Atoms and atomic structure

The atom

For many years it was speculated that the ultimate state of subdivision of matter was discrete indivisible particles termed atoms. In the early nineteenth century John Dalton gave quantitative support to these ideas by assigning a self-consistent set of masses to atoms of elements. Elements are substances which cannot be decomposed by ordinary types of chemical change, or made by chemical union. However, subsequent work by Faraday, Crookes and Goldstein pointed out the electrical nature of atoms and that they are divisible. The simplest picture of an atom now shows a central nucleus surrounded by a cloud of electrons.

Electrons

When a neutral atom is supplied with sufficient energy it will ionise yielding positively and negatively charged fragments. The former, the positive ion, is characteristic of the particular parent atom, whilst the negative fragment, the electron, is the same irrespective of the parent atom. The charge and mass of electrons can be determined by studying their behaviour in electrical and magnetic fields. Early experiments revealed the ratio of charge to mass, e/m, which for all electrons is 1.7588 × 108 coulombs per gram (Cg−1).

To evaluate the mass of an electron, experiments were carried out to determine the applied potential required to prevent oil droplets carrying negative charges from settling due to gravity.