotten (or never knew) the roman numerals from 1 to 10 are:

1	I	6	VI
2	II	7	VII
3	III	8	VIII
4	IV	9	IX
5	V	10	X

Latin Names

If a metal ion has only two possible charges, there is an older way to label the charge which is still in use (although less and less).

This method involves taking the latin root of the element's name and adding the ending "ic" for the higher charge or "ous" for the lower charge. (That's easy to remember because ic and high both have the letter i in them and ous and low both contain the letter o.
 
Here are some of the commonly used elements, their latin roots and the roots with the two possible endings.

element	root	higher charge	lower charge
Copper	cuprum	cupric	cuprous
cobalt	cobalt	cobaltic	cobaltous
gold	aurum	auric	aurous
nickel	nickel	nickelic	nickelous
tin	stannum	stannic	stannous
lead	plumbum	plumbic	plumbous
iron	ferrum	ferric	ferrous

Of course to use this you will need to be able to determine the charge on the ion.
So the compound NiSO₄ would be called either nickel II sulfate or nickelous sulfate, while the compound Ni₂(SO₄)₃ would be called either Nickel III sulfate or nickelic sulfate.