When the atom went quantum
Bohr's revolutionary atomic theory turns 100
Before Niels Bohr, atoms baffled science’s brightest brains.
For millennia, atoms had been phantoms, widely suspected to exist but remaining stubbornly invisible — though not indivisible, as their name (Greek for “uncuttable”) originally implied. By the start of the 20th century, physicists knew that atoms had electrically charged parts; the favorite model envisioned blobs of positively charged pudding studded with negatively charged plums (actually, electrons). That image was challenged, though, when Ernest Rutherford showed in 1911 that the positive pudding was all crammed into a massive dense core, or nucleus, surrounded at a distance by the electron plums (SN: 5/7/11, p. 30).