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Everything you can touch, taste, smell or see is made of atoms: the air you breathe, the water you drink, the screen you’re reading this on, and you. An atom is the smallest unit of an element that still has that element’s chemical identity. Cut a piece of gold in half again and again, and eventually you’d reach a single gold atom. Split that atom, and what’s left is no longer gold.
This guide explains what atoms are, what’s inside them, and why such tiny things explain so much about the world.
Where the word comes from
The word atom comes from the ancient Greek atomos, meaning “uncuttable”. Around 2,400 years ago, the philosophers Leucippus and Democritus argued that matter couldn’t be divided forever; eventually you’d reach tiny, indivisible particles. It was a brilliant guess, but it wasn’t based on experiments, and for about two thousand years most thinkers preferred other ideas.
The modern atomic theory began with John Dalton in the early 1800s, who used careful measurements of how elements combine to argue that each element is made of its own kind of atom. Later experiments showed that atoms aren’t actually uncuttable: they contain even smaller particles. The name stuck anyway. See the history of atomic models.
What’s inside an atom?
An atom has two main regions:
- A tiny, dense nucleus at the centre, containing protons and neutrons.
- A much larger surrounding region where electrons are found.
| Particle | Where it is | Relative charge | Relative mass |
|---|---|---|---|
| Proton | nucleus | +1 | 1 |
| Neutron | nucleus | 0 | 1 |
| Electron | around the nucleus | −1 | about 1/1836 (almost 0) |
A few things stand out:
- Almost all the mass is in the nucleus, because protons and neutrons are each about 1,836 times heavier than an electron.
- Protons and electrons have equal and opposite charges. In a neutral atom, the number of electrons equals the number of protons, so the charges cancel.
- Neutrons have no charge, but they add mass and help hold the nucleus together.
For a deeper look at the three particles, see protons, neutrons and electrons.
Atoms are mostly empty space
If an atom were enlarged to the size of a football stadium, the nucleus would be about the size of a small pea in the centre, and the electrons would be spread thinly through the rest of the stadium. The nucleus is roughly 100,000 times smaller than the atom as a whole.
So why doesn’t your hand pass straight through a table? Because the electrons in the atoms of your hand and the table repel each other strongly. It’s electrical repulsion, not solid “stuff”, that makes objects feel solid. See how small is an atom? for more mind-bending comparisons.
Where are the electrons?
Early models pictured electrons orbiting the nucleus like planets around the Sun. Modern quantum theory gives a different picture: electrons don’t follow neat paths. Instead, we can only describe regions where an electron is likely to be found, called orbitals.
For many purposes, a simpler model works well: electrons are arranged in shells (energy levels) at increasing distances from the nucleus.
- The first shell holds up to 2 electrons.
- The second holds up to 8.
- The third can hold up to 18, though in the first 20 elements it’s filled with 8 before the fourth shell starts.
The electrons in the outermost shell (valence electrons) largely decide how an atom reacts. See how to find valence electrons and electron configuration rules.
What makes one element different from another?
The number of protons in the nucleus defines the element. This number is the atomic number:
- 1 proton → hydrogen
- 6 protons → carbon
- 8 protons → oxygen
- 79 protons → gold
Change the number of protons and you change the element. That’s why the periodic table is arranged in order of atomic number.
The number of neutrons can vary slightly between atoms of the same element, giving different isotopes (for example, carbon-12 and carbon-14). The number of electrons can change when atoms gain or lose electrons to form ions. Neither of these changes the element. See isotopes, ions, isomers and allotropes and what is an ion?
How atoms make everything
There are 118 known elements, but millions of different substances. That’s because atoms join together:
- Molecules form when atoms share electrons in covalent bonds. A water molecule is two hydrogen atoms bonded to one oxygen atom (H₂O).
- Ionic compounds form when atoms transfer electrons, making positive and negative ions that attract each other, as in sodium chloride (NaCl), table salt.
- Metals are made of atoms that share a “sea” of electrons.
See ionic vs covalent bonds and metallic bonding.
In a chemical reaction, atoms are rearranged into new combinations. They aren’t created or destroyed, which is why chemical equations must balance. See how to balance chemical equations.
How do we know atoms exist?
Atoms are far too small to see with ordinary light microscopes, yet the evidence for them is overwhelming:
- Chemical laws: elements always combine in fixed ratios by mass, which is exactly what you’d expect if they’re made of atoms of fixed mass.
- Brownian motion: tiny pollen grains in water jiggle randomly because they’re bombarded by moving water molecules. In 1905, Albert Einstein explained this mathematically, and experiments by Jean Perrin confirmed it, allowing the size of molecules to be estimated.
- X-ray crystallography reveals the regular arrangement of atoms in crystals.
- Scanning tunnelling and atomic force microscopes can now produce images of individual atoms on surfaces, and even move them one by one.
How many atoms are there?
A single drop of water contains roughly 1.7 × 10²¹ molecules, and three times as many atoms. Chemists count such enormous numbers using the mole: one mole contains about 6.022 × 10²³ particles. See Avogadro’s number.
Common misconceptions
- “Atoms are alive” or “atoms are tiny cells.” Cells are made of trillions of atoms; atoms themselves aren’t alive.
- “Atoms of a substance have its properties.” A single copper atom isn’t orange or shiny, and a water molecule isn’t wet. Colour, shininess, wetness and hardness come from huge numbers of atoms interacting.
- “Atoms expand when things get hot.” Substances expand because their particles move faster and further apart, not because each atom gets bigger.
- “Electrons orbit like planets.” It’s a useful early picture, but electrons are better described by orbitals, regions of probability.
Key takeaways
- An atom is the smallest unit of an element that keeps its chemical identity.
- Atoms contain a tiny, dense nucleus (protons and neutrons) surrounded by electrons.
- The number of protons (atomic number) defines the element; neutrons give isotopes; gaining or losing electrons makes ions.
- Atoms are mostly empty space; electron repulsion makes objects feel solid.
- Atoms combine by bonding to form every substance, and are rearranged, not destroyed, in chemical reactions.
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