Thomson's Atomic Model

J.J. Thomson's discovery of the electron came from studying the rays produced inside an evacuated glass tube when a high voltage was applied between two electrodes. The cathode (negative electrode) emitted rays that travelled to the anode and produced a visible glow where they struck the glass beyond it. By measuring how these cathode rays were deflected by electric and magnetic fields, Thomson showed they were streams of negatively charged particles -- far lighter than any known atom -- which he named electrons. This was a direct demonstration that Dalton's 'indestructible' atom in fact contains smaller constituent particles.

Thomson proposed his atomic model in 1903 to accommodate this discovery. In his picture, an atom is a sphere of uniformly distributed positive charge, roughly the size of the whole atom, with the much smaller, much lighter electrons embedded inside it -- rather like raisins studded through a pudding, which is why the model is often called the 'plum-pudding model'. The total positive charge of the sphere exactly balances the total negative charge of the embedded electrons, so the atom as a whole is electrically neutral. Because the positive charge is spread through a large, essentially rigid sphere, only the small, mobile electrons can be knocked loose and emitted -- never the positive charge itself -- which is consistent with cathode rays being purely negative. The model could also qualitatively account for the formation of ions (atoms with extra or missing electrons) and ionic compounds. …