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While every effort has been made to follow citation style rules, there may be some discrepancies.
Please refer to the appropriate style manual or other sources if you have any questions.
electron
, one of the three basic
subatomic particles
—along with
protons
and
neutrons
—that make up atoms, the basic building blocks of all
matter
and
chemistry
. The negatively charged electrons circle an
atom’s
central nucleus, which is formed by positively charged protons and the electrically neutral particles called neutrons. (The nucleus of the ordinary hydrogen atom is an exception, containing only one proton and no neutrons.) Like opposite ends of a
magnet
that attract one another, the negative electrons are attracted to a positive force, which binds them to the nucleus. The nucleus is small and
dense
compared with the electrons, which are the lightest charged particles in nature. The electrons circle the nucleus in
orbital
paths called
shells
, each of which holds only a certain number of electrons.
The electron was discovered in 1897 by the English physicist
J.J. Thomson
during investigations of
cathode rays
. His discovery of electrons, which he initially called corpuscles, played a pivotal role in revolutionizing knowledge of atomic structure. Under ordinary conditions electrons are bound to the positively charged nuclei of
atoms
by the attraction between opposite electric charges. In a neutral atom the number of electrons is identical to the number of positive charges on the nucleus. Any atom, however, may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole; these charged atoms are known as
ions
. Not all electrons are associated with atoms; some occur in a free state with
ions
in the form of matter known as
plasma
.
Within any given atom, electrons move about the nucleus in an orderly arrangement of
orbitals
, the attraction between electrons and nucleus overcoming repulsion among the electrons that would otherwise cause them to fly apart. These orbitals are organized in concentric shells proceeding outward from the nucleus with an increasing number of subshells. The electrons in orbitals closest to the nucleus are held most tightly; those in the outermost orbitals are shielded by intervening electrons and are the most loosely held by the nucleus. As the electrons move about within this structure, they form a
diffuse
cloud of negative charge that occupies nearly the entire volume of the atom. The arrangement of electrons in orbitals and shells around the nucleus is referred to as the
electronic configuration
of the atom. This electronic configuration determines not only the size of an individual atom but also the chemical activity of the atom. The classification of
elements
within groups of similar elements in the
periodic table
, for example, is based on the similarity in their electron structures.
Within the
field
of
particle physics
, there are two ways of classifying electrons. The electron is a
fermion
, a type of particle named after the
Fermi-Dirac statistics
that describe its behaviour. All fermions are characterized by half-integer values of their
spin
, where spin corresponds to the
intrinsic
angular momentum
of the particle. The concept of spin is embodied in the
wave equation
for the electron formulated by
P.A.M. Dirac
. The Dirac wave equation also predicts the existence of the
antimatter
counterpart of the electron, the
positron
. Within the fermion group of subatomic particles, the electron can be further classified as a
lepton
. A lepton is a subatomic particle that reacts only by the
electromagnetic
,
weak
, and
gravitational
forces; it does not respond to the short-range
strong force
that acts between
quarks
and binds protons and neutrons in the atomic nucleus.
The lightest stable
subatomic particle
known, the electron carries a negative charge of 1.602176634 × 10
−19
coulomb
, which is considered the basic unit of
electric charge
. The rest
mass
of the electron is 9.1093837015 × 10
−31
kg
, which is only
1
/
1,836
the mass of a
proton
. An electron is therefore considered nearly massless in comparison with a proton or a
neutron
, and the electron mass is not included in calculating the
mass number
of an atom.
