Building Atoms from the Standard Model
Also known as: Quark composition of matter · Atom builder
Everything you've ever touched is three particles: up quarks, down quarks, and electrons. Two ups and a down make a proton (+1); one up and two downs make a neutron (0). The proton count Z picks the element, the neutron count N picks the isotope, and the electron count sets the charge. Stray too far from the stable Z–N balance and the weak force flips a quark — beta decay — to restore it.
Build real atoms from the Standard Model: dial protons (uud), neutrons (udd), and electrons, and the simulation assembles the element, isotope, and ion — with live quark counts, shell filling, and a stability verdict (stray off the valley of stability and it tells you exactly which beta decay fixes it). Flip the lepton slider to swap every electron for a muon and watch the whole atom shrink into an exotic muonic atom.
Equivalent forms
The periodic table, all of chemistry, and you — built from a three-letter alphabet: u, d, e.
Murray Gell-Mann and George Zweig independently proposed in 1964 that protons and neutrons are made of fractionally-charged 'quarks' — an idea so strange Gell-Mann himself hedged on whether they were 'real'. Five years later, electrons fired down the two-mile SLAC accelerator bounced off hard, point-like lumps inside the proton, exactly as Rutherford's alphas had revealed the nucleus in 1911. The lumps were quarks. The Standard Model has passed every test since, capped by the Higgs boson in 2012.
- Isotope dating: carbon-14 (6p, 8n) is neutron-rich -decays with a 5,730-year half-life — the clock of archaeology.
- Muonic atoms: swap the electron for a muon heavier) and the orbit shrinks — used to measure the proton's radius and to catalyze fusion.
- PET scans decay of fluorine-18 — a proton turning into a neutron inside your body, emitting a positron.
- Heavy-ion facilities (FRIB, CERN-ISOLDE) build never-seen isotopes nucleon by nucleon to map the limits of existence.
- Antimatter atoms: CERN traps antihydrogen (antiproton + positron = ūūd̄ + to test whether antimatter falls the same way.
- “Protons are fundamental.” — They are bags of quarks and gluons; in fact % of the proton's mass is gluon-field energy, not quark mass action).
- “Atoms are mostly matter.” — They are mostly empty: if the nucleus were a marble, the electron cloud would span a stadium.
- “You can pull a quark out.” — Confinement: stretching the gluon field costs so much energy it snaps into new quark–antiquark pairs. No one has ever seen a lone quark.
- “Electrons orbit like planets.” — They exist as standing probability waves (orbitals); the planetary picture fails the uncertainty principle.
- Quark charges: the up carries +2e/3, the down (confirmed by deep-inelastic scattering structure functions).
- Proton : charge . Neutron : charge .
- A nucleus with Z protons and N neutrons therefore contains 2Z+N up quarks and Z+2N down quarks, bound by gluons.
- Adding Z electrons (charge each, no quark substructure) neutralizes the atom; fewer or extra electrons make ions.
- Stability: the weak interaction converts ̄ₑ decay) when N is too large, and ₑ when Z is too large, steering nuclei toward the valley of stability.
Limiting cases
What if…
Protons would decay into neutrons instead of vice versa — no hydrogen, no stars as we know them, no chemistry. Our existence hangs on mass difference.
The atom shrinks . In hydrogen, the muon orbits so close it samples the proton's interior — the 2010 muonic-hydrogen experiment found a proton radius 4% smaller than expected, a real puzzle that took a decade to resolve.
Quark inventory of a helium-4 atom
- Up quarks: .
- Down quarks: .
- Nuclear charge: ; electrons contribute .
- Total charge — a neutral helium atom (the nucleus alone is an alpha particle).
Why tritium decays
- is far above the stability line for light nuclei).
- The weak force flips one d quark in a neutron to u: ̄ₑ.
- New nucleus: , — helium-3, which is stable.