Describe atoms as the building blocks of matter and distinguish between elements, compounds and mixtures with everyday examples

What this dot point is asking

This dot point wants you to know that everything is made of tiny building blocks called atoms, and to tell the difference between three important groups of substances: elements, compounds and mixtures. You should be able to give an everyday example of each and explain in simple words how they are different. The big idea is that atoms are the basic pieces, elements are made of just one kind of atom, compounds are different atoms joined together, and mixtures are different substances simply mixed but not joined.

The answer

Atoms are the building blocks

Everything around you is made of incredibly small pieces called atoms. Atoms are far too small to see, even with an ordinary microscope. They are the basic building blocks of all matter, in the same way that bricks are the building blocks of a wall. There are about a hundred different kinds of atom, and everything in the world is built from these kinds joined and arranged in different ways.

Elements: one kind of atom

An element is a substance made of only one kind of atom. Because it contains just one type of atom, an element cannot be broken down into anything simpler by ordinary means.

Everyday examples of elements include oxygen (the gas you breathe in), gold (a metal used in jewellery), copper (the metal in electrical wires) and iron (used to make steel). Each of these is made of only one kind of atom. All the known elements are listed in a chart called the Periodic Table.

Compounds: different atoms joined together

A compound is a substance made of two or more different elements that are chemically joined together. The word "joined" is important: the atoms are locked together in a chemical bond, not just sitting next to each other.

When elements join to make a compound, the compound usually behaves very differently from the elements it was made from. For example, water is a compound made of hydrogen and oxygen joined together. Hydrogen on its own is a gas that can burn, and oxygen on its own is a gas that helps things burn, but joined together they make water, which puts out fires. Another example is salt (sodium chloride), made of the elements sodium and chlorine joined together.

Mixtures: substances mixed but not joined

A mixture is made of two or more substances that are simply mixed together but not chemically joined. Because they are not joined, the parts of a mixture keep their own properties and can be separated again without a chemical reaction.

Everyday examples include air (a mixture of gases such as oxygen and nitrogen), salty seawater (salt mixed in water), and a fruit salad (different fruits mixed together). In each case you could separate the parts again using a physical method.

The key difference

The simplest way to remember the difference: an element is one kind of atom, a compound is different atoms chemically joined, and a mixture is different substances mixed but not joined. A compound needs a chemical reaction to split it up; a mixture can be separated easily, for example by filtering or evaporating.

Examples in context

Example 1. The label on a drink bottle. Bottled mineral water is mostly the compound water, with small amounts of dissolved minerals mixed in, which makes the whole drink a mixture. The label even lists the different minerals, showing they are mixed in rather than joined into one new substance.

Example 2. Rusting iron. A garden gate made of the element iron slowly turns to rust when it reacts with oxygen and water in the air. Rust is a new compound, with different properties from shiny iron: it is flaky, weak and reddish-brown. This shows how an element can chemically join with others to form a compound.

Try this

• Cue. State whether gold is an element, a compound or a mixture, and explain why. Gold is an element because it is made of only one kind of atom.

• Cue. A fizzy drink is sugar, water and carbon dioxide gas all together. Is it an element, a compound or a mixture? It is a mixture, because the different substances are mixed together but not chemically joined.

• Cue. Explain why water is a compound and not a mixture. Water is a compound because it is two different elements, hydrogen and oxygen, chemically joined together, and it can only be split apart by a chemical reaction.