The electron was discovered in 1897 by J.J. Thompson. Thompson found that passing a electric current through a very low-pressure gas resulted in a flow of particles from the negative electrode, or cathode, to the positive electrode, the anode. As opposites attract this implied that the particles were negatively charged. This charge also meant that the particle stream could be deflected by a magnetic field. These particles exhibited exactly the same properties irrespective of the gas used. Subsequent experiments have shown electrons to have exceedingly low, almost negligible mass: a proton weighs 1840 times as much as an electron.
Further experiments with gases found that stream of particles is also produced in the opposite direction to the stream of electrons, that is from the anode to the cathode. This implied that these particles were positively charged, which was verified by their deflection in a magnetic field that was in the opposite direction to the electrons. The mass of these particles differed according to the gas used, with the lightest particles being generated from hydrogen, the lightest element. These particles were protons and were deflected by a magnetic field far less than electrons, suggesting that protons are far more massive.
Neutrons were discovered by James Chadwick in 1932. Chadwick found that bombarding beryllium with alpha particles generated a stream of fast-moving particles that could penetrate several inches of lead. Chadwick also found that these particles were not deflected by electric or magnetic fields, suggesting they had no charge. Unlike the proton and the electron, the neutron is unstable outside of the nucleus. The neutron decays with a half life of 13 minutes to form a proton and electron, and because of its constituents the neutron is slightly heavier than the proton.
The atom comprises a nucleus orbited by one or more electrons. Hydrogen comprises a nucleus of a single proton orbited by a single electron. All of the elements feature more than one proton in the nucleus, together with one or more neutron; in most atoms the number of protons and neutrons in the nucleus is more or less equal.
It used to be believed that the movement of electrons around the nucleus was analogous to the movement of planets around a star. In the 1930s the development of quantum physics -- with work by scientists such as Schrodinger, Planck, Heisenberg and Einstein -- showed that electrons exist at discrete energy levels around the nucleus.