Why do certain elements and compounds have Roman numerals behind them i.e. copper (II) sulphate (VI)?



Why do certain elements and compounds have Roman numerals behind them i.e. copper (II) sulphate (VI)?

In: Chemistry

The purpose of these Roman numerals is to indicate the oxidation state of the element in the context of the discussion. Metals can exist in various elemental oxidation states. An oxidation state in the simplest terms can be described as the charge of the element (+1, +2, +3, and so on). When an element has a charge is is called an ion. Metals can achieve different charges or oxidation states because of the number of electrons in their outer electron cloud shell. Due to the nature of their electron shell cloud they can either give or receive electrons therefore creating a charge. In a chemical equation the Roman numeral is used to tell the reader what oxidative state the metal is in so that they may understand the state of the metal.

The numbers represent something called the *oxidation state* of the ion in question.

Because oxidation, chemically speaking, involves the loss of electrons, the *oxidation state* can be understood to answer the question “How many electrons has this ion lost?” or more generally (since oxidation states can be negative) “What is the electric charge on this ion?”

Copper(II) sulfate has the chemical formula CuSO4. The sulfate ion, -SO4, has a charge of ^-2 , so the copper ion has a charge of ^+2 to balance it. Its oxidation state is therefore +2.

EDIT: Incidentally, this scheme was adopted because copper (and a lot of other elements) can have different oxidation states. Copper(I) oxide, for example, is Cu2O, and Copper(II) oxide is CuO.

Some chemicals, typically metals can have different numbers of bonds. One is vanadium which can bond to oxygen and make VO2 or VO4 or VO6

We could give each of these a different names, but doing so would require memorizing a lot of different compound names. Instead, we used the Roman numerals to explain the amount of electrons bonded. VO2 would be vanadium (II) oxide and we can use this to easily differntiate names