The planetary model of the atom

One can say with hindsight that it was only to be expected that quantisation would play a major role in the structure of the atom. Rapid developments were taking place in the first and second decades of the century in building up an atomic model, which would help to visualise atomic structure and to explain its function. The fact that there is a structure was demonstrated in 1911 by Sir Ernest Rutherford (1871-1937). Together with his students Hans Geiger (1882-1945) and Ernest Marsden (1889-1970), he discovered that the mass of an atom is concentrated in a very small nucleus which is positively charged. Electrons, which carry negative electric charge, occupy the outer layers.

Since positive and negative charges attract one another there must be some mechanism which keeps the electrons from falling into the nucleus. Soon afterwards, Niels Bohr (1885-1962) and Arnold Sommerfeld (1868-1951) put forward the planetary model of the atom, in which the electrons orbit the nucleus in a similar fashion to planets orbiting the sun in the Copernican model of the solar system. Just as the earth does not plunge into the sun, in spite of the attractive gravitational force, electrical attraction serves to keep the electron in orbit, without crashing into it into the nucleus.

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