Atoms and Elements

Understand atoms as the building blocks of matter, and tell elements, compounds and mixtures apart.

About 3 min read
  • Describe what an atom is
  • Explain what an element is
  • Use chemical symbols
  • Tell elements and compounds apart
Before you start

You know that all matter is made of tiny particles. Now you will find out what those particles are made of.

Atoms

Everything around you is built from atoms. Atoms are so small that millions could fit across the width of a human hair. There are about 118 known types of atom. All atoms of the same element have the same number of protons, and this gives the element its identity.

Definition: atom

The smallest particle of an element that can take part in a chemical reaction.

Elements and symbols

An element contains only one type of atom. Each element has a symbol of one or two letters. The first letter is always a capital, and the second is lower case. For example, O is oxygen, Fe is iron and Na is sodium. The periodic table lists all the elements.

Definition: element

A substance made of only one type of atom.

Compounds

A compound contains atoms of two or more elements chemically bonded together. Water, H₂O, has two hydrogen atoms bonded to one oxygen atom. A compound can have properties that are very different from the elements in it.

Read a chemical formula

How many atoms of each element are in one unit of CO₂?

C has no small number after it, so there is 1 carbon atom.

The small 2 follows O, so there are 2 oxygen atoms.

Answer: 1 carbon atom and 2 oxygen atoms, which is 3 atoms in total.

Write symbols carefully

Co is cobalt, but CO is carbon monoxide. Capital letters change the meaning.

Exam focus
Say it like this“A compound contains two or more elements chemically bonded together.”
Not like this“A compound is lots of things mixed.” — Say chemically bonded, because a mixture is not bonded.
Common mistakeConfusing a compound with a mixture. Only the atoms in a compound are chemically bonded.

Practice

What is the symbol for sodium?
Na.
Is oxygen gas, O₂, an element or a compound?
An element, because it contains only oxygen atoms.
How many atoms are in one unit of CH₄?
5 atoms: 1 carbon and 4 hydrogen.
Going further
Iron and sulfur are heated to form iron sulfide. Explain why this is a compound.
The iron and sulfur atoms have chemically bonded to form a new substance with different properties.

Quick check

Atoms and elements quick check

What does an element contain?

Its atoms all have the same proton number, so nothing else is present.

Which is a compound?

Each unit has hydrogen and oxygen atoms bonded together, so it contains two elements.

Which is the correct symbol for iron?

The symbol comes from ferrum, the Latin name for this metal.
Key takeaways
  • Atoms are the building blocks of matter.
  • An element contains one type of atom.
  • A compound contains bonded atoms of different elements.
  • Chemical symbols must be written exactly.
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