1Build your atom
Drag a particle into the atom to add it. Drag one out of the atom to remove it.
Simplified model! This is a Bohr-style picture: electrons sit on rings that we fill 2, 8, 8, 18, 18, 32, 32. Real atoms are fuzzy clouds, and real electrons fill sub-shells in the aufbau order (1s, 2s, 2p, 3s, 3p, 4s, 3d …), so outer rings start filling earlier than shown. Sizes are not to scale: a real nucleus is about 100,000× smaller than the atom.
Particle counts illustrate charge arithmetic. They do not establish that an ion can exist freely or remain bound. The ring-filling scheme is a drawing convention, not a ground-state electron configuration.
Mass number
Mass number A = protons + neutrons. Electrons are so light (about 1/1836 of a proton) that we don't count them.
2Is this isotope in the snapshot?
Isotopes are versions of an element with different numbers of neutrons. We check yours against the nuclide table (isotopes.csv, NUBASE2020/AME2020).
This is a dated evaluation, not a live discovery list. NUBASE includes measured and extrapolated properties; a listed entry is not proof that every property was measured. AMDC data notes · NUBASE2020 evaluation.
Stable or radioactive? Stable here means no decay has been observed; it is not proof of an infinite lifetime. A radioactive nucleus can decay. Its half-life is the time for half the nuclei in a large sample to decay, on average.
Half-life units used in the file: s = seconds, m = minutes, h = hours, d = days, y = years; ky = 1,000 years, My = million years, Gy = billion years. Tiny ones: ms = 1/1,000 s, us = millionth, ns, ps, fs, as, zs, ys (each 1,000× smaller). A # means the table's value is an estimate, not a measurement.
Decay words: β⁻ = a neutron converts to a proton with electron emission. β⁺ = a proton converts to a neutron with positron emission. EC = electron capture by the nucleus. α = helium-4 nucleus emission. IT = an excited isomer releases energy to reach a lower state. SF = spontaneous fission (the nucleus splits). n / p = neutron or proton emission.
3Element facts
Straight from the data files. If a value is blank there, you'll see not in file.
Category and room-temperature phase come from PubChem's periodic-table CSV (sources/pubchem.csv, columns GroupBlock and StandardState); everything else from elements.csv.
4Periodic table
Tap an element to jump to it. You get its protons, the same number of electrons (neutral), and the neutrons of its common or longest-lived isotope.
The table's shape is the standard layout. The two bottom rows (lanthanides and actinides) belong in the gaps marked 57–71 and 89–103. Colours are PubChem's GroupBlock categories.
5The valley of stability
Each dot is one listed ground-state isotope: protons across, neutrons up. The stable ones form a narrow "valley". Other listed nuclei are radioactive or have unreported half-lives; arbitrary particle counts need not form bound nuclei. Your atom is the red ring. Tap a dot to build that isotope.
6Challenges
Can you build these? Each one is checked against the data files. A ✓ appears when you've done it.