Electron

The electron is a negatively charged subatomic particle which is an important component of the atoms which make up ordinary matter. The electron is fundamental; that means it is not believed to be made up of smaller constituents. The size of the charge on the electron has for many years been considered the fundamental unit of charge found in nature. Recently, however, considerable evidence has been found to indicate that particles classified as mesons and baryons are made up of objects called quarks, which have charges of either 2/3 or 1/3 the charge on the electron. For example, the neutrons and protons, which make up the nuclei of atoms, are baryons. However, scientists have never been able to observe an isolated quark, so for all practical purposes the charge on the electron can still be considered the fundamental unit of charge found in nature. The mass of the electron is about onethousandth that of the smallest atom.

All atoms found in nature have a positively charged nucleus about which the negatively charged electrons move. The atom is electrically neutral and thus the positive electrical charge on the nucleus (contributed by protons) has the same magnitude as the negative charge due to all the electrons. The electrons are held in the atom by the attractive force exerted on them by the positively charged nucleus. They move very rapidly about the nucleus; their exact location at any moment in time is expressed in terms of probability. Thus, for a given electron, it can be more likely that it will be found in some locations than in others. The total probabilities can be drawn as an electron cloud, which is densest nearer to the nucleus and progressively less dense at an increasing distance from the nucleus. Some of the electrons in a typical atom can be quite close to the nucleus, while others can be at distances which are many thousands of times larger than the diameter of the nucleus. Thus, the electron cloud determines the size of the atom. It is the outermost electrons that determine the chemical behavior of the various elements. The size and shape of the electron clouds around atoms can only be explained utilizing a field of physics called quantum mechanics.

In metals, some of the electrons are not tightly bound to atoms and are free to move through the metal under the influence of an electric field. It is this situation that accounts for the fact that most metals are good conductors of electricity and heat.

Quantum theory also explains several other rather strange properties of electrons. Electrons behave as if they were spinning, which allows them to behave like little magnets. The way electrons are arranged in some materials, such as iron, causes these materials to be magnetic. The existence of the positron, the antiparticle of the electron, was predicted by French physicist Paul Dirac in 1930. To predict this antiparticle, he used a version of quantum mechanics which included the effects of the theory of relativity. The positron’s charge has the same magnitude as the electron’s charge but is positive. Dirac’s prediction was verified two years later when the positron was observed experimentally by Carl Anderson in a cloud chamber used for research on cosmic rays. The positron does not exist for very long in the presence of ordinary matter because it soon comes in contact with an ordinary electron and the two particles annihilate, producing a gamma ray with an energy equal to the energy equivalent of the two electron masses, according to Einstein’s famous equation E = mc².

Robert L. Stearns