Element

An element is a substance made up of only one kind of atom, which cannot be broken down into simpler substances by chemical means. Malay: unsur · Chinese: 元素.

What makes a substance an element is that every atom in it has the same proton number, the same number of protons in the nucleus. Each element has its own chemical symbol and its own place in the Periodic Table, arranged in order of increasing proton number.

Example. Oxygen (O), carbon (C) and copper (Cu) are elements. A sample of copper wire contains only copper atoms, so no ordinary chemical reaction can turn it into something simpler. Oxygen gas is still an element even though it exists as O₂ molecules, because both atoms in each molecule are the same kind of atom.

Confusion to avoid. Do not confuse an element with a compound or a mixture. A compound contains two or more different elements chemically combined in a fixed ratio, such as water (H₂O) or carbon dioxide (CO₂), and it can be broken down by chemical means into its elements. A mixture contains different substances that are only physically together and can be separated by physical methods. The single-kind-of-atom test is what marks an element apart: one type of atom means an element, two or more chemically joined means a compound.

In the exam you may be asked to classify a given substance as an element, a compound or a mixture, and the answer always comes back to this definition. Metals such as copper and iron, and non-metals such as oxygen and carbon, are all elements, because each is built from a single kind of atom.

Elements are the building blocks of the whole Matter and the Atomic Structure chapter and of the Periodic Table that follows it. Because the proton number defines the element, the moment you know an atom’s proton number you know exactly which element it is, a link students in our online lessons practise until it is automatic.