Atoms and molecules, governed by the interaction of each atom or molecule with its neighbours. In contrast , light emitted from rarefied gases heated in a flame, or excited electrically in a glow tube such as the familiar neon sign or mercury vapour light has only certain discrete wavelengths. The spectrum appears as a series of bright lines. In such gases, the average spacing between atoms is large.
Hence, the radiation emitted can be considered due to individual atoms rather than because of interactions between atoms or molecules. In the early nineteenth century it was also established that each element is associated with a characteristic spectrum of radiation, for example, hydrogen always gives a set of lines with fixed relative position between the lines. This fact suggested an intimate relationship between the internal structure of an atom and the spectrum of radiation emitted by it. In , Johann Jakob Balmer ( – ) obtained a simple empirical formula which gave the wavelengths of a group of lines emitted by atomic hydrogen.
Since hydrogen is simplest of the elements known, we shall consider its spectrum in detail in this chapter. Ernst Rutherford ( – ), a former research student of J. J. Thomson, was engaged in experiments on ( -particles emitted by some radioactive elements.
In , he proposed a classic experiment of scattering of these ( -particles by atoms to investigate the atomic structure. This experiment was later performed around by Hans Geiger ( – ) and Ernst Marsden ( – , who was year-old student and had not yet earned his bachelor’s degree). The details are discussed in Section . .
The explanation of the results led to the birth of Rutherford’s planetary model of atom (also called the nuclear model of the atom ). According to this the entire positive charge and most of the mass of the atom is concentrated in a small volume called the nucleus with electrons revolving around the nucleus just as planets revolve around the sun. Rutherford’s nuclear model was a major step towards how we see the atom today. However, it could not explain why atoms emit light of only discrete wavelengths.
How could an atom as simple as hydrogen, consisting of a single electron and a single proton, emit a complex spectrum of specific wavelengths? In the classical picture of an atom, the electron revolves round the nucleus much like the way a planet revolves round the sun. However, we shall see that there are some serious difficulties in accepting such a model.