What Is a Compound?

Introduction

There are only about a hundred naturally occurring elements, yet the world contains millions of different substances: water, salt, sugar, chalk, plastics, proteins and many more. How can so few building blocks make so much variety? The answer is that elements can join together chemically to form compounds . Just as the twenty-six letters of the alphabet combine into countless words, the elements combine into an enormous number of compounds, each with its own properties.

Core explanation

Definition. A compound is a pure substance made of two or more different elements that are chemically combined in a fixed ratio. The atoms of the different elements are held together by chemical bonds.

Examples.

- Water , H₂O: hydrogen and oxygen, with two hydrogen atoms for every oxygen atom. - Carbon dioxide , CO₂: carbon and oxygen, with two oxygen atoms for every carbon atom. - Sodium chloride , NaCl: sodium and chlorine, in a 1 : 1 ratio. - Calcium carbonate , CaCO₃: calcium, carbon and oxygen, found in chalk and limestone.

Elements versus compounds. An element contains only one type of atom. A compound contains at least two types of atom, bonded together. Oxygen gas, O₂, has two atoms per molecule, but both are oxygen, so it is an element. Carbon monoxide, CO, also has two atoms per molecule, but they are different, so it is a compound.

Formed by chemical reactions. Compounds are made when elements react. When magnesium burns in air, magnesium atoms combine with oxygen atoms to form magnesium oxide, a white powder. This is a chemical change: a new substance is formed, and energy is usually given out or taken in.

Separated only by chemical reactions. Because the elements in a compound are bonded, they cannot be separated by physical methods such as filtering, evaporating or distilling. Boiling water simply turns it into steam, which is still H₂O. To get hydrogen and oxygen back, a chemical reaction is needed, such as passing electricity through the water.

A compound is a single substance. Although it contains more than one element, a compound is a pure substance with its own fixed composition and its own properties, such as a definite melting point and boiling point.

Step-by-step reasoning

To decide whether a substance is an element or a compound:

1. Look at its formula or particle diagram. 2. Count the different kinds of atom (different symbols). 3. If there is only one kind, it is an element, even if the atoms are joined in molecules. 4. If there are two or more kinds bonded together, it is a compound.

Visual explanation

In a particle diagram, an element might be shown as a box of identical red circles, either alone or joined in pairs. A compound is shown as a box of identical particles, each made of joined circles of different colours, such as one red circle bonded to two white circles for water. Every particle in the compound box is the same.

Real-world analogy

A compound is like a cake. Flour, eggs, sugar and butter go in, but once baked they have been changed and joined into something new. You cannot simply pick the eggs back out of a cake, just as you cannot pick hydrogen out of water by physical means.

Real-world example

Table salt is the compound sodium chloride. Sodium is a soft, reactive metal, and chlorine is a toxic green gas, yet the compound they form is safe to sprinkle on food. Sugar, the compound sucrose, is made from carbon, hydrogen and oxygen — three elements that separately are a black solid and two colourless gases.

Why?

Why can't a compound be separated by filtering or distilling? Physical methods work by using differences in properties, such as particle size or boiling point, between separate substances. In a compound, the atoms of different elements are bonded inside the same particle, so there are no separate substances to pull apart. Only breaking the bonds in a chemical reaction can release the elements.

Common misconception

"Any substance with a formula containing a number is a compound." O₂, N₂ and S₈ all contain numbers but are elements, because each contains only one type of atom. The test is the number of different elements, not the number of atoms.

Worked example

Question: Classify each as an element or a compound: H₂, NH₃, Fe, CH₄, O₃.

Reasoning: H₂ contains only hydrogen. NH₃ contains nitrogen and hydrogen. Fe is iron only. CH₄ contains carbon and hydrogen. O₃ contains only oxygen.

Answer: Elements: H₂, Fe, O₃. Compounds: NH₃, CH₄.

Quick check

1. Which elements are combined in carbon dioxide? Answer: Carbon and oxygen.

Exam focus

A complete definition must include three ideas: two or more different elements, chemically combined (bonded), in a fixed ratio. Particle diagrams are a favourite question type; check whether each particle contains different atoms bonded together.

Advanced insight

More than 200 million different compounds have been recorded by chemists, and thousands of new ones are made every day. Most contain carbon, because carbon atoms can bond to each other in long chains and rings. The study of these carbon compounds forms a whole branch of chemistry called organic chemistry.

Summary

A compound is a pure substance made of two or more different elements chemically combined in a fixed ratio. Examples include water, H₂O, carbon dioxide, CO₂, and sodium chloride, NaCl. Compounds are formed by chemical reactions and can only be broken back into their elements by chemical reactions, not by physical separation methods. Counting the different kinds of atom distinguishes a compound from an element.

Practice questions

1. Define the term compound. Answer: A pure substance made of two or more different elements chemically combined in a fixed ratio. 2. Explain why ozone, O₃, is an element and not a compound. Answer: It contains only one type of atom, oxygen, even though three atoms are bonded together. 3. Name the elements present in calcium carbonate, CaCO₃. Answer: Calcium, carbon and oxygen. 4. Why can hydrogen not be obtained from water by boiling it? Answer: Boiling is a physical change; the water molecules stay intact as steam, so a chemical reaction is needed to separate the elements.