The History of Atomic Theory. Due to the constantly developing scientific theories, the model of the atom has changed significantly over the years from Democritus with his views of indivisible tomos to Thomsons plum pudding model and scientists

During experiments with cathode rays around the turn of the 20th century, JJ Thomson discovered the electron. He realised that these cathode rays were a stream of particles and that they all had negative charges and small masses. They could also be deflected by electrical fields and magnets and Thomson concluded that the atom was further divisible, disproving the theory of Dalton and the Greeks. He decided that electrons came from within the atoms – his theory being that an atom was made up of electrons floating in a ‘sea’ of positive charge, with the atom’s overall charge being neutral. This became known as the ‘plum-pudding’ model, created between 1897 and 1906.

This model was short-lived, however. In 1909, work began that led to the downfall of the ‘plum-pudding’ model. Hans Geiger and Ernest Marsden, two of Ernest Rutherford’s students, were carrying out a series of experiments at the University of Manchester. Some α (alpha) particles were directed at a sheet of gold foil and measured the particles’ deflection. Rutherford predicted that the α particles would not be deflected by Thomson’s plum pudding model.

However, as shown in the above diagram, whilst most particles were not deflected, the centremost particles were. This indicated that there was a small, concentrated positive charge in the centre of the atom. In 1911, he proposed the nuclear atom. He said that the positive charge and most of the atom’s mass is concentrated in the centre, in a nucleus. Between the orbiting electrons and the nucleus would be most of the atom’s volume. Finally, the overall charge must be neutral.

In 1913, Niels Bohr modified Rutherford’s model slightly to change the electrons’ orbital paths. If they did not follow these paths, they would veer off course towards the nucleus. The nucleus is orbited by the electron á la the sun and its planets, earning it the name ‘planetary atom.’ He suggested what is now used as the GCSE model: two electrons on the innermost shell with eight, eighteen and thirty-six occupying the shells further outward.

Also in 1913, Henry Moseley discovered a link between X-ray frequencies and atomic number which was explained by another discovery by Rutherford – the proton was discovered in 1918, which gives the atom its atomic number. During the 1920s two physicists, Louis de Broglie and Erwin Schrödinger suggested that particles and electrons respectively had wave properties. De Broglie also worked on the theory of wave-particle duality, which proposed that a particle could behave as either a wave or particle at any time.

The theory of atomic orbitals was also devised by Schrödinger. The periodic table was split into four sections: sharp, principal, diffuse and fundamental. These sections indicated energy levels and each orbital could contain two electrons. Each energy level can hold a different number of orbitals: the s level holds one, the p level holds three (px, py, pz), the d level holds five orbitals (dz2, dxz, dyz, dxy, dx2-y2) and the f level holds seven. Instead of electronic configuration being shown as ‘shell 1, shell 2, shell 3 etc,’ they are shown as a list of orbital arrangements: for example, Oxygen is shown as 1s2 2s2 2p4.

Neutrons were finally discovered in 1932 by Englishman James Chadwick. He used high energy α particles to bombard lighter elements and found that a new form of radiation was emitted. This radiation was made up of particles with the same mass as a proton but had no charge. Different elements have different isotopes; meaning that they have a different number of neutrons – for the periodic table, the weighted average of known isotopes is taken.

Scientists are continuously working on creating new theories and strengthening old ones. Chemistry is constantly evolving and the model of the atom is still changing today as technology improves. Whilst it is true that science has come a long way since the four elements of the Greeks, it is likely that in the future the atom’s model will become even more complex.