An electron is a stable and negatively charged subatomic particle that also acts as the carrier of electricity. Each electron carries one unit of negative charge (1.602 x 10-19 coulomb) and has a mass of just about 1/1836th of a proton. Electrons are found both not permanently attached to atoms and within the nucleus.
Quantum mechanics states that electrons can not be distinguished on the basis of any intrinsic property, so all electrons have the same mass, the same electric charge, and the same spin, so they can freely interchange their positions within a system without causing a noticeable change.
Who discovered electrons?
The possibility of electrons was predicted by Richard Laming (1838-1851), and other scientists. Irish physicist G. Johnstone Stoney (1874) coined the term ‘electron’ in 1891, to refer to the unit of charge in his experiments. In 1897, English physicist Joseph John Thomson discovered electrons while conducting experiments with cathode-ray tubes. He called electrons "corpuscles".
Thomson directed cathode rays between two parallel aluminum plates to the end of a tube, where they could be observed as luminescence on the glass. When the top aluminum plate was negative, the rays moved down; when the top plate was positive, the rays moved up. This deflection was proportional to the difference in potential between the plates, demonstrating that cathode rays were negatively charged particles.
From this, Thomson made the following hypotheses:
- Cathode rays are made up of negatively charged particles also known as corpuscles.
- Corpuscles are elementary particles.
- Other than corpuscles, an atom doesn’t have any constituent particles.
Today, we know that the third hypothesis is not accurate, but this discovery of the electron revolutionized physics and paved the way for developments concerning electricity, gravitation, electromagnetism, thermal conductivity, and many other areas. For his work, Thomson was awarded the 1906 Nobel Prize in Physics.
Prior to Thomson, scientists such as Richard Fleming had previously predicted the possible existence of electrons. The ancient Greeks are said to have discovered that when amber is rubbed with fur, it attracts small objects. The Greek word for amber, ‘elektron’ was used for the force that caused this attraction.
Electron symbol, charge, and other subatomic particles
Protons and electrons have equal, but opposite charges. Electrons are attracted to positively charged particles, such as protons. Whether or not a substance has a net electric charge is determined by the balance between the number of electrons and the positive charge of atomic nuclei. If there are more electrons than positive charges, a material is said to be negatively charged. If there is an excess of protons, the object is considered to be positively charged. If the number of electrons and protons is balanced, a material is said to be electrically neutral.
The radius of an electron is approximately 2 x 10-10 cm. Neutrons and protons, together known as nucleons, form 99.9% of the total atomic mass of an atom, and as compared to these particles, electrons have negligible mass value, therefore, the mass of electrons is not considered when the mass number of an atom is calculated.
The symbol for an electron is ‘e–’ and for proton is ‘p+’ but, interestingly, protons are not the true antiparticles to electrons. The antiparticle of the electron is the positron, which has an electric charge of +1 e, a spin of 1/2 (the same as the electron), and has the same mass as an electron.
Positrons are not found in nature but are formed during the decay of nuclides that have an excess of protons in their nucleus. When decaying takes place, these radionuclides emit a positron and a neutrino.
Number of protons, electrons, and neutrons in an atom
For any element, the atomic mass number is the total number of protons and neutrons in the nucleus. It is measured in the atomic mass units (amu).
Atomic Mass Number = (Number of Protons) + (Number of Neutrons)
Whereas, the atomic number is the number of protons only. For example, the atomic number of carbon is six, therefore, carbon has six protons in its nucleus and six electrons in the energy orbits surrounding the nucleus.
Electrons are described as surrounding the nucleus of an atom in shells. These are not actual structures but are regions of probability.
Atomic Number = Number of Protons
However, in the case of charged atoms also known as ions, the number of protons and electrons differ and depends on the charge on the atom. The number of neutrons for an atom can be easily calculated by subtracting the number of protons from the total atomic mass number.
Number of Neutrons = Atomic Mass Number - Number of Protons
The nature of the electric charge on any substance is defined by the number of protons and electrons in its nuclei. If the number of protons exceeds the number of electrons, then the substance is positively charged. Where there are more electrons than protons, the substance is said to have an overall negative charge. Any substance is said to be balanced or electrically neutral when the number of protons and electrons is equal.
The wave nature of electrons
French physicist Louis De Broglie proposed the wave nature of electrons in his 1924 Ph.D. thesis. He stated that if light and radiation can show dual behavior, then the matter can also exist as both particle and wave.
De Broglie was influenced by Albert Einstein’s theory of relativity and the photoelectric effect. Twenty years earlier, Einstein has proposed the idea that matter on the atomic scale might exhibit the properties of a wave and a particle. This idea of the dual nature of light was just beginning to gain scientific acceptance when de Broglie extended the idea to include matter.
According to De Broglie’s hypothesis, any moving object, whether macroscopic or microscopic has its own wavelength, and this wavelength is inversely proportional to the size of the object.
In the years that followed, the American physicists, Clinton Davisson and Lester Germer conducted electron diffraction experiments that further confirmed the dual nature of matter given by De Broglie. In 1929, De Broglie received the Nobel Prize in Physics for his exceptional contribution to quantum physics.
Bonus: interesting facts about electrons
- Electrons are elementary particles, which means they can not be divided further. However, if it has been shown that a mass of electrons artificially confined in a small space at temperatures close to absolute zero, will split into quasiparticles called spinons and holons. A third quasi-particle, the orbiton, has also been found. The orbiton is a collective excitation of electrons in a 1D solid that behaves just like an electron – with orbital angular momentum but with no spin or electric charge.
- The world of quantum physics witnessed a historical experiment in the year 2018, The scientists at Imperial College, London managed to stop an electron on its path with the help of a Gemini Laser, equipment with brightness a million times higher than that of the sun’s surface.
- Bonding between individual atoms occurs when the electrons of one atom interact with the electrons present in the outer shell of another atom and form bonds either by sharing or donating electrons.
- Electricity is the result of electrons becoming exchanged in a stream of charged particles by the means of a conducting medium such as metal. This stream is also known as electric current.
- The electron microscope uses a beam of electrons to image objects. The specimen is specially prepared and placed inside a vacuum chamber and instead of lenses, a series of coil-shaped electromagnets are used. The coils bend the electron beams to produce magnification between 1 and 50 million times.
- In 2019, the researchers at Lawrence Berkeley National Laboratory created a superfast electron detector to detect defects occurring at atomic levels. This device is equipped with a 4D camera and can process 4TB of data per minute.