Electrons, protons and neutrons

Dalton thought atoms could not be divided. Experiments with electricity soon showed that atoms contain even smaller particles.

Electrons. In a discharge tube (a glass tube with two metal electrodes and gas at very low pressure), a high voltage produces cathode rays, which travel from the cathode (negative electrode) to the anode in straight lines. Without electric or magnetic fields they go straight; with fields they bend, the way a negative charge would. They are streams of negatively charged particles, now called electrons. Their properties are the same whatever gas or electrode is used, so electrons must be present in all atoms.

In 1897, J.J. Thomson measured how much the rays bent in electric and magnetic fields, and from this found the ratio of charge to mass of the electron: e/mₑ = 1.758820 × 10¹¹ C kg⁻¹.

Millikan's oil drop experiment (1906-14) measured the charge itself: –1.6 × 10⁻¹⁹ C (now –1.602176 × 10⁻¹⁹ C). Putting the two results together gives the electron's mass: mₑ = 9.1094 × 10⁻³¹ kg.

Protons and neutrons. A discharge tube with a perforated cathode also shows canal rays (positive rays) travelling the other way. The lightest positive particle, from hydrogen, is the proton: charge +e, mass 1.6726 × 10⁻²⁷ kg, nearly 2000 times the electron's. In 1932, Chadwick bombarded a thin sheet of beryllium with α-particles and found a neutral particle: the neutron.

Every electric charge is a whole number of electron charges. To count the electrons, divide by 1.602 × 10⁻¹⁹ C. For example, a charge of –1.282 × 10⁻¹⁸ C is 1.282 × 10⁻¹⁸ ÷ 1.602 × 10⁻¹⁹ = 8 electrons.