What Is an Atom?

An atom is the smallest unit of an ordinary chemical element. Almost everything you can see, touch, taste or smell is made of atoms: your body, the chair you are sitting on, the air you breathe, the stars in the sky. Atoms are unbelievably tiny. A single drop of water contains about 1.5 sextillion atoms (that is 1.5 followed by 21 zeros). For thousands of years people wondered what would happen if you kept cutting something in half. The ancient Greek thinker Democritus guessed (correctly) that eventually you would reach a piece so small that it could not be cut anymore. He called it atomos, meaning "uncuttable". That is where the word atom comes from.

  • What it isSmallest unit of an elementBuilding block of all matter
  • SizeApprox. 0.1 nanometres1 million times thinner than a hair
  • PartsProtons, neutrons, electronsPlus mostly empty space
  • Atoms in a drop of water1.5 sextillion1.5 followed by 21 zeros
  • Different elements118 knownEach has a unique atom type
  • Word originGreek "atomos"Meaning "uncuttable"

What is inside an atom?

Even though atoms are the smallest piece of an element, they are themselves made of even smaller things called subatomic particles. There are three main types:

  • Protons: positively charged particles found in the centre (nucleus) of the atom. The number of protons tells you what element the atom is.
  • Neutrons: particles with no charge, also found in the nucleus. They add mass and help hold the nucleus together.
  • Electrons: tiny, negatively charged particles that whizz around the nucleus in fuzzy clouds called shells or orbitals.

If an atom were the size of a football stadium, the nucleus would be the size of a pea sitting in the middle. The electrons would be like tiny gnats buzzing around in the stands. The rest is empty space. That is right: atoms (and so everything made of them, including you) are mostly nothing at all.

Fact If you removed all the empty space from the atoms in every human on Earth, the entire human race would fit inside a sugar cube. The reason solid things feel solid is not because atoms touch each other but because their electron clouds repel one another with electrical force.

How big is an atom?

Atoms are so small that they are very hard to imagine. A typical atom is about 0.1 nanometres across. A nanometre is one billionth of a metre. To get a feeling for that:

  • A human hair is about 1 million atoms thick.
  • If you lined up 10 million atoms in a row, the line would be about 1 millimetre long.
  • A pinhead contains about 5 thousand million million million atoms (5 followed by 21 zeros).

No light microscope can ever see an atom directly. Visible light has wavelengths much bigger than atoms, so it just rolls over them like sea waves over a pebble. Scientists need very special instruments called scanning tunnelling microscopes to "feel" the shapes of individual atoms by detecting electrical effects.

Atoms and elements

Every type of element is made from one specific kind of atom. The element hydrogen is made from hydrogen atoms, which have 1 proton. The element helium is made from helium atoms, which have 2 protons. Carbon has 6 protons. Gold has 79. The number of protons is called the atomic number, and it is the single most important fact about any atom. Change the number of protons and you change the element.

There are 118 known elements, each with its own type of atom and its own properties. They are listed in order of atomic number on the periodic table. The first 94 occur naturally on Earth; the rest have only been made in special labs and are highly radioactive.

Did you know? Every atom in your body was once cooked inside a star. The hydrogen in your water came from just after the Big Bang nearly 14 billion years ago. The carbon, oxygen and nitrogen that make up most of your tissues were forged in stars billions of years ago, blown into space when those stars died, and eventually became part of Earth and life. You are, quite literally, made of stardust.

Joining atoms together

Atoms rarely sit alone. Most of the time they join with other atoms to form molecules. They join using chemical bonds, which are like invisible springs made of electrical force. When atoms bond:

  • Two hydrogen atoms join with one oxygen atom to make water (H2O).
  • One carbon atom joins with two oxygen atoms to make carbon dioxide (CO2).
  • Six carbon atoms link in a ring with attached hydrogens to start building sugar and plastics.

The number of possible combinations is almost endless. From only around 90 useful elements, nature builds millions of different substances by combining atoms in different ways.

Try this Build atoms out of beads or playdough at home. Use one colour for protons, a different colour for neutrons, and tiny beads for electrons. Build a hydrogen atom (1 proton, 0 neutrons, 1 electron). Build a helium atom (2 protons, 2 neutrons, 2 electrons). Then try carbon (6, 6, 6) and oxygen (8, 8, 8). Notice how the nucleus gets bigger but always stays much smaller than the orbital paths the electrons fly around in.
Deeper dive: how we discovered the atom

The idea of the atom is over 2,400 years old, but for most of history people had no way to test it. The first experimental evidence appeared in 1803 when British scientist John Dalton showed that chemicals always combine in fixed ratios (a clue that they were made of indivisible units). For a century, scientists thought atoms were tiny solid balls.

Then in 1897, JJ Thomson discovered the electron, the first known subatomic particle, by studying glowing rays inside vacuum tubes. Suddenly atoms had to have an inside, not just an outside.

In 1911, Ernest Rutherford ran a famous experiment in which he fired tiny positively charged particles at a thin sheet of gold foil. Most went straight through (suggesting atoms were mostly empty), but a few bounced back at sharp angles. Rutherford concluded the atom must have a tiny, dense, positively charged centre. He called it the nucleus.

In 1913, Danish scientist Niels Bohr showed that electrons could only orbit at certain fixed distances, like steps on a ladder. This explained the strange rainbow patterns of light given off by glowing gases.

Finally, in the 1920s, the new theory of quantum mechanics revealed that electrons are not tiny solid balls at all but fuzzy clouds of probability that surround the nucleus. The picture of the atom you see in textbooks today (a small nucleus surrounded by orbiting electrons) is a useful simplification of something much stranger and more beautiful underneath.

For more, see protons, neutrons and electrons and what is a molecule.