The electron was discovered in 1897 as the first elementary particle ever. It carries an elementary charge, the smallest charge that can be observed isolated, which is thus used as a unit of measurement for other particles’ charge.
While the concept of electricity had been known for some time, the nature of electricity only became clear when J.J. Thomson discovered the electron. He observed that particles produced by a glowing cathode could be deflected by a magnetic field. Later it was shown that electrons are the charge carriers for electric currents flowing in conductors.
The discovery of the electron revolutionised the concept of atom (from the Greek "atomos", "indivisible"). It is not an indivisible entity as it was long assumed. Today we know that atoms are made of a central, positively charged nucleus surrounded by electrons. Originally it was thought that electrons moved in circular paths around the atomic nucleus (N. Bohr), but the introduction of quantum mechanics (E. Schrödinger) and the realisation that elementary particles also behave as waves (particle-wave duality), led to today’s understanding of the atomic structure: the position of an electron has a probability definition and orbits are defined in terms of charge density.
The sign shows a simple atomic model, where the electrons move around the nucleus in orbits located at different energy levels as well as a few examples of the quantum mechanical representations of the electrons in an atom. There the enclosed areas reflect the regions in which the electron is most likely to be located.
