Protons, Neutrons and Electrons

Every atom is built from three kinds of tiny particle: protons, neutrons and electrons. Together they decide what an atom is and how it behaves. The protons and neutrons huddle together in the centre, called the nucleus, while the electrons fly around the outside in fuzzy clouds called shells. Although every atom has the same three kinds of particle, it is the number of each that makes one element different from another.

  • ProtonsPositive charge (+)Live in the nucleus
  • NeutronsNo charge (0)Live in the nucleus
  • ElectronsNegative charge (-)Whizz around the nucleus
  • Proton mass1 atomic unitAround 1,836 times an electron
  • Neutron massApprox. 1 atomic unitSlightly more than a proton
  • Electron mass1/1,836 of a protonAlmost nothing

Protons

Protons sit in the centre of every atom. They have a positive electrical charge (+1) and a mass of 1 atomic mass unit (amu). The number of protons in an atom is the single most important fact about it. It decides what element the atom is, and it is called the atomic number.

  • 1 proton = hydrogen (H)
  • 2 protons = helium (He)
  • 6 protons = carbon (C)
  • 8 protons = oxygen (O)
  • 79 protons = gold (Au)
  • 92 protons = uranium (U)

Change the number of protons and you literally change the element. Adding one proton to a lead atom (82 protons) would turn it into bismuth (83 protons). This is what medieval alchemists tried (and failed) to do when they hoped to turn lead into gold. Modern particle accelerators can actually do it, but they cost millions of pounds to make a few atoms of gold.

Neutrons

Neutrons also live in the nucleus, alongside the protons. They are almost the same size and mass as protons (1 amu) but they have no charge at all. That is why they are called "neutrons" (from "neutral").

Neutrons act like glue. Without them, the positive protons in the nucleus would repel each other (because like charges push apart) and the atom would fly apart. Neutrons help hold the nucleus together using a force called the strong nuclear force.

The number of neutrons in an atom can vary, even within the same element. Atoms of the same element with different numbers of neutrons are called isotopes. For example, carbon usually has 6 neutrons (carbon-12), but some carbon atoms have 7 (carbon-13) or 8 (carbon-14). These are all still carbon, just heavier versions.

Fact The total number of protons + neutrons in an atom is called its mass number. So an atom of carbon with 6 protons and 8 neutrons has a mass number of 14, written as carbon-14. The mass number is roughly equal to the atoms mass in atomic units, because electrons weigh almost nothing.

Electrons

Electrons are the smallest of the three subatomic particles. They have a negative electrical charge (-1) and almost no mass (only 1/1,836 of a proton). They are not in the nucleus. They fly around the outside at almost the speed of light, in fuzzy clouds called shells or orbitals.

Each shell can hold a maximum number of electrons:

  • 1st shell: up to 2 electrons
  • 2nd shell: up to 8 electrons
  • 3rd shell: up to 18 electrons
  • 4th shell: up to 32 electrons

Electrons fill the lowest shells first. Atoms are happiest (most stable) when their outer shell is full. This is the secret of all chemistry: atoms swap, share or steal electrons in order to fill their outer shell. That is exactly how chemical bonds form.

Charge and balance

In a normal, neutral atom, the number of protons equals the number of electrons. The positive charges balance the negative charges and the atom has no overall charge.

If an atom loses or gains an electron, the balance breaks and the atom becomes a charged particle called an ion:

  • Lose an electron (more protons than electrons) and the atom becomes a positive ion (cation).
  • Gain an electron (more electrons than protons) and the atom becomes a negative ion (anion).
Did you know? Electricity is just electrons in motion. When you flip a light switch, you are pushing electrons through metal wires. In your laptop battery, chemical reactions push electrons one way to power the chip. In a thunderstorm, billions of electrons jump suddenly from cloud to ground in a single dazzling spark. The whole modern world runs on understanding electrons.

Where do they live?

To understand the inside of an atom, imagine a tiny, dense ball at the centre (the nucleus, with protons and neutrons), surrounded by mostly empty space, with electrons buzzing around at huge distances.

  • If the atom were the size of a football stadium, the nucleus would be the size of a pea in the centre.
  • The electrons would be like tiny gnats darting around in the back rows.
  • The space in between would be nothing at all.
Try this Draw a chart for the first 10 elements (hydrogen through neon). For each, write the atomic number (= protons), the most common mass number, the neutrons (= mass minus protons) and the electrons (= protons in a neutral atom). You will see a pattern: as the atomic number increases, the number of each particle increases too. Notice how electrons start filling shells: hydrogen 1, helium 2 (first shell full), lithium 2 in shell 1 then 1 in shell 2, and so on.
Deeper dive: are protons and neutrons really the smallest things?

For a long time, scientists thought protons and neutrons were the most basic, indivisible parts of matter. Then, in the 1960s, physicists smashing particles together at very high speeds in the new particle accelerators found something amazing: protons and neutrons themselves are made of even smaller particles called quarks.

Each proton is made of 3 quarks: two "up" quarks and one "down" quark. Each neutron is also made of 3 quarks: two "down" and one "up". The quarks are held together by particles called gluons, which carry the strong nuclear force.

Quarks have never been seen alone. The strong force gets stronger the further apart they pull, like a stretched elastic band, so they can never escape from their proton or neutron. Six different types of quark exist in nature, with strange names: up, down, charm, strange, top and bottom. The protons and neutrons that build our world need only the lightest two (up and down).

Electrons, on the other hand, really do seem to be fundamental. As far as we can tell, they are not made of anything smaller. Modern physics describes them as tiny points of charge with no measurable size. So the next time you switch on a light, you are pushing around some of the most basic building blocks of reality.

For more, see ions and isotopes.