The smallest bits of matter with chemical and physical properties

What are atoms?

Atoms are the building blocks of ordinary matter.

The term “atom” comes from the Greek word for indivisible because we once believed atoms were the smallest things in the universe and could not be divided into smaller parts.

In fact, our ideas on atoms have changed a lot over the years as scientists discovered more and more about the atom.

We now know an atom is made up of three particles (called subatomic particles): the proton, neutron and electron.

Protons and neutrons make up the centre of the atom, called the nucleus. Together, we call protons and neutrons: nucleons. They both have approximately the same mass (1.67 x 10-27 kg or 0.00000000000000000000000000167 kg).

Electrons move around the nucleus in a small “cloud”. The term cloud can be a bit misleading, we just mean the electrons “hover” around the nucleus. Electrons are only about 1/1800 the mass of a nucleon (proton or neutron).

Electrons carry a negative charge. Protons carry a positive charge.

In a normal (electrically neutral) atom, the number of protons and electrons are the same.


We group atoms into elements. All atoms in a specific element have the same chemical and physical properties.

Atoms of the same element always have the same number of protons. So, each element is assigned an atomic number equal to the number of protons in its nucleus.

The atoms are listed in order of atomic number on the Periodic Table of elements.

While atoms are made up of protons, neutrons and electrons, most of the volume of an atom (more than 99%) is actually empty space.

So, why don’t solid objects just pass through one another? Because the electron clouds repel one another, resulting in the impression of solid matter.

So what?

Atoms explain all the properties of ordinary matter. We cannot understand physics or chemistry without atoms.

Atoms undergo amazing changes, decaying into smaller atoms or combining to form an infinite variety of more complicated structures.

By studying atoms in this way, we can harness the energy of nuclear reactions, better understand the world around us and improve our technologies.

Atoms are critically important in chemical reactions. They cannot be destroyed (or created) in chemical reactions and this means we can make all sorts of important predictions.

What else?

In 2013, an excited hydrogen atom was directly observed for the first time. This was achieved using a quantum microscope – a special device that allows scientists to observe things on the atomic scale.

Want more?

Richard Feymann on why atoms may be one of the most important scientific ideas of all time:

Brick by Brick

Atomic and subatomic physics explained in LEGO®️.


Meeting the particle accelerators unlocking the atom.


Jim Al-Khalili presents this series on the atom.

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