Valency: The Combining Power of an Element

Introduction

Why is water H₂O and not HO or H₃O? Why is methane CH₄ rather than CH₂? Atoms do not join in any number they like: each element has a characteristic combining power , called its valency . Valency is the key that unlocks formula writing. Once you know the valencies of the elements involved, you can predict the formula of a compound instead of memorising it.

Core explanation

Definition. The valency of an element is the number of hydrogen atoms that one atom of the element combines with (or replaces). Hydrogen is the reference, and its valency is 1.

Examples from hydrogen compounds.

Compound Formula H atoms per atom of the other element Valency --- --- --- --- Hydrogen chloride HCl 1 chlorine 1 Water H₂O 2 oxygen 2 Ammonia NH₃ 3 nitrogen 3 Methane CH₄ 4 carbon 4

Valency as bonds. In molecules, valency equals the number of covalent bonds an atom forms. In a structural diagram each bond is a line. Hydrogen always has one line, oxygen two, nitrogen three and carbon four. So water can be drawn H–O–H, and in carbon dioxide the carbon forms two double bonds, O=C=O, using all four of its bonds.

Valency from other partners. Not every element forms a simple hydrogen compound. Chlorine, with valency 1, can be used as a second reference: one chlorine atom is "worth" one hydrogen atom. Sodium forms NaCl, so sodium has valency 1; calcium forms CaCl₂, so calcium has valency 2; aluminium forms AlCl₃, so aluminium has valency 3. Oxygen, with valency 2, can also be used: MgO shows magnesium has valency 2.

Valency in ionic compounds. For ions, the valency equals the size of the charge: Na⁺ has valency 1, Ca²⁺ has valency 2, O²⁻ has valency 2, Cl⁻ has valency 1. Valency itself has no sign; it is simply a number.

Why valency gives fixed formulae. In any compound, the combining power must be "used up" exactly on both sides. In CaCl₂, one calcium atom provides 2 units of combining power, and two chlorine atoms provide 1 each, making 2 in total. The numbers balance, and that is why the formula is fixed — the idea behind the law of constant composition.

Step-by-step reasoning

To find an element's valency from a formula with hydrogen or chlorine:

1. Find a compound of the element with hydrogen or chlorine only. 2. Count the H or Cl atoms per single atom of the element. 3. That number is the valency (for example, PH₃ gives phosphorus 3). 4. Check by drawing bonds: each atom should have its valency number of lines.

Visual explanation

Picture each atom as a ball with "hands" sticking out. Hydrogen has one hand, oxygen two, nitrogen three and carbon four. A molecule forms when every hand holds exactly one other hand. Carbon's four hands each grip a hydrogen's single hand, giving CH₄ with no hand left empty.

Real-world analogy

Valency is like the number of plug sockets on an extension lead. A lead with four sockets can power four devices, one with two sockets only two. Each device has one plug. You cannot connect more devices than sockets, and a good design leaves none spare.

Real-world example

Carbon's valency of 4 is the reason for the enormous variety of organic compounds, from the methane in a gas cooker to the proteins in your muscles. Four bonds per carbon atom allow chains, branches and rings, giving millions of different molecules built from a few elements.

Why?

Why do elements have particular valencies? It depends on the electrons in the outer shell. Atoms tend to gain, lose or share electrons until they reach a stable arrangement, often eight outer electrons. Oxygen has six outer electrons, so it needs two more and forms two bonds; nitrogen has five and forms three.

Common misconception

"Valency is the number of atoms in a molecule." It is not. Valency belongs to one element and describes its combining power. Water has three atoms, but oxygen's valency is 2 and hydrogen's is 1.

Worked example

Question: Silicon forms the hydride SiH₄ and sulfur forms H₂S. State the valency of silicon and of sulfur.

Reasoning: In SiH₄, one silicon atom combines with four hydrogen atoms, so silicon has valency 4. In H₂S, one sulfur atom combines with two hydrogen atoms, so sulfur has valency 2.

Answer: Silicon 4; sulfur 2.

Quick check

1. Phosphorus forms PCl₃. What is its valency in this compound? Answer: 3, because one phosphorus atom combines with three chlorine atoms, each of valency 1.

Exam focus

Be able to define valency in words and deduce it from a formula containing hydrogen, chlorine or oxygen. Remember that oxygen has valency 2, so in an oxide you must double the count: in Al₂O₃, the valency of aluminium is 3, not 1.5.

Advanced insight

Modern chemistry often uses "oxidation state" rather than valency, because oxidation states carry a sign and handle ions and complex compounds more consistently. Valency remains a powerful starting idea: it was the concept that led Kekulé and Couper, in 1858, to propose that carbon is tetravalent, founding structural organic chemistry.

Summary

Valency is the combining power of an element: the number of hydrogen atoms (or chlorine atoms) one atom combines with, which in molecules equals the number of covalent bonds it forms. Hydrogen and chlorine have valency 1, oxygen 2, nitrogen 3 and carbon 4. Balancing combining power on both sides explains why each compound has a fixed formula.

Practice questions

1. Define valency. Answer: The combining power of an element, equal to the number of hydrogen atoms that one atom of the element combines with. 2. What is the valency of nitrogen in ammonia, NH₃? Answer: 3. 3. Magnesium forms MgCl₂. State the valency of magnesium and explain. Answer: 2, because one magnesium atom combines with two chlorine atoms, each of valency 1. 4. How many bonds does each carbon atom form in carbon dioxide, O=C=O? Answer: Four, as two double bonds, matching carbon's valency of 4. 5. Using oxygen's valency of 2, deduce the valency of calcium from the formula CaO. Answer: 2, because one calcium atom combines with one oxygen atom of valency 2.