Variable Valency and Roman Numerals

Introduction

Iron forms two chlorides. One gives a pale green solution; the other gives a yellow-brown one. Copper forms two oxides: one black, one red. If you called both iron compounds "iron chloride", nobody would know which you meant. Elements like these show variable valency — they can combine with different numbers of atoms in different compounds. Chemists handle this by adding a Roman numeral to the name, turning a vague name into a precise one.

Core explanation

Variable valency. Most main-group metals have one valency: sodium is always 1, magnesium always 2. Many transition metals, however, can lose different numbers of electrons and so form ions with different charges. Iron forms Fe²⁺ and Fe³⁺; copper forms Cu⁺ and Cu²⁺. Each ion has its own set of compounds with different formulae, colours and properties.

Roman numerals in names. The valency is written as a Roman numeral in brackets straight after the metal's name, with no space: iron(II), iron(III), copper(I), copper(II). You say "iron two", "copper one" and so on. This is called Stock notation.

Name Ion Example compound Formula Appearance --- --- --- --- --- Iron(II) chloride Fe²⁺ iron(II) chloride FeCl₂ pale green solid Iron(III) chloride Fe³⁺ iron(III) chloride FeCl₃ dark brown solid Iron(III) oxide Fe³⁺ iron(III) oxide Fe₂O₃ red-brown (rust) Copper(I) oxide Cu⁺ copper(I) oxide Cu₂O red solid Copper(II) oxide Cu²⁺ copper(II) oxide CuO black solid Copper(II) sulfate Cu²⁺ copper(II) sulfate CuSO₄ white (blue when hydrated)

The numeral is not a count of atoms. Iron(III) oxide is Fe₂O₃. The "III" does not mean three iron atoms or three oxygen atoms; it means each iron has valency 3. The formula follows from balancing: two iron atoms give 2 × 3 = 6 units of combining power; three oxygen atoms give 3 × 2 = 6. They match.

When to include a numeral. Use a Roman numeral for metals with more than one common valency, such as iron, copper, lead (II and IV), tin (II and IV) and chromium. Do not use one for metals with a single valency: write sodium chloride, not "sodium(I) chloride", and zinc oxide, not "zinc(II) oxide".

Older names. You may see "ferrous" for iron(II) and "ferric" for iron(III), or "cuprous" and "cupric" for copper(I) and copper(II). In these older names, -ous marked the lower valency and -ic the higher. The Roman numeral system is clearer and is now preferred.

Step-by-step reasoning

To find the Roman numeral from a formula such as CuCl₂:

1. Identify the other element and its valency: chlorine, 1. 2. Find its total combining power: 2 × 1 = 2. 3. Divide by the number of metal atoms: 2 ÷ 1 = 2. 4. Write the numeral: copper(II) chloride.

Visual explanation

Picture an iron atom as a ball with two hands in one compound and three hands in another. In FeCl₂ the two-handed version holds two chlorine atoms; in FeCl₃ the three-handed version holds three. The Roman numeral on the name is simply a label counting the hands in use.

Real-world analogy

Variable valency is like a player who can play two positions. The team sheet has to say which one — "Smith (defence)" or "Smith (midfield)" — so everyone knows the role. The Roman numeral does the same job for iron or copper.

Real-world example

Rust is mainly hydrated iron(III) oxide, which is why it is reddish-brown. Iron tablets taken for anaemia usually contain iron(II) compounds, such as iron(II) sulfate, because the body absorbs iron(II) more easily. Same element, different valency, very different uses.

Why?

Why can transition metals have more than one valency? Their outer electrons sit in shells that are close in energy, so losing two or three electrons can both give stable ions. Main-group metals such as sodium lose a set number of electrons easily and then stop, giving one valency.

Common misconception

"Copper(II) oxide contains two copper atoms." It does not: its formula is CuO, with one copper and one oxygen. The (II) describes the valency (ion charge) of copper, not the number of atoms.

Worked example

Question: Name the compound Fe₂O₃ using Stock notation.

Reasoning: Oxygen has valency 2, and there are three oxygen atoms, giving 3 × 2 = 6 units. These are shared between two iron atoms: 6 ÷ 2 = 3. Each iron has valency 3.

Answer: Iron(III) oxide.

Quick check

1. What is the formula of copper(II) chloride? Answer: CuCl₂.

Exam focus

Examiners test both directions: naming a compound such as FeCl₃ or PbO₂ with the correct numeral, and writing a formula from a name such as copper(I) oxide. Always show the valency calculation, and never add a numeral to sodium, magnesium, aluminium or zinc compounds.

Advanced insight

Strictly, the Roman numeral shows the oxidation state of the metal, which for simple ions equals the ion charge. Some elements have many oxidation states: manganese ranges from +2 in MnCl₂ to +7 in potassium manganate(VII), KMnO₄, the purple compound used in some laboratory tests. The numeral system works for all of them.

Summary

Variable valency means an element can have more than one combining power. Iron (II or III) and copper (I or II) are common examples. A Roman numeral in brackets after the metal's name shows which valency is present, such as iron(III) oxide, Fe₂O₃. The numeral is not a count of atoms, and it is left out for metals with only one valency.

Practice questions

1. Name FeCl₂ and FeCl₃. Answer: Iron(II) chloride and iron(III) chloride. 2. Write the formula of copper(I) oxide. Answer: Cu₂O. 3. Explain why the name "iron chloride" is not precise enough. Answer: Iron has two valencies and forms both FeCl₂ and FeCl₃, so the name must show which one is meant. 4. Why is magnesium oxide not written as magnesium(II) oxide? Answer: Magnesium has only one valency (2), so no Roman numeral is needed. 5. Lead forms PbO₂. What is the valency of lead in this compound? Answer: 4, because two oxygen atoms provide 2 × 2 = 4 units of combining power for one lead atom; the name is lead(IV) oxide.