01 200 years of people being slightly wrong
Every model was the best guess with the evidence at the time. Drag the slider through time.
02 Rutherford's gold foil (try it)
Fire alpha particles (+2 charge) at a thin sheet of gold. Thomson predicted they'd all go straight through. Watch what actually happens.
straight through: 0 · deflected a bit: 0 · bounced back: 0
→ conclusion: atom is mostly empty space with a tiny, dense, positive nucleus.
03 Bohr: electrons on shells
Electrons sit in fixed energy levels. Give one energy and it jumps up, then falls back and gives off light of one exact colour (that's the idea behind line spectra, covered in the Emission Spectra lesson).
hydrogen, electron in n=1 (ground state)
04 Quantum model: the electron cloud
Schrödinger + Heisenberg: we can't know exactly where an electron is, only where it's likely to be. Each dot below is one "where was it?" snapshot. They pile up into an orbital.
05 Ok but how tiny is "tiny"?
Rutherford said the nucleus is tiny. Like, really tiny. Slide to blow a hydrogen atom up to a size you can picture.
06 The 3 particles + the numbers
| particle | charge | relative mass | where |
|---|---|---|---|
| proton | +1 | 1 | nucleus |
| neutron | 0 | 1 | nucleus |
| electron | −1 | 1/1836 (≈0) | shells |
tap an isotope:
carbon-12: 6 p, 6 n. The common one (~99%).
notation: 2311Na → 11 protons, 12 neutrons, 11 electrons
07 Build-an-atom lab
Drag particles from the trays onto the atom (protons & neutrons → middle, electrons → anywhere around). Drag them off to remove. Tap also works.
charge: 0
mass number: 0
config: –
challenge: build carbon-12, sodium, oxide ion O²⁻, carbon-14