Note · Fundamental concepts
What is an atom? Protons, neutrons and electrons, explained
An atom is the smallest unit of a chemical element: a tiny nucleus of protons and neutrons with electrons around it. What each part does, and how a century of experiments found it.
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Contents · 7 sections
An atom is the smallest unit of a chemical element that still has that element's properties. Each one is a tiny, dense nucleus of protons and neutrons with a cloud of electrons around it. The number of protons decides what the atom is: every atom with 6 protons is carbon, and every atom with 79 is gold. The electrons decide how it behaves with other atoms, and so all of chemistry.
In short
- An atom has three kinds of parts: positive protons and neutral neutrons packed into the nucleus, and negative electrons spread around it.
- The proton count, called the atomic number, fixes the element. Neutrons can vary, which gives isotopes of the same element.
- Electrons are not little planets on orbits. They occupy fuzzy regions called orbitals, and the old planetary picture is wrong in the ways that matter most.
What is inside an atom?
Three particles make up every atom except the simplest form of hydrogen, which has no neutron.
| Particle | Where it is | Charge | Mass (kg) | Mass compared with an electron |
|---|---|---|---|---|
| Proton | Nucleus | +1 | 1.6726 × 10⁻²⁷ | About 1,836 |
| Neutron | Nucleus | 0 | 1.6749 × 10⁻²⁷ | About 1,839 |
| Electron | Around the nucleus | −1 | 9.109 × 10⁻³¹ | 1 |
The charges of a proton and an electron are equal and opposite, as precisely as any experiment can measure, so an atom with as many electrons as protons is electrically neutral. An atom that gains or loses electrons becomes an ion, with an overall charge. Protons and neutrons are themselves made of smaller particles called quarks, but the electron has no known parts at all.
Something has to hold the nucleus together, since its protons are all positive and repel each other. That job falls to the strong nuclear force, which is about a hundred times stronger than the electric repulsion at the distance between neighboring protons but fades to almost nothing beyond the width of a few of them. It is one of the four fundamental forces.
How big is an atom?
Atoms are typically 0.1 to 0.5 nanometers across, a nanometer being a billionth of a meter. A human hair is roughly 70,000 nanometers wide, so hundreds of thousands of atoms would fit side by side across it. The nucleus is far smaller again, around 20,000 to 60,000 times narrower than the atom, depending on the element.
Picture an atom blown up to the size of a sports stadium: its nucleus would be about the size of a pea in the middle, and yet it holds more than 99.9% of the atom's mass. The electrons fill the rest of the stadium, not as specks flying around in a vacuum, but as a spread-out cloud. That cloud is what gives an atom its size and what touches another atom when two of them meet.
A worked example: does a carbon atom weigh what its parts weigh?
Carbon-12, the most common form of carbon, has 6 protons, 6 neutrons and 6 electrons. Its atomic number is 6 (the protons) and its mass number is 12 (protons plus neutrons). Add up the parts, using the atomic mass unit u, which is defined as one twelfth of a carbon-12 atom's mass:
- Six protons: 6 × 1.007276 u = 6.043656 u.
- Six neutrons: 6 × 1.008665 u = 6.051990 u.
- Six electrons: 6 × 0.000549 u = 0.003294 u.
- Total for the separate parts: 12.098940 u.
- A carbon-12 atom, by definition, weighs exactly 12 u.
The atom is about 0.099 u, or 0.8%, lighter than its parts. That missing mass is the energy that was released when the nucleus formed. Using E = mc², with 1 u equal to about 931.5 million electronvolts (MeV) of energy, 0.099 u is about 92 MeV, the binding energy of carbon-12. To pull the nucleus apart, you would have to put those 92 MeV back. Differences of this kind, between how tightly different nuclei are bound, are the energy source of both fission reactors and the Sun.
The same arithmetic explains isotopes. Carbon-14 has 6 protons, so it is still carbon, but 8 neutrons. That combination is unstable, and carbon-14 slowly decays into nitrogen, which is the basis of radiocarbon dating and a subject for the radioactivity note.
How did scientists work out what atoms are like?
Nobody has seen inside an atom directly. Each model below was forced by an experiment the one before it couldn't explain.
| When | Model | What changed it |
|---|---|---|
| About 400 BCE | Matter is made of uncuttable pieces, atomos | Greek philosophers Leucippus and Democritus, by argument rather than experiment |
| 1803–1808 | Each element is made of identical atoms that combine in fixed whole-number ratios | John Dalton, from the weights of elements in chemical compounds |
| 1897–1904 | Atoms contain tiny negative electrons, set in a spread of positive charge, the "plum pudding" | J. J. Thomson discovered the electron with cathode rays in 1897 |
| 1909–1911 | Nearly all the mass and positive charge sit in a tiny central nucleus | Hans Geiger and Ernest Marsden saw about 1 in 8,000 alpha particles bounce back off metal; Ernest Rutherford explained it in May 1911 |
| 1913 | Electrons circle the nucleus only on certain allowed orbits, with fixed energies | Niels Bohr, to explain the exact colors of light hydrogen gives off |
| 1926 | Electrons are waves described by probabilities, filling orbitals rather than orbits | Erwin Schrödinger's wave equation and the rest of [[what-is-quantum-mechanics |
| 1932 | The nucleus also holds neutral neutrons | James Chadwick |
Rutherford later said the result was "as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you," as the American Institute of Physics' account of the discovery records. If the atom's positive charge were spread thin, as Thomson's model had it, nothing could have rebounded. Only a tiny, heavy, concentrated center could.
Where does the planetary picture of the atom go wrong?
The familiar image, electrons circling a nucleus like planets around a Sun, is Bohr's 1913 model, and it is still on many logos. It got one big thing right: electrons in an atom can only have certain energies, and jumping between them gives off or absorbs light of exact colors. That earned Bohr the 1922 Nobel Prize.
It is wrong about almost everything else. An electron moving in a circle should radiate light and spiral into the nucleus within a tiny fraction of a second, yet atoms are stable. The model also fails for any atom with more than one electron. In the quantum picture, an electron has no path at all. It has an orbital, a three-dimensional region describing where it is likely to be found, and some orbitals are spheres while others are shaped like dumbbells or clover leaves. The planetary picture is a useful first step, not a description of a real atom.
Why do atoms matter for everything else?
The number of protons sets the element, and the arrangement of electrons, especially the outermost ones, sets how it reacts. That is why the periodic table can group elements by behavior. When atoms share or swap outer electrons, they bind into molecules, and everything from water to DNA is built that way. The nucleus matters in its own right: splitting heavy nuclei powers nuclear reactors, and fusing light ones powers the stars.
Check yourself
- Without looking back, explain why the number of protons, not neutrons or electrons, decides which element an atom is.
- Why does a carbon-12 atom weigh less than its six protons, six neutrons and six electrons added together?
- What did the gold-foil experiment show that the plum-pudding model could not explain?
Sources
- Rutherford's Nuclear World: Alpha particles and the atom — American Institute of Physics
- Niels Bohr, Nobel Prize in Physics 1922 — Nobel Prize Outreach
- James Chadwick, Nobel Prize in Physics 1935 — Nobel Prize Outreach
- Proton mass — NIST, CODATA 2022
- Geiger–Marsden experiments — Wikipedia
- Atom — Wikipedia