The short answer
An atom has a small, dense nucleus made of protons and neutrons, surrounded by electrons. Protons and neutrons are not fundamental: each is made of smaller particles called quarks. Electrons, as far as current experiments show, are not made of anything smaller. Together, protons, neutrons, electrons and quarks are the main examples of subatomic particles.
The three familiar particles
| Particle | Electric charge | Approximate mass | Where it is found |
|---|---|---|---|
| Proton | +1 | 1 atomic mass unit | Nucleus |
| Neutron | 0 (neutral) | About the same as a proton, slightly larger | Nucleus |
| Electron | −1 | About 1/1,836 of a proton | Surrounding the nucleus |
The electron is thousands of times lighter than the proton, so nearly all of an atom’s mass is in its nucleus. Charge is measured in units of the charge of a proton, with the electron carrying the same amount of charge but opposite sign. A neutral atom has equal numbers of protons and electrons, so the charges cancel.
Which element is it?
The number of protons, the atomic number, determines what element an atom is. One proton means hydrogen, six means carbon, eight means oxygen. Change the number of protons and you have a different element.
The number of neutrons can vary without changing the element. Carbon atoms with 6, 7 or 8 neutrons are all carbon, but they are different atomic isotopes with slightly different masses, and some are radioactive. The periodic table is arranged by proton count for this reason.
The electrons, especially the outermost ones, decide how an atom interacts with others. Atoms can gain, lose or share electrons, which is the basis of chemical bonding.
How small and how empty
Atoms are tiny: a typical atom is about 10⁻¹⁰ meters across, or one ten-billionth of a meter. A nucleus is far smaller, roughly 100,000 times narrower than the whole atom. If an atom were scaled up to the size of a large stadium, the nucleus would be about the size of a small object near the center, such as a marble.
That does not mean an atom is empty in the sense of containing nothing. The region around the nucleus is occupied by the electrons’ probability distribution, and the electromagnetic forces involved are strong. The common phrase “atoms are mostly empty space” is a rough picture, and matter feels solid because of those forces and because of quantum rules that stop electrons from being squeezed into the same states.
Electrons are not tiny planets
Early models pictured electrons circling the nucleus like planets around the Sun. Modern quantum physics replaces that with orbitals: regions where an electron is likely to be found. Electrons occupy specific energy levels, and an atom can absorb or emit light when an electron moves between them. The exact position of an electron is not defined in the way a planet’s is, and the shape of each orbital comes from a wave description of the electron.
Inside the nucleus: quarks and gluons
In the 1960s, experiments and theory showed that protons and neutrons have internal structure. Each is made of three quarks. Quarks come in several types, called flavors, but ordinary matter uses two:
- The up quark has charge +2/3.
- The down quark has charge −1/3.
A proton is two up quarks and one down quark: 2/3 + 2/3 − 1/3 = +1. A neutron is one up quark and two down quarks: 2/3 − 1/3 − 1/3 = 0. These sums match the known charges.
Quarks are held together by the strong force, carried by particles called gluons. A curious feature is that single quarks are never observed alone under ordinary conditions. Pulling quarks apart produces new particles instead. Interestingly, most of a proton’s mass comes not from the masses of its three quarks, which are small, but from the energy of the strong-force interactions inside it, a consequence of mass-energy equivalence. Physicists accept this account, though the details are technical.
What holds an atom together
Several forces act at once:
- Electromagnetism attracts negatively charged electrons to the positively charged nucleus.
- The strong force binds quarks into protons and neutrons, and a leftover effect of it holds protons and neutrons together in the nucleus, overcoming the electric repulsion between protons.
- The weak force is involved in some forms of radioactive decay, in which a neutron can change into a proton or the reverse.
Gravity acts too, but it is negligible at the scale of a single atom.
Other particles you may hear about
Experiments reveal many more particles than the ones inside ordinary atoms:
- Neutrinos are very light, electrically neutral particles that are produced in some nuclear reactions, including in the Sun, and that rarely interact with matter.
- Antiparticles have the same mass as their counterparts but opposite charge. A positron is the antiparticle of the electron.
- Heavier quarks and electron-like particles appear in cosmic rays and particle accelerators but decay quickly.
The Standard Model organizes all of these, and the best current evidence is that up quarks, down quarks and electrons are the ones that make up the atoms of everyday matter.
What is still open
The Standard Model works well but does not explain everything. It has nothing to say about dark matter, which appears to outweigh ordinary matter in the universe, and it does not include gravity. So the picture of the atom described here is well tested, but it is part of a larger picture that is not complete.