Question

I am really confused by naming compounds. The reason I am confused is because some compounds, you would use Roman numerals and some you don’t.

Example 1, NiPO4 would be nickel(III) phosphate.

Example 2, Li2SO3 would be lithium sulfite.

I don’t get how example 1 used Roman numerals and example 2 didn’t. How do you know when to use Roman numerals?

Answer 1

Consider a compound Ni₃( PO₄)₂ . This compound contain a metal ion nickel. Ni exists in two variable oxidation states namely 0 and +2. Nickel is a transition metal element. So for naming compounds containing metal ion having variable oxidation states Roman numerals are used. To write name first name of metal is written then in parenthesis oxidation state ( or positive charge) is mentioned by Roman numerals and lastly name negatively charged ion or group.

So the name of compound is Nickel(II) phosphate.

Consider a compound Li₂SO₃ . It contain Li a alkali metal and it shows only one oxidation state i e +1 . Such compounds are named according to nomenclature system of simple ionic compounds. The name of this compound is Lithium sulphite.