Roman numerals (numbers) are sometimes used in formula e.g. Lead(II) chloride.
When they are used, they are there to indicate the valency number (VN) of the element they are next to. e.g. the VN of lead in the example above is 2.
Roman numerals are commonly used to indicate a different valency than the Group would give e.g. Lead is in Group 4 so we might have expected a valency of 4.
Also between Groups 2 and 3 in the table is a large block of elements, sometimes called the Transition elements. While these elements often have a valency of 2 and in some courses you are expected to assume a valency of 2 unless otherwise stated. Roman numerals are often used to show the valency for these elements.
In each these examples, the second element has a VN dependant on its Group.
|Compound||Lead(II) chloride||Gold(III) oxide||Iron(II) sulfide||Silver(I) sulfide|
|Symbols||Pb Cl||Au O||Fe S||Ag S|
|Valency number||2 1||3 2||2 2||1 2|
Lead chloride would have the formula PbCl4 if no valency was suggested for lead by a Roman numeral.