Orbel

Meet Orbel, a proton whose electron moves much farther out with each step up in energy.

In Niels Bohr's model of hydrogen, the electron can only orbit at certain distances. Each allowed orbit has a whole number n. The smallest, n = 1, has a radius called the Bohr radius, a₀.

The orbit radius grows as rₙ = n²a₀. With a₀ ≈ 5.29 × 10⁻¹¹ m, the n = 2 orbit is about 0.212 nm from the centre, four times wider. The n = 3 orbit is nine times wider, about 0.476 nm.

Modern quantum mechanics replaces neat orbits with clouds of probability, but the Bohr radius is still the most likely distance of the electron from the nucleus in the ground state.

It sets the natural size scale for atoms. Most atoms, whatever their element, are only a few Bohr radii across, around a tenth of a nanometre or so. Larger orbits need more energy, and the electron jumps between them by absorbing or giving out light.