The Physics
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Diameter, Radius of an Electron

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World Book Encyclopedia. Chicago: World Book. "The diameter of an electron is less than 1/1000 the diameter of a proton. A proton has a diameter of approximately 1/25,000,000,000,000 inch (0.000000000001 mm)." < 10−18 m
Mac Gregor, Malcolm H. The Enigmatic Electron. Boston: Klurer Academic, 1992: 4-5. "Rc = 3.86 × 10−11 cm
Rqmc = 6.64 × 10−11 cm
Rqmc = 6.70 × 10−11 cm"
4–7 × 10−13 m
Pauling, Linus. College Chemistry. San Francisco: Freeman, 1964: 57, 4-5. "The radius of the electron has not been determined exactly but it is known to be less than 1 × 10−13 cm" < 10−15 m
"Ro = 2.82 × 10−13 cm" 2.82 × 10−15 m

An electron is a negatively charged subatomic particle. They are responsible for the formation of chemical compounds. Electrons are considered to be fundamental units of matter (they are not made up of smaller units). Although scientists have been studying electrons for quite a while, the exact diameter of an electron is unknown. According to Malcolm H. Mac Gregor,

The electron is a point-like particle-that is, a particle with no measurable dimensions, at least within the limitations of present-day instrumentation. However, a rather compelling case can be made for an opposing viewpoint: namely, that the electron is in fact a large particle which contains an embedded point-like charge.

The electron was the first subatomic particle to be discovered. It was discovered in 1897 by a British physicist named Sir Joseph John Thomson. Later, in 1913, an American Physicist by the name of Robert A. Millikan obtained an accurate measurement of the electron's charge. Recent studies show that the charge of an electron is 1.60218 × 10−19 coulombs. The mass of an electron is known to be 9.10939 × 10−31 kilograms.

In all neutral atoms there are the same number of electrons as protons. Electrons are bound to the nucleus by electrostatic forces. Most of the volume of an atom is occupied by electrons, even though they barely contribute to the atomic mass. Neils Bohr, Wolfgang Pauli, and others discovered the pattern in which electrons are distributed throughout an atom in the 1920s. Electrons are arranged at various distances from the nucleus, and are arranged in energy levels called shells. The average distance of outer electrons from the nucleus is a few tens of nanometers in all atoms. In heavy atoms inner electrons are much closer to the nucleus. The number of electrons in outermost shell determines the chemical behavior of that atom. If an atom combines with another atom to form a molecule, than the electrons in the outermost shell are transferred from one atom to another or shared between atoms.

Danny Donohue -- 2000