Discovery of Protons
Scientific investigations, like detective stories, introduce evidence from many sources before the mystery is 'solved'. Research work in chemistry and electricity could not, alone, supply all the facts needed to make the next scientific model of the atom. Some parts of the theories of light, heat, magnetism and electrostatics (the branch of science which deals with electricity at rest) were concerned in the process. Mathematics, too, contributed many ideas without which progress would not have been possible.
The great 'detective' who finally brought all the evidence together, and at the same time encouraged so many others to follow in his footsteps, was a New Zealander, Ernest Rutherford (1871-1937). Following Thomson at Cambridge University, Rutherford, in a series of brilliant yet quite simple experiments, uncovered new evidence which suggested that the atom was, like a miniature solar system, mostly empty space.
The mass of the atom, he suggested, was concentrated in a central nucleus or 'sun', and the electrons rotated around this sun in miniature orbits.
The nucleus contained a positive charge which attracted the negative electrons, normally preventing them flying off into space. Hydrogen, the lightest nucleus, had one unit of positive charge, equal exactly to the negative charge of the single electron rotating around it. Rutherford called the nucleus of the hydrogen atom a proton, and this became the new building block of the Periodic Table.
There was, however, a very serious difficulty in accepting this new model of the atom, since it did not agree with certain well established laws of electricity.
It was a Danish physicist, Niels Bohr (1885-1962), who had the courage to suggest that it was not the new model that was at fault but the old laws. He insisted that these laws could not apply to the microscopic world of the atom.
The German physicist Max Planck (1858-1947) had already suggested that energy, like matter, existed in 'lumps', or quanta as he called these minutely small particles of energy. His Quantum Theory (the Latin word quantum means a definite amount) was used by Niels Bohr to justify his new atomic model.