Atoms and Molecules

All the matter in the Universe that we know of is made of tiny, invisible particles called atoms. Atoms are crammed into everything you can see - from the skin on your fingers to the words in this book - and things you can’t see too, such as the air you breathe and the cells rushing through your blood.

The idea of atoms is one of the oldest in science. Some Ancient Greek philosophers thought atoms were the smallest possible bits of matter and couldn't be broken down further. They chose the name atom because it means "uncuttable" or "indivisible." This early concept of atoms lived on for thousands of years, until, in the early 20th century, ingenious scientists "split the atom," smashing it apart into even tinier particles.

When atoms combine, they make bigger clumps called molecules. Although there are only about 100 types of atoms, together they can form millions of different types of molecules.

The structure of an atom

Inside an atom are even tinier particles of matter called protons, neutrons, and electrons. Protons and neutrons are found in the center of an atom, which is called the nucleus, and are locked tightly together by powerful forces. Electrons spin up, down, and around outside the nucleus within areas called electron shells.

The nucleus

A cluster of protons and neutrons make up the nucleus of an atom. It makes up most of an atom’s weight, but only takes up a tiny amount of its volume. Outside the nucleus, more than 99.99 percent of an atom is just empty space. If an atom were the size of a football stadium, the nucleus would be the size of a pea in the center of it, and the electrons would be zooming around the outer stands.


Protons have a positive electric charge, and are attracted to electrons.


These are around the same size as protons. Neutrons are neutral, which means they don’t have an electric charge and aren’t attracted to electrons or protons.


These particles are about 2,000 times lighter than protons and neutrons, and orbit the nucleus at almost the speed of light. They have a negative electric charge.

Electron shells

An atom usually has an equal number of negatively charged electrons and positively charged protons. Some big atoms have more than 100 of both. The carbon atom shown here has six of each. Electrons are arranged in layers (called electron shells) around the nucleus, like satellites around a planet. Bigger atoms have more shells than smaller ones. Imagining electrons in shells helps us to understand how atoms join together to form molecules.

Making molecules

Atoms can join together - or bond - to make molecules. A molecule can be made of the same atoms or different ones. Gases such as hydrogen have simple molecules made of just two atoms, while plastics can be made from endlessly repeating molecules made up of thousands of atoms joined together in very long lines.

Water (H2O)

A molecule of water is made from two hydrogen atoms joined to one oxygen atom.

Carbon dioxide (CO2)

A carbon dioxide molecule is made from two atoms of oxygen joined to one atom of carbon.

How atoms bond

Atoms bind together to make molecules using their electrons, which they give, take, or share with each other. Three main types of bonds hold atoms together.

Ionic bond

One atom gives electrons to another. The first atom becomes positively charged and the second atom becomes negatively charged, so the atoms attract and lock.

Covalent bond

Two atoms share their outer electrons by making their outer electron shells overlap.

Metallic bond

Metals bond into what’s called a crystal lattice by sharing their outer electrons in a giant cloud.