On the Constitution of Atoms and Molecules
Electrons can't spiral inward — they live only on fixed rungs, and light is the jump between them.
Bohr found the rule that keeps atoms from collapsing — and turned the coloured lines in a glowing gas into the atom's fingerprint.
The big idea
By 1913 physicists knew the atom was mostly empty: a tiny, heavy nucleus with electrons around it, like planets round a sun. But there was a fatal flaw. An electron whirling around a nucleus should, by the known laws of electricity, constantly leak energy as light and spiral into the centre in less than a billionth of a second. Atoms should not exist. Yet here we are.
Bohr's answer was radical and simple: the electron is allowed only on certain fixed “rungs” around the nucleus, never in between. On a rung it loses no energy. It can change only by jumping from one rung to another — and when it drops to a lower rung, the leftover energy flies off as a single particle of light of one exact colour. Each element has its own ladder of rungs, so each gives off its own unique set of colours: a barcode written in light.
How it came about
Niels Bohr was a 27-year-old Dane who had just spent a few months in Manchester with Ernest Rutherford, the man who had discovered the nucleus. The instability problem haunted him, and so did a set of numbers the spectroscopists had: the wavelengths of hydrogen's coloured lines fit a strange little formula, found by a Swiss schoolteacher named Balmer in 1885, that nobody could explain.
The breakthrough came when a colleague pointed Bohr to Balmer's formula. “As soon as I saw Balmer's formula,” he later said, “the whole thing was immediately clear.” He realised the whole number in Balmer's formula was simply counting his rungs. He combined Rutherford's nucleus, Planck's quantum of energy and Balmer's numbers — and out came hydrogen's spectrum, exactly, with the right constant calculated from the mass and charge of the electron. He published it in three papers in 1913.
Why it mattered
Two things. First, it rescued the atom: a clear rule for why matter is stable, and why it glows in sharp colours instead of a smear. Second, and bigger, it showed that the quantum — Planck's idea that energy comes in lumps — was not a quirk of hot furnaces but the governing law of the atom itself. After Bohr you could no longer treat the inside of an atom with ordinary physics. The strange new quantum world was real, and it was everywhere.
A way to picture it
Think of a staircase, not a ramp. On a ramp you can stand at any height; on a staircase only on the steps. An electron in an atom is on a staircase: it can sit on step 1, 2, 3… but never halfway. To climb it must absorb exactly the energy of one step or more; when it falls, it releases exactly that energy as a flash of light of a single colour. Different atoms have staircases with differently spaced steps — which is why a neon sign is red, a sodium lamp is orange, and starlight split into its colours tells us what stars are made of. Use the tool below to pick a jump and see which colour comes out.
Where it sits
Bohr's atom is the hinge between two ages. Behind it stand Planck (1900) and Einstein (1905), who first found that energy and light come in quanta; in front of it stand Heisenberg and Schrödinger, who in 1925–26 built full quantum mechanics and replaced Bohr's neat orbits with fuzzier “probability clouds.” Bohr's specific picture — electrons on circular tracks — turned out to be wrong in detail, yet his core idea, discrete energy levels and quantum jumps between them, is exactly right and remains the backbone of how we understand light and matter. It is the atom you still meet first in school.
the energy radiation from an atomic system does not take place in the continuous way assumed in ordinary electrodynamics, but that it, on the contrary, takes place in distinctly separated emissions
(1) That the dynamical equilibrium of the systems in the stationary states can be discussed by help of the ordinary mechanics, while the passing of the systems between different stationary states cannot be treated on that basis.
(2) That the latter process is followed by the emission of a homogeneous radiation, for which the relation between the frequency and the amount of energy emitted is the one given by Planck's theory.