The late 1800s and early 1900s saw the emergence of a revolutionary new model of the atom. An atom that could no longer be conceived as having no structure of its own, but it had to be understood as a complex of particles. Of these particles, the electron was the one that precisely marked the affirmation of the atomic model of matter. Indeed, the belief in the electron as a particle also presupposed a firm belief in the existence of atoms. The existence of ions, evoked in Arrhenius’ theory of electrolytic dissociation, confirmed the presence of positive and negative charges due to the transfer of electrons from atom to atom.