Specimen 01// hydrogen, orbital
An electron does not circle the nucleus on a track. The cloud shows where you would probably find it: each dot is one possible position, and denser means more likely. Hydrogen's electron sits in the 1s orbital, a sphere.
▸ electron configuration
fig. 1 — one electron, one orbital, half full. That empty half is why hydrogen bonds so readily.
Specimen 01// hydrogen, nucleus
Fly in far enough and the cloud thins out around a single proton. The most common form of hydrogen has no neutrons at all.
▸ nucleus
fig. 2 — scale break: the nucleus is drawn thousands of times too large. At true scale it would be invisible.
Specimen 02// neon, orbitals
Neon's ten electrons fill 1s (2), 2s (2) and all three 2p orbitals (6). The p orbitals are dumbbells pointing along x, y and z. A full set of them adds up to a perfectly round cloud, and a full outer shell is why neon barely reacts.
▸ electron configuration
fig. 3 — the sizes between shells are schematic; the shapes within each orbital are the real probability distributions.
Specimen 02// neon, nucleus
Neon-20, the common isotope, packs ten protons (orange) and ten neutrons (grey) together. The strong force holds them at a range far shorter than the size of the electron clouds around them.
▸ nucleus
fig. 4 — equal numbers of protons and neutrons is typical for light, stable nuclei.
Specimen 03// sodium, orbitals
Sodium is neon plus one electron. The second shell is full, so the newcomer goes into 3s, a much bigger sphere. The ten inner electrons shield it from most of the nucleus's pull, so it is easy to pull away. That is why sodium reacts so readily.
▸ electron configuration
fig. 5 — same core as neon (fig. 3), plus one electron in the outermost orbital.
Specimen 03// sodium, nucleus
Sodium-23 has eleven protons and twelve neutrons. As nuclei get heavier they need proportionally more neutrons to stay together against the protons' electrical repulsion.
▸ nucleus
fig. 6 — the neutron-to-proton ratio climbs as atoms get heavier.
The rules// how orbitals fill
Behind them is one picture: neon's three 2p orbitals, equal in energy and at right angles to each other. Electrons fill them one at a time before any pairing up.
▸ filling order
fig. 7 — break Pauli and the state is impossible. Break Aufbau or Hund and you get an excited, higher-energy atom.
Live// build your own atom
Drag to any electron count. Orbitals fill by the three rules, and the cloud density follows the electron count.
▸ the three 2p orbitals (they fill only after 2s is full)
// drag the slider: every atom from Z=1 to Z=11 fills the same way.