Any of various self-contained units of matter or energy. Discovery of the electron in 1897 and of the atomic nucleus in 1911 established that the atom is actually a composite of a cloud of electrons surrounding a tiny but heavy core. By the early 1930s it was found that the nucleus is composed of even smaller particles, called protons and neutrons. In the early 1970s it was discovered that these particles are made up of several types of even more basic units, named quarks, which, together with several types of leptons, constitute the fundamental building blocks of all matter. A third major group of subatomic particles consists of bosons, which transmit the forces of the universe. More than 200 subatomic particles have been detected so far, and most appear to have a corresponding antiparticle (see antimatter).
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Subatomic particles include the atomic constituents electrons, protons, and neutrons. Protons and neutrons are composite particles, consisting of quarks. A proton contains two up quarks and one down quark, while a neutron consists of one up quark and two down quarks; the quarks are held together in the nucleus by gluons. There are six different types of quark in all ('up', 'down', 'bottom', 'top', 'strange', and 'charm'), as well as other particles including photons and neutrinos which are produced copiously in the sun. Most of the particles that have been discovered are encountered in cosmic rays interacting with matter and are produced by scattering processes in particle accelerators. There are dozens of known subatomic particles.
The idea of a particle is one which had to undergo serious rethinking in light of experiments which showed that the smallest particles (of light) could behave just like waves. The difference is indeed vast, and required the new concept of wave-particle duality to state that quantum-scale "particles" are understood to behave in a way which resembles both particles and waves. Another new concept, the uncertainty principle, meant that analyzing particles at these scales required a statistical approach. All of these factors combined such that the very notion of a discrete "particle" has been ultimately replaced by the concept of something like wave-packet of an uncertain boundary, whose properties are only known as probabilities, and whose interactions with other "particles" remain largely a mystery, even 80 years after quantum mechanics was established.
Through the work of Albert Einstein, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature. This phenomenon 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; we can't detect wave properties of macroscopic objects due to their small wavelengths.
Interactions between particles have been scrutinized for many centuries, and a few simple laws underpin how particles proceed in collisions and interactions. The most angelic of these are the conservation of energy and momentum which facilitate us to elucidate calculations between particle interactions on scales of magnitude which diverge between planets and quarks. These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in Philosophiae Naturalis Principia Mathematica originally published in 1687.
Electrons, which are negatively charged, have a mass of 1/1836 of a hydrogen atom, the remainder of the atom's mass coming from the positively charged proton. The atomic number of an element counts the number of protons. Neutrons are neutral particles with a mass almost equal to that of the proton. Different isotopes of the same nucleus contain the same number of protons but differing numbers of neutrons. The mass number of a nucleus counts the total number of nucleons.
Chemistry concerns itself with the arrangement of electrons in atoms and molecules, and nuclear physics with the arrangement of protons and neutrons in a nucleus. The study of subatomic particles, atoms and molecules, their structure and interactions, involves quantum mechanics and quantum field theory (when dealing with processes that change the number of particles). The study of subatomic particles per se is called particle physics. Since many particles need to be created in high energy particle accelerators or cosmic rays, sometimes particle physics is also called high energy physics.
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. Among the standard model particles the existence of the Higgs boson remains to be verified— this is seen as the primary physics goal of the accelerator called the Large Hadron Collider in CERN. All currently known particles fit into the standard model.