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"Subatomic" means anything smaller than or inside an atom.
Electrons, Protons, and Neutrons
The electron was the first subatomic particle discovered (J.J. Thomson, 1897). It is fundamental — it has no internal structure. Electrons orbit the nucleus in quantum orbitals (probability clouds, not fixed paths). They are stable and carry negative charge.
The proton was identified by Ernest Rutherford (1917-1920). It lives in the nucleus and carries positive charge. It is not fundamental — it is made of two up quarks and one down quark (uud). As far as we’ve observed, protons do not decay.
The neutron was discovered by James Chadwick (1932). It also lives in the nucleus but carries no charge. Like the proton, it is composite — one up quark and two down quarks (udd). A free neutron is unstable and decays in about 14.6 minutes, but inside a nucleus it is stable.
Properties
| Property | Electron | Proton | Neutron |
|---|---|---|---|
Mass (kg) |
9.109 × 10⁻³¹ |
1.673 × 10⁻²⁷ |
1.675 × 10⁻²⁷ |
Mass (amu) |
0.00055 |
1.00728 |
1.00866 |
Charge |
-1 |
+1 |
0 |
Spin |
½ |
½ |
½ |
Location |
Orbitals around nucleus |
Nucleus |
Nucleus |
Fundamental? |
Yes |
No (uud) |
No (udd) |
Stable? |
Yes |
Yes (lifetime > 10³⁴ years) |
Only inside nuclei |
Charge radius |
Point-like (< 10⁻¹⁸ m) |
0.842 fm |
0.84 fm |
1 femtometer (fm) = 10⁻¹⁵ meters.
Fundamental vs Composite Particles
A fundamental particle has no internal structure — it cannot be broken into smaller components. The Standard Model lists 17 fundamental particles: 6 quarks, 6 leptons, 4 gauge bosons, and the Higgs boson.
A composite particle is built from fundamental particles bound together:
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Proton = up + up + down quarks (uud)
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Neutron = up + down + down quarks (udd)
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Pion = 1 quark + 1 antiquark
How do we know protons aren’t fundamental? Deep inelastic scattering experiments at SLAC (Stanford Linear Accelerator Center), 1967-1973. Jerome Friedman, Henry Kendall, and Richard Taylor fired high-energy electrons at protons and found them scattering off point-like objects inside — initially called "partons," later identified as quarks. Nobel Prize in Physics: 1990.
The Mass Puzzle
The three quarks inside a proton have a combined mass of roughly 9.4 MeV/c². But the proton’s total mass is 938.3 MeV/c². The quarks account for only about 1% of the mass.
Where does the other 99% come from? The energy of the gluon field binding the quarks together, and the kinetic energy of quarks and gluons. Mass from pure energy — \(E = mc^2\) in action. This was confirmed by lattice QCD calculations (Budapest-Marseille-Wuppertal collaboration, published in Science, 2008).
How They Combine to Make Atoms
The nucleus (protons + neutrons) is held together by the residual strong force — a spillover effect of the strong force between quarks. This force operates at femtometer scales (~1-3 fm) and overpowers the electromagnetic repulsion between protons at those distances.
Electrons are bound to the nucleus by electromagnetic attraction. They occupy quantum orbitals — probability clouds, not fixed orbits.
Key definitions:
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Atomic number (Z) = number of protons. This defines the element.
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Mass number (A) = protons + neutrons.
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Isotopes = same number of protons, different number of neutrons (e.g., carbon-12 has 6 neutrons, carbon-14 has 8).
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Ions = atoms that have gained or lost electrons.
Scale
The proton radius is ~0.84 fm. The hydrogen atom’s electron cloud extends to roughly 53,000 fm (the Bohr radius, 5.29 × 10⁻¹¹ m). That is a ratio of about 63,000:1.
If the nucleus were a marble (1 cm), the electron cloud would extend about 630 meters — roughly six football fields. The atom is overwhelmingly empty space.
Sources
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Particle Data Group, Review of Particle Physics, https://pdg.lbl.gov. Source of all numerical values used here — masses, charges, charge radii, lifetimes.
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National Institute of Standards and Technology, CODATA Recommended Values of the Fundamental Physical Constants, https://physics.nist.gov/cuu/Constants. The reference for electron mass, proton mass, and the Bohr radius.
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David Griffiths, Introduction to Elementary Particles, 2nd ed. (Wiley-VCH, 2008). Undergraduate textbook covering all of this material in depth.
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Discovery of the electron: J.J. Thomson, "Cathode Rays," Philosophical Magazine 44, 293 (1897).
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Discovery of the neutron: James Chadwick, "The Existence of a Neutron," Proceedings of the Royal Society A 136, 692 (1932).
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Quark structure of the proton: M. Breidenbach et al., "Observed Behavior of Highly Inelastic Electron-Proton Scattering," Physical Review Letters 23, 935 (1969). Friedman, Kendall, and Taylor’s Nobel Lectures (1990) tell the same story for non-specialists.
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Mass from the gluon field (the 99% puzzle): S. Dürr et al. (Budapest-Marseille-Wuppertal collaboration), "Ab Initio Determination of Light Hadron Masses," Science 322, 1224 (2008). The lattice QCD calculation that confirmed the proton’s mass really does come from the energy of the strong field, not the bare quark masses.
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Free neutron lifetime (~877 s, ~14.6 minutes): Particle Data Group world average. UCNτ Collaboration, "Improved neutron lifetime measurement with UCNτ," Physical Review Letters 127, 162501 (2021).
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Proton stability bound: Super-Kamiokande Collaboration, "Search for proton decay via p → e⁺π⁰ and p → μ⁺π⁰…," Physical Review D 102, 112011 (2020). Current lower bound on proton lifetime: > 2.4 × 10³⁴ years.
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Bohr radius and atomic scale: Niels Bohr, "On the Constitution of Atoms and Molecules," Philosophical Magazine 26 (1913). The original derivation, still the cleanest path to the number.
