1. Historical perspective

In the years before Bohr formulated his theory of the atom, based upon the principles of quantum physics, some steps had been made on the understanding of the atomic structure. We list here some important contributions:

Mendeleev had developed a concept for arranging the known chemical elements based on their mass. Order was given in terms of increasing mass, while the elements were further arranged according to the ordering principle of chemical behaviour. The columns in the table of the elements relate to chemical valence. In 1869 the Periodic Table was not yet complete.
D.I. Mendeleev

Avogadro had conceived the idea that gasses consist of discrete particles and had established the law that equal volumes of gas at equal pressure and temperature contain the SAME number of such particles, although the actual number was not yet determined. Avogadro's hypothesis was not accepted by many physicists for a long time.
A. Avogadro.

The work on the analysis of the "Brownian motion" turned out to be decisive for the acceptance of the particle hypothesis. First the botanist Brown had looked at grains of pollen moving in a liquid through observation by a microscope. Then Einstein developed his theory of the Brownian motion based on the ideas of diffusion and random walk. Look here for Brownian motion in action. As a final outcome Einstein deduced the important relation between the Boltzman constant k_B and the universal gas constant R:

k_B = R/N_A
where N_A is Avogadros number. This lead J.B. Perrin to a measurement of this important value yielding:
N_A = 6.0221 x 10²³ molecules/mole, which is not far off from the true value.
J.B. Perrin Nobel Prize laureate 1926

It was realized for some time that electrostatic charges were important for the building blocks of matter. This followed from Faradays experiments on electrolysis , from which it was deduced that ions move in a liquid as charged particles, and from the experiments on radioactivity in which electrically charged particles were emitted.
M. Faraday

Thomson's experiments on cathode rays were important for the determination of some properties of the constituents of matter. The charged particles, emitted from a cathode, were deflected in a combination of crossed static electric and magnetic fields, and detected on the phosphorent screen (see Fig). Hence impinging charged particles could be made visible by the light emitted by the screen.


The experiment is based upon an analysis of the kinematics of the particles, deflected by a field as explained here. A value can be determined for the e/m ratio: e/m = 1.75 x 10¹¹ C/kg.
for the small charged particle which was named the "electron". The electron was discovered (as an elementary particle) in 1897.
J.J. Thomson Nobel Prize laureate 1906

The results on cathode rays and their interpretation, sometimes referred to as " mysterious rays", were long debated among scientists ( click here).

After the ratio (e/m) was determined Millikan performed his famous oil-drop experiment from which the two values of e and m could be unravelled. In fact Millikan also proved the discreteness of charge in his experiment of 1906.
R.A. Millikan Nobel Prize laureate 1923

Based on these concepts Thomson developed a model for the atom consisting of the electrons as negatively charged particles of low mass and some substance that should carry positive charge and nearly all the mass within the atom. Since the elements were arranged according to their mass nemuber A, the atoms were thought to consist of A positive particles and A electrons in a structure as shown below. Note that the atomic number Z does not play a role yet.

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Atomic Models

Thomson's model Rutherford's model

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After his studies into radioactivity Rutherford set out, in the first decade of the 20th century, to perform scattering experiments with alpha-particles, to investigate the structure of the atom. A full theoretical treatment on the physical concepts and scattering cross sections can be found here.
Two important conclusions could be drawn from the angular distribution of the scattered particles:
- almost all the particles were transmitted through a foil of metal without any deflection
- a very few particles were scattered backwards into the detector
From detailed analyses of the scattering kinematics of charged particles (known as Rutherford scattering) it was concluded that the atom consists of:

merely a void
with a heavy positively charged nucleus at the centre carrying all the mass of the atom within a size of a few fm (10^-15 m)
electrons outside the nucleus


E. Rutherford Nobel Prize laureate 1908 (Chemistry)

There are however a number of shortcomings to this planetary model of an atom bound by classical electromagnetic forces:

the problem of stability with accelerated electrons in orbit loosing energy
the model gives no indication of the size of the atom
the model gives no explanation for the characteristic spectroscopy of atoms

All these contributions, mentioned above, lead jointly to a conception of the structure of atoms. There is however another branch of research that has in the past, and as of today, taught us most about the structure of atoms and molecules.

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Last change: 6 February 2001