Atomic Models

The discovery of the first subatomic particle, the electron, resulted from experiments involving the effects of electricity on matter. In the late 1800s, the cathode ray tube was developed and used in several investigations. A cathode ray tube is a partially evacuated glass tube containing a gas at low pressure. At one end of the tube is a cathode, at the other end, an anode. The cathode and anode are attached to a voltage source. The voltage source creates a current that can be passed through the gas trapped inside. Early experiments showed that the current caused the surface of the tube directly opposite the cathode to glow. It was hypothesized that a stream of particles originating at the cathode and moving toward the anode caused the glow. This stream of particles was called a cathode ray. When a paddle wheel was placed in the tube, it rolled from cathode to anode, which showed that the particles making up the cathode ray had mass. When exposed to a magnetic field, the cathode ray was deflected in the same manner as an electric current, which has a negative charge. Therefore, it was concluded that the particles that compose a cathode ray not only had mass but also a negative charge.

English physicist Joseph John Thomson (1856–1940) confirmed these findings in 1897. Thomson performed a series of experiments in which he was able to determine the ratio of the charge of the particles that make up the cathode ray to their mass by measuring the deflection of the rays with varying magnetic and electric fields. Thompson performed the same experiments using different metals for the cathode and anode as well as different gases inside the tube. His experiments demonstrated that the amount of deflection could be predicted mathematically. Thomson found that the charge-to-mass ratio was always the same, regardless of the materials used. He then concluded that all cathode rays are made up of the same particles, which were later named electrons by another English physicist, G. Johnstone Stoney (1826–1911).

The American physicist Robert A. Millikan (1868–1953) of the University of Chicago performed experiments that further confirmed Thomson's results. In 1909, he discovered that the mass of one electron is approximately one two-thousandth that of a hydrogen atom through his "oil droplet" experiments. He used x rays to give oil droplets a negative charge. He then sprayed these droplets through an apparatus, allowing them to fall between two electrically charged plates. He varied the charge on the two plates and measured how this change affected the droplets' rate of fall. Using these data, he calculated that the charge of every oil droplet was a multiple of the same number each time, and concluded that this must be the charge of a single electron. Using this number, and Thompson's charge-to-mass ratio, he was able to calculate the mass of one electron. He reasoned that since cathode rays show the same deflection for any gas used, electrons must be present in the atoms of all elements. Since electrons have a negative charge, and all atoms are electrically neutral, there must also be a positive charge present in the atom. Further, because the mass of an electron is so much smaller than that of an entire atom, there must be other particles present in the atom to account for the majority of its mass. These results were the first to show that atoms are indeed divisible and were the basis for the first atomic model.