What is an Electron | Definition, Discovery, Examples

Definition of Electron

An electron is a negatively charged subatomic particle. It can be either free (not attached to any atom) or bound to the nucleus of an atom.

what is electron

Electrons in atoms exist in spherical shells of various radii, representing energy levels. The larger the spherical shell, the higher the energy contained in the electron.

Charge on Electron

charge on electron

The charge on a single electron is considered as the unit electrical charge. It is assigned negative polarity. The charge on an electron is equal but opposite to the positive charge on a proton or hole.

Electrical charge quantity is not usually measured in terms of the charge on a single electron because this is an extremely small charge. Instead, the standard unit of electrical charge quantity is the coulomb, symbolized by C, representing about 6.24 x 1018 electrons.

The electron charge, symbolized by e, is about 1.60 x 10-19C. The mass of an electron at rest symbolized me, is approximately 9.11 x 10-31 kilogram (kg).

Electrons moving at an appreciable fraction of the speed of light, for example, in a particle accelerator, have greater mass because of relativistic effects.

Discovery of Electron

discovery of electron

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.

Cathode Ray Experiment

cathode ray experiment

J. J. Thomson constructed a glass tube that was partially evacuated, i.e., much of the air was pumped out of the tube. Then he applied a high electrical voltage between two electrodes at either end of the tube.

He detected that a stream of the particle (ray) was coming out from the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray, and the whole construction is called a cathode ray tube.

Properties of Cathode Ray Particle

  • They travel in straight lines.
  • They are independent of the material composition of the cathode.
  • Applying an electric field in the path of the cathode ray deflects the ray towards the positively charged plate. Hence cathode-ray consists of negatively charged particles.

Electron Configuration

electron configuration

Electron configuration tells us how the electrons are arranged around the nucleus in energy levels or shells. The electrons in an atom occupy the lowest available energy levels that are the innermost available shells.

  1. The first level or shell can only contain a maximum of 2 electrons.
  2. The energy levels or shells after this can hold a maximum of 8 electrons.
  3. Electronic configuration is written as a series of numbers. For example, oxygen is 2, 6, and aluminum is 2, 8, 3.

The periodic table arranges the elements in terms of their electronic structure. Elements in the same group have the same number of electrons in their outermost shell. Therefore, they have similar properties.

From left to right, across each period, a particular energy level is gradually filled with electrons. In the next period, the next energy level is filled, etc.

Characteristics of Electrons

  1. The smallest unit in the composition of matter is the atom. The atom is composed of a nucleus at the center and one or more electrons orbiting around the nucleus. The nucleus consists of protons and neutrons, collectively called nucleons. The protons are positively charged particles with a mass of 1.00728 amu, and the neutrons are electrically neutral particles with a mass of 1.00867 amu.
  2. The electrons are negatively charged particles with a mass of 0.000549 amu. The protons and neutrons are about 1836 times heavier than the electrons. The number of electrons is equal to the number of protons, thus resulting in a neutral atom of an element. The electronic arrangements determine the chemical properties of an element, whereas the nuclear structure dictates the stability and radioactive transformation of the atom.

Importance of Electron

  1. Electrons are also important to electricity. Electricity is basically the exchange of electrons in a stream called a current through a conducting medium. In most cases, the medium is an acid, metal, or similar conductor. In the case of static electricity, a stream of electrons travels through the medium of air.
  2. Understanding the electron has allowed for a better understanding of some of the most important forces in our universe, such as the electromagnetic force. Understanding its workings has allowed scientists to work out concepts such as potential differences and the relationship between electrical and magnetic fields.
  3. Electrons are also important for the bonding of individual atoms together. Without this bonding force between atoms, the matter would not interact in the many reactions and forms we see every day. This interaction between the outer electron layers of an atom is called atomic bonding. It can occur in two forms. One is covalent bonding, where atoms share electrons in their outer orbits. The other is ionic bonding, where an atom gives up electrons to another atom. In either case, bonding must meet specific rules. We won’t go into great detail, but each electron orbit or energy level can only hold so many electrons. Atoms can only bond if there is room to share or receive extra electrons on the outermost orbit of the atom.

Rest Mass of Electron

rest mass of electrons

The electron rest mass (symbol: me) is the mass of a stationary electron, also known as the electron’s invariant mass. It is one of the fundamental constants of physics. It has a value of about 9.109×10-31 kilograms or about 5.486×10-4 Dalton’s, equivalent to an energy of about 8.187×10-14 joules or about 0.5110 MeV.


  • Electrons are one of three main types of particles that make up atoms, consisting of smaller, simpler particles. Electrons are fundamental particles that do not consist of smaller particles. They are a type of fundamental particle called leptons. All leptons have an electric charge of −1 or 0. Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of −1 , which is equal but opposite to the charge of a proton, which is +1 . All atoms have the same number of electrons as protons, so the positive and negative charges “cancel out,” making atoms electrically neutral.
  • Electrons are found outside the nucleus. Because opposite electric charges attract one another, negative electrons are attracted to the positive nucleus. This force of attraction keeps electrons constantly moving through the otherwise space around the nucleus. The figure below is a common way to represent the structure of an atom. It shows the electron as a particle orbiting the nucleus, similar to how planets orbit the sun. However, this is an incorrect perspective, as quantum mechanics demonstrates that electrons are more complicated.

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