What is an element in chemistry?
An element is a pure substance made of only one kind of atom. It cannot be broken down into anything simpler by a chemical reaction. Gold, oxygen and carbon are elements, and each of the 118 known elements has its own box on the periodic table.
Everything is made of atoms
Everything around you, from your desk to the air you breathe, is made of tiny particles called atoms. They are far too small to see, even with an ordinary microscope.
Atoms come in different kinds. An element is a substance whose atoms are all the same kind. A bar of pure gold contains only gold atoms, and a balloon filled with pure helium contains only helium atoms.
So what makes two atoms the same kind? It is the number of protons, the tiny particles with a positive charge in the center of every atom. Every carbon atom has 6 protons and every oxygen atom has 8. This number is called the atomic number, and no two elements share it.
Elements are the simplest substances
You can melt, freeze, cut or grind an element, and it is still the same element. Melted gold is still gold. No chemical reaction can split an element into simpler substances.
Water is different. Using electricity, chemists can split water into two gases, hydrogen and oxygen. Because water can be broken down like this, it is not an element. Hydrogen and oxygen cannot be split any further, so they are elements.
Compounds: elements joined together
A compound forms when atoms of two or more different elements join together with chemical bonds. Water is a compound of hydrogen and oxygen. Its formula, H₂O, shows that it always has two hydrogen atoms for every oxygen atom.
The recipe of a compound never changes. Pure water from a glacier and pure water from a kettle have exactly the same two-to-one ratio.
A compound can be completely different from the elements that make it. Sodium is a soft, shiny metal that fizzes and darts around on water, and chlorine is a poisonous yellow-green gas. Joined together, they make sodium chloride (NaCl), the table salt you sprinkle on food.
Mixtures: mixed but not joined
In a mixture, two or more substances are mixed together but not chemically bonded. Each one keeps its own properties, and the amounts can change. You can stir a little sugar or a lot of sugar into tea, and you still have sweet tea.
Because nothing is bonded, mixtures can be separated by simple physical methods. A magnet pulls iron filings out of sand, a filter catches sand from water, and boiling away the water from salt water leaves the salt behind.
Air is a mixture too. It is mostly nitrogen and oxygen, which are both elements, plus small amounts of argon, carbon dioxide and water vapor.
How to tell them apart
Ask three questions. Is there only one kind of atom? Then it is an element. Are different elements bonded together in a fixed ratio? Then it is a compound. Are different substances simply mixed and easy to separate? Then it is a mixture.
Watch out for oxygen gas, O₂. Its atoms travel in pairs, but both atoms in each pair are oxygen, so oxygen gas is still an element. Carbon dioxide, CO₂, contains two different elements bonded together, so it is a compound.
Of the 118 known elements, about 94 are found naturally on Earth, some only in tiny traces. Scientists made the rest in laboratories. The four newest, including oganesson (118), were named in 2016.
Key points
- An element is a pure substance made of only one kind of atom.
- The number of protons in an atom, its atomic number, decides which element it is.
- A compound is two or more elements chemically bonded in a fixed ratio, such as water (H₂O).
- A mixture is two or more substances mixed but not bonded, such as air or salt water, and it can be separated by physical methods.
- There are 118 known elements, and about 94 of them occur naturally on Earth.
Check yourself
Three quick questions on this lesson.
1. Which of these is an element?
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Oxygen — Oxygen is made of only one kind of atom, while water and table salt are compounds and air is a mixture.2. What makes a substance a compound rather than a mixture?
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Its elements are chemically bonded in a fixed ratio — In a compound, atoms of different elements are bonded in a fixed ratio, while the parts of a mixture are only mixed together.3. What decides which element an atom belongs to?