Since when do names include numbers?

Copper II Sulfate, Lead IV Oxide, Nickel III Hydroxide...What's with the Roman numerals in those names for chemical compound?

Some elements can only have one charge, so you can just use their name without needed to add anything to explain what the charge is. Sodium is always 1+ charge, so you can just name the compounds sodium chloride, sodium oxide, etc.

But some elements can have different charges at different times. Copper is sometimes 1+ and sometimes 2+, so you can't just call something copper oxide. You wouldn't know if that was CuO or Cu2O becuase you wouldn't know what charge to criss-cross. So we use a Roman numeral in the name to show what charge the metal has in this particular substance.

So CuO would be copper II oxide and Cu2O would be copper I oxide. Remember that the Roman numeral doesn't tell us what the subscripts are, they tell us the charge. It's easy to get confused and get the names backwards.

So which elements need these Roman numerals? All of the transition metals except for zinc, cadmium and silver can have more than one charge and need Roman numerals. Those three always have the same charge (silver is 1+ and zinc and cadmium are 2+) so they don't need Roman numerals. Also tin and lead, even though they aren't transition metals, can have 2+ or 4+ charges and need Roman numerals.