Concept of Atoms

By the nineteenth century, enough evidence had accumulated in favour of the atomic hypothesis of matter. In 1897, experiments on electric discharge through gases carried out by J. J. Thomson showed that atoms of different elements contain negatively charged constituents called electrons, and these electrons are identical for all atoms. Since atoms as a whole are electrically neutral, the atom must also contain positive charge to neutralise the negative charge of electrons.

The first model of the atom was proposed by J. J. Thomson in 1898. According to this model, the positive charge of the atom is uniformly distributed throughout the volume of the atom, and the negatively charged electrons are embedded in it like seeds in a watermelon.

Plum pudding model: Thomson's atomic model in which electrons are embedded in a uniformly distributed positive charge.

This model was also described pictorially as the plum pudding model of the atom. Later studies showed that the actual distribution of electrons and positive charge is very different from this proposal.

Condensed matter, such as solids and liquids, and dense gases at all temperatures, emit electromagnetic radiation with a continuous distribution of wavelengths, though with different intensities. This radiation is considered to arise from oscillations of atoms and molecules, governed by the interactions 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 a neon sign or mercury vapour lamp, has only certain discrete wavelengths. Its spectrum appears as a series of bright lines.

In rarefied gases, the average spacing between atoms is large. Therefore, the emitted radiation can be considered to arise from individual atoms rather than from interactions between atoms or molecules.

In the early nineteenth century, it was established that each element is associated with a characteristic spectrum of radiation. For example, hydrogen always gives a set of lines with fixed relative positions between the lines.

This fact suggested a close relationship between the internal structure of an atom and the spectrum of radiation emitted by it.

In 1885, Johann Jakob Balmer obtained a simple empirical formula that gave the wavelengths of a group of lines emitted by atomic hydrogen. Since hydrogen is the simplest known element, its spectrum is considered in detail in this chapter.

• Hydrogen gives a specific set of spectral lines.
• Balmer's work provided an empirical formula for the wavelengths of a group of hydrogen lines.

Ernst Rutherford (1871–1937) was a New Zealand-born British physicist who did pioneering work on radioactivity. He discovered alpha-rays and beta-rays. Along with Frederick Soddy, he created the modern theory of radioactivity. He studied the emanation of thorium and discovered a new noble gas, an isotope of radon, now known as thoron.

By scattering alpha rays off metal foils, he discovered the atomic nucleus and proposed the planetary model of the atom. He also estimated the approximate size of the nucleus.

Ernst Rutherford, who had earlier been a research student of J. J. Thomson, was conducting experiments on alpha particles emitted by certain radioactive elements. In 1906, he proposed a classic experiment in which these alpha particles were scattered by atoms to investigate atomic structure.

This experiment was later carried out around 1911 by Hans Geiger and Ernst Marsden. The explanation of the results led to Rutherford's planetary model of the atom, also known as the nuclear model.

According to Rutherford's model, the entire positive charge and most of the mass of the atom are concentrated in a small volume called the nucleus. Electrons orbit the nucleus just as planets orbit the Sun.

• The positive charge is concentrated in the nucleus.
• Most of the mass of the atom is concentrated in the nucleus.
• Electrons revolve around the nucleus.
• This model is also called the nuclear model of the atom.

Rutherford's nuclear model was a major step towards the modern picture of the atom. However, it could not explain why atoms emit light only of discrete wavelengths.

The source raises an important difficulty: 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, the electron revolves around the nucleus much like a planet revolves around the Sun, but the source notes that there are serious difficulties in accepting such a model.