In the physical sciences, subatomic particles are the particles smaller than an atom.[1] (although some subatomic particles have mass greater than some atoms). There are two types of subatomic particles: elementary particles, which according to current theories are not made of other particles; and composite particles.[2] Particle physics and nuclear physics study these particles and how they interact.[3]

The elementary particles of the Standard Model include:[4]

Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles.

Composite subatomic particles (such as protons or atomic nuclei) are bound states of two or more elementary particles. For example, a proton is made of two up quarks and one down quark, while the atomic nucleus of helium-4 is composed of two protons and two neutrons. Composite particles include all hadrons: these include baryons (such as protons and neutrons) and mesons (such as pions and kaons).


In particle physics, the concept of a particle is one of several concepts inherited from classical physics. This describes the world we experience, and (for example) describes how matter and energy behave at the molecular scales of quantum mechanics. For physicists, the word "particle" means something rather different from the common meaning of the term, reflecting the modern understanding that at the quantum scale particles behave very differently from what much of everyday experience would lead us to expect.

The idea of a particle underwent serious rethinking in light of experiments that showed that light could behave like a stream of particles (called photons) as well as exhibit wave-like properties. This led to the new concept of wave-particle duality to reflect that quantum-scale "particles" behave like both particles and waves. Another new concept, the uncertainty principle, states that analysis of particles at these scales requires a statistical approach. In more recent times, wave-particle duality has been shown to apply not only to photons but to increasingly massive particles.[5]

All of these factors ultimately combined to replace the notion of discrete "particles" with the concept of "wave-packets" of uncertain boundaries, whose properties are known only in terms of probabilities, and whose interactions with other "particles" remain largely a mystery, even 80 years after the establishment of quantum mechanics.


In Einstein's hypotheses, energy and mass are analogous: the energy of a particle equals its mass times the speed of light squared(E = mc^2 \,\!). That is, mass can be expressed in terms of energy and vice versa. Consequently, there are only two known mechanisms by which energy can be transferred. These are particles and waves. For example, light can be expressed as both particles and waves. This paradox is known as the Wave–particle Duality Paradox.[6]

Through the work of Albert Einstein, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature.[7] This has been verified not only for elementary particles but also for compound particles like atoms and even molecules. In fact, according to traditional formulations of non-relativistic quantum mechanics, wave–particle duality applies to all objects, even macroscopic ones; although wave properties of macroscopic objects cannot be detected due to their small wavelengths.[8]

Interactions between particles have been scrutinized for many centuries, and a few simple laws underpin how particles behave in collisions and interactions. The most fundamental of these are the laws of conservation of energy and conservation of momentum, which let us make calculations of particle interactions on scales of magnitude that range from stars to quarks.[9] These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in Philosophiae Naturalis Principia Mathematica, originally published in 1687.

Dividing an atom

The negatively-charged electron has a mass equal to 11836 of that of a hydrogen atom. The remainder of the hydrogen atom's mass comes from the positively charged proton. The atomic number of an element is the number of protons in its nucleus. Neutrons are neutral particles having a mass slightly greater than that of the proton. Different isotopes of the same element contain the same number of protons but differing numbers of neutrons. The mass number of an isotope is the total number of nucleons (neutrons and protons collectively).

Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and molecules. Nuclear physics deals with how protons and neutrons arrange themselves in nuclei. The study of subatomic particles, atoms and molecules, and their structure and interactions, requires quantum mechanics. Analyzing processes that change the numbers and types of particles requires quantum field theory. The study of subatomic particles per se is called particle physics. Since most varieties of particle occur only as a result of cosmic rays, or in particle accelerators, particle physics is also called high-energy physics.


In 1905, Albert Einstein demonstrated the physical reality of the photons, hypothesized by Max Planck in 1900, in order to solve the problem of black body radiation in thermodynamics.

In 1874, G. Johnstone Stoney postulated a minimum unit of electrical charge, for which he suggested the name electron in 1891.[10] In 1897, J. J. Thomson confirmed Stoney's conjecture by discovering the first subatomic particle, the electron (now denoted e). Subsequent speculation about the structure of atoms was severely constrained by Ernest Rutherford's 1907 gold foil experiment, showing that the atom is mainly empty space, with almost all its mass concentrated in a (relatively) tiny atomic nucleus. The development of the quantum theory led to the understanding of chemistry in terms of the arrangement of electrons in the mostly empty volume of atoms. In 1918, Rutherford confirmed that the hydrogen nucleus was a particle with a positive charge, which he named the proton, now denoted p+. Rutherford also conjectured that all nuclei other than hydrogen contain chargeless particles, which he named the neutron. It is now denoted n. James Chadwick discovered the neutron in 1932. The word nucleon denotes neutrons and protons collectively.

Neutrinos were postulated in 1931 by Wolfgang Pauli (and named by Enrico Fermi) to be produced in beta decays of neutrons, but were not discovered until 1956. Pions were postulated by Hideki Yukawa as mediators of the residual strong force, which binds the nucleus together. The muon was discovered in 1936 by Carl D. Anderson, and initially mistaken for the pion. In the 1950s the first kaons were discovered in cosmic rays.

The development of new particle accelerators and particle detectors in the 1950s led to the discovery of a huge variety of hadrons, prompting Wolfgang Pauli's remark: "Had I foreseen this, I would have gone into botany". The classification of hadrons through the quark model in 1961 was the beginning of the golden age of modern particle physics, which culminated in the completion of the unified theory called the standard model in the 1970s. The discovery of the weak gauge bosons through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics. As of early 2012, of all the particles in the Standard Model, only the existence of the Higgs boson remained to be verified. On July 4th, 2012, CERN announced the discovery of a new particle, compatible with the Standard Model Higgs boson, through experiments conducted with the Large Hadron Collider. Further experiments continued and in March 2013 it was tentatively confirmed that the newly discovered particle was a Higgs Boson.

See also

Physics portal


Further reading

General readers
  • Feynman, R.P. & Weinberg, S. (1987). Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures. Cambridge Univ. Press.
  • Oerter, Robert (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume.
  • Schumm, Bruce A. (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. John Hopkins Univ. Press. ISBN 0-8018-7971-X.
  • Coughlan, G. D., J. E. Dodd, and B. M. Gripaios (2006). The Ideas of Particle Physics: An Introduction for Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.

External links

  • The Standard Model.
  • Particle chart.
  • University of California: Particle Data Group.
  • Annotated Physics Encyclopædia: Quantum Field Theory.
  • Jose Galvez: Chapter 1 Electrodynamics (pdf).