Roman Numerals in Chemistry

As if chemistry is not complicated enough for some of us, we sometimes run into Roman numerals in chemistry formulas. For example, should you write copper(I) chloride or copper(II) chloride? Is it iron(II) sulfide or iron(III) sulfide? Why are Roman numerals used in chemistry? How do you know when an element needs a Roman numeral?

What Do Roman Numerals After Elements Mean?

In chemistry nomenclature (writing names systematically), Roman numerals are used for a specific group of elements. These elements are called transition metals. And the Roman numerals indicate the charges that these metals carry in a compound.

Why Do Transition Metals Require Roman Numerals?

Transition metals are elements that change the way they bond. Metals normally form ions. This means that when bonding, they give away electrons and gain a positive charge. However, the transition metals may change their charges. While the net charge of an ionic compound should be equal to zero, transition metals change the number of electrons they give away, based on the element they are bonding with. This happens to neutralize the net charge.

For example, in one ionic compound a transition metal may give away one electron and thus, gain +1 (I) charge. However, in other compounds it might give two or three electrons and gain +2 (II) or +3 (III) charges, respectively. This is also called the oxidation states of these metals.

This variation in charges is the reason that Roman numerals are used chemistry. The Roman numerals after an ion indicate the charges and therefore help name the compound.

Which Metals Require a Roman Numeral in the Name?

As stated above, transition metals require Roman numerals when naming the compound they have formed. There are 38 elements that are called ‘transition metals’ and they are found in the middle of the periodic table, in groups 3 through 12.

Three of the transition metals -iron, cobalt, and nickel- are the only elements known to produce a magnetic field. The transition metals are listed as follows:

Scandium, Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, Copper, Zinc, Yttrium, Zirconium, Niobium, Molybdenum, Technetium, Ruthenium, Rhodium, Palladium, Silver, Cadmium, Hafnium, Tantalum, Tungsten, Rhenium, Osmium, Iridium, Platinum, Gold, Mercury, Rutherfordium, Dubnium, Seaborgium, Bohrium, Hassium, Meitnerium, Ununnilium, Unununium, Ununbium.

What Transition Metals Don’t Need Roman numerals?

The above list of transition metals contain three exceptions in terms of Roman numeral assignment. These are aluminum, zinc and silver. These metals exist in only one ion; therefore, they cannot give away more than one ion. Thus, even though they are transition metals, aluminum, zinc and silver are only (I) and never require Roman numerals written after their names.

How Do You Know When an Element Needs a Roman Numeral?

Naming a compound that requires Roman numerals is easier than it sounds. You just need a periodic table and some attention.

Let’s say that you are asked to write down the name of an ionic compound. The first thing you should do is to check if the compound contains a transition metal. If one of the above transition metals is there, it means that you won’t automatically know the charge. You have to check the charge of the non-metal to figure out the charge of the transition metal and add the equivalent Roman numeral.

For example, FeCl contains Iron (Fe) and Chlorine (Cl). However, the name will not be automatically ‘Iron Chloride’. This is because the transition metal (iron) needs to give away 2 charges to balance the non-metal’s (chlorine) -2 charge. Therefore, FeCl2 = Iron(II) Chloride.

Examples of Chemical Compounds That Need Roman Numerals

One example would be copper and chlorine. Copper can bond with chlorine in two different ways: It can either be CuCl or CuCl2. If they are combined with a one-to-one relationship, this means that chlorine has a charge of -1. To make the compound neutral, copper should have a charge of +1. What follows is that CuCl should be named as copper(I) chloride. Meanwhile, in CuCl2, the two chlorine ions have a charge of -2. Therefore, the copper ion should have a charge of +2. Thus, CuCl2 should be named copper(II) chloride.

Let’s consider the example of lead and oxygen. Lead(II) oxide (PbO), lead(II,IV) oxide (Pb3O4) or lead(IV) oxide (PbO2) are all variations of lead oxide. The different Roman numerals indicate the different charges that lead has had to take in order to neutralize the net charge of the compound.

How Are Compounds Named?

Ionic compounds are made up of metal cations (positive ions) and non-metal anions (negative ions). They are named by first the cation, then the anion. The cation takes exactly the same name as its element. On the other hand, the anion is named by removing the last syllable and adding -ide.

For example, in NaCl, Na is sodium and Cl is chlorine. Thus, the compound is named sodium chloride (not sodium chlorine). Similary, NaF would be named as sodium fluoride (not sodium fluorine). If a transition metal is used in the formula, the charge of the metal ion should be calculated to insert the required Roman numeral immediately after the transition metal. This will indicate the oxidation number.