Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [44]
This drastic reduction in electron numbers revealed that most of the weight of an atom was due to the diffuse sphere of positive charge. Suddenly, what Thomson had originally invoked as nothing more than a necessary artifice to produce a stable, neutral atom took on a reality of its own. But even this new, improved model could not explain alpha particle scattering and failed to pin down the exact number of electrons in a particular atom.
Rutherford believed that alpha particles were scattered by an enormously strong electric field within the atom. But inside J.J.'s atom, with its positive charge evenly distributed throughout, there was no such intense electric field. Thomson's atom simply could not send alpha particles hurtling backwards. In December 1910, Rutherford finally managed to 'devise an atom much superior to J.J.'s'.45 'Now,' he told Geiger, 'I know what the atom looks like!'46 It was nothing like Thomson's.
Rutherford's atom consisted of a tiny positively-charged central core, the nucleus, which contained virtually all the atom's mass. It was 100,000 times smaller than the atom, occupying only a minute volume, 'like a fly in a cathedral'.47 Rutherford knew that electrons inside an atom could not be responsible for the large deflection of alpha particles, so to determine their exact configuration around the nucleus was unnecessary. His atom was no longer the 'nice hard fellow, red or grey in colour, according to taste' that he once, tongue-in-cheek, said he had been brought up to believe in.48
Most alpha particles would pass straight through Rutherford's atom in any 'collision', since they were too far from the tiny nucleus at its heart to suffer any deflection. Others would veer off course slightly as they encountered the electric field generated by the nucleus, resulting in a small deflection. The closer they passed to the nucleus, the stronger the effect of its electric field and the greater the deflection from their original path. But if an alpha particle approached the nucleus head-on, the repulsive force between the two would cause it to recoil straight back like a ball bouncing off a brick wall. As Geiger and Marsden had found, such direct hits were extremely rare. It was, Rutherford said, 'like trying to shoot a gnat in the Albert Hall at night'.49
Rutherford's model allowed him to make definite predictions, using a simple formula he had derived, about the fraction of scattered alpha particles to be found at any angle of deflection. He did not want to present his atomic model until it had been tested by a careful investigation of the angular distribution of scattered alpha particles. Geiger undertook the task and found alpha particle distribution to be in total agreement with Rutherford's theoretical estimates.
On 7 March 1911, Rutherford announced his atomic model in a paper presented at a meeting of the Manchester Literary and Philosophical Society. Four days later, he received a letter from William Henry Bragg, the professor of physics at Leeds University, informing him that 'about 5 or 6 years ago' the Japanese physicist Hantaro Nagaoka had constructed an atom with 'a big positive centre'.50 Unknown to Bragg, Nagaoka had visited Rutherford the previous summer as part of a grand tour of Europe's leading physics laboratories. Less than two weeks after Bragg's letter, Rutherford received one from Tokyo. Nagaoka wrote offering his gratitude 'for the great kindness you showed me in Manchester' and pointing out that in 1904 he had proposed a 'Saturnian' model of the atom.51 It consisted of a large heavy centre surrounded by rotating rings of electrons.52
'You will notice that the structure assumed in my atom is somewhat similar to that suggested by you in your paper some years ago', acknowledged Rutherford in