Early models of the atom

Thomson's discharge experiments of 1897 showed that atoms of every element contain electrons, and that these electrons are identical from one element to the next. A whole atom is electrically neutral, so it must also carry an equal amount of positive charge.

Thomson's own model (1898): the positive charge is spread evenly through the whole volume of the atom, with the electrons embedded in it like seeds in a watermelon. It is known as the plum pudding model. Later experiments showed the charges are arranged very differently.

Solids, liquids and dense gases at any temperature give out a continuous spread of wavelengths, with different intensities. This radiation comes from atoms and molecules oscillating under the pull of their neighbours.

A rarefied gas heated in a flame or excited in a glow tube (a neon sign, a mercury vapour lamp) gives out only certain discrete wavelengths: a spectrum of bright lines. Its atoms are far apart, so the light comes from individual atoms, not from interactions between them.

Every element has its own characteristic spectrum; hydrogen always gives lines at fixed relative positions. This hinted that the spectrum is tied to the atom's internal structure. In 1885 Balmer found a simple empirical formula for the wavelengths of one group of hydrogen lines.

Rutherford, a former research student of Thomson, proposed an alpha-particle scattering experiment in 1906. Geiger and Marsden performed it around 1911; Marsden was then a 20-year-old student without his bachelor's degree.

From its results came Rutherford's nuclear (planetary) model: all the positive charge and most of the mass sit in a tiny central nucleus, with electrons revolving round it like planets round the sun. The model could not explain why atoms emit only discrete wavelengths.