Valency and Combining Power

Introduction

Why is water H₂O and not HO or H₃O? Why is table salt NaCl but magnesium chloride MgCl₂? The answer is valency — the combining power of each element. Once you know an element's group, you can work out its valency, and with two valencies you can predict the formula of a simple compound without memorising it. This page brings together valence electrons, ions and group numbers into one practical tool.

Core explanation

What valency means. The valency of an element is the number of electrons its atom loses, gains or shares when it forms bonds. In simple terms, it tells you how many "hands" an atom has to hold on to other atoms. Hydrogen has a valency of 1: one hydrogen atom can bond to one other atom. Oxygen has a valency of 2, so one oxygen atom can bond to two hydrogen atoms, giving H₂O.

Valency from the group number. For main-group elements:

Group 1 2 3 4 5 6 7 0 --- --- --- --- --- --- --- --- --- Valency 1 2 3 4 3 2 1 0 Examples Na, K Mg, Ca Al C, Si N, P O, S Cl, Br Ne, Ar

For Groups 1 to 4, valency equals the group number. For Groups 5 to 7, valency equals 8 minus the group number, which is the number of electrons needed to fill the outer shell. Group 0 has valency 0 because the noble gases do not normally combine.

Link with ion charges. For ionic compounds, valency is simply the size of the ion's charge: Na⁺ has valency 1, Mg²⁺ has 2, O²⁻ has 2. For compounds made only of non-metals, valency is the number of covalent bonds an atom usually forms: carbon forms 4, nitrogen 3, oxygen 2, hydrogen and chlorine 1.

Predicting formulae. In a compound, the combining power on each side must balance. Total valency of one element = total valency of the other.

- Sodium (1) and chlorine (1): one of each balances → NaCl - Magnesium (2) and chlorine (1): one Mg needs two Cl → MgCl₂ - Aluminium (3) and oxygen (2): 2 Al give 6, 3 O give 6 → Al₂O₃ - Carbon (4) and hydrogen (1): one C needs four H → CH₄

The swap-and-drop method. Write the two valencies, swap them over to become the subscripts, then simplify if possible. For aluminium oxide, Al valency 3 and O valency 2 → Al₂O₃. For calcium oxide, Ca 2 and O 2 → Ca₂O₂, which simplifies to CaO. A subscript of 1 is not written.

Limits of the method. Transition metals can have more than one valency (iron can be 2 or 3), so their compounds are named with Roman numerals, such as iron(III) oxide. Some main-group elements also show more than one valency in certain compounds. The simple rules work well for the common compounds of the main-group elements.

Step-by-step reasoning

To predict the formula of the compound of potassium and sulfur:

1. Potassium is in Group 1: valency 1. 2. Sulfur is in Group 6: valency 8 − 6 = 2. 3. Swap: K gets 2, S gets 1. 4. Formula: K₂S. Check: 2 × 1 = 2 and 1 × 2 = 2, so the combining powers balance.

Visual explanation

Draw each atom as a ball with a number of hooks equal to its valency: hydrogen with one hook, oxygen with two, nitrogen with three, carbon with four. Link the hooks so that none are left empty. A carbon ball needs four hydrogen balls to fill its hooks, giving CH₄. The interactive periodic table can display the usual valency of each main-group element.

Real-world analogy

Valency is like the number of hands a person has to hold in a circle dance. If some dancers had four hands and others one, you would need four one-handed dancers around every four-handed dancer so that no hand is left empty. Formulae describe how many of each "dancer" are needed.

Real-world example

Nitrogen has a valency of 3 and hydrogen a valency of 1, so they combine as NH₃, ammonia. Ammonia is made on a huge scale to produce fertilisers that help feed a large share of the world's population. Knowing valencies lets chemists predict and check the formulae of such important compounds.

Why?

Why does valency come from the group number? Atoms combine to reach a full outer shell. A Group 2 atom must lose 2 electrons; a Group 6 atom must gain or share 2. The number of electrons needed to do this sets the number of bonds or the ionic charge, which is the valency.

Common misconception

"Valency is the same as the number of valence electrons." For Groups 1 to 4 they match, but for Groups 5 to 7 they do not. Chlorine has 7 valence electrons but a valency of 1, because it needs only one more electron to fill its shell.

Worked example

Question: Predict the formula of the compound formed between calcium and nitrogen.

Reasoning: Calcium, Group 2, valency 2. Nitrogen, Group 5, valency 8 − 5 = 3. Swap: Ca gets 3, N gets 2, giving Ca₃N₂. Check: 3 × 2 = 6 and 2 × 3 = 6.

Answer: Ca₃N₂ (calcium nitride).

Quick check

1. What is the valency of an element in Group 6? Answer: 2, because it needs two more electrons to fill its outer shell.

Exam focus

Expect questions asking you to "deduce the formula" of a compound from the positions of two elements. Show the valencies, swap them and simplify, then check that the combining powers balance. Do not confuse valency with the number of valence electrons for Groups 5 to 7.

Advanced insight

Modern chemists often use oxidation states instead of valency, because oxidation states carry a sign and can describe elements with several combining powers. Sulfur, for example, can have oxidation state −2 in H₂S but +6 in SO₃. Valency remains a quick, reliable guide for the common compounds of main-group elements.

Summary

Valency is an element's combining power: the number of electrons an atom loses, gains or shares when bonding. For main-group elements it is the group number for Groups 1 to 4 and 8 minus the group number for Groups 5 to 7; noble gases have valency 0. Balancing valencies, often by swapping and simplifying, predicts simple formulae such as NaCl, MgCl₂, Al₂O₃ and CH₄.

Practice questions

1. Define valency. Answer: The combining power of an element: the number of electrons its atom loses, gains or shares when it forms bonds. 2. State the valency of phosphorus and explain how you worked it out. Answer: 3; phosphorus is in Group 5, so it needs 8 − 5 = 3 more electrons to fill its outer shell. 3. Predict the formula of the compound formed between aluminium and chlorine. Answer: AlCl₃ (aluminium valency 3, chlorine valency 1). 4. Predict the formula of magnesium oxide and show how you simplified it. Answer: Mg valency 2 and O valency 2 give Mg₂O₂, which simplifies to MgO. 5. Hydrogen and sulfur combine to form a compound. Predict its formula. Answer: H₂S (hydrogen valency 1, sulfur valency 2).