Inside an atom there are two main things, the nucleus and the electrons.
The nucleus lies in the center of the atom, it is composed of protons(p) and neutrons(n).
The electrons orbit the atom's nucleus and along with protons and neutrons, make up atoms and molecules.
Here is a diagram of inside a carbon atom:
All the particles (protons, neutrons, quarks, and electrons) are constantly in motion.
But protons and neutrons are not fundamental -- they are comprised of more fundamental particles called quarks.
While we know for sure that quarks and electrons are smaller than 10 to the power of (-18) meters, it is possible that they have no size at all. It is also possible that quarks and electrons are not fundamental but are composites of more fundamental particles.
Physicists look for undiscovered particles in order to understand how the universe works. They always wonder if the new particles, as well as known particles, are truly fundamental.
Physicists have discovered about 200 particles (most of which are not fundamental.) To keep track of these particles, they name them with letters from the Roman and Greek alphabets.
Matter Particles:
The Standard Model says that most matter particles we know of are actually composites of more fundamental particles called quarks. There is also another class of fundamental matter particles called leptons (an example of a lepton is the electron).
What makes the Standard Model so comprehensive is that all observed particles can be explained by:
There are six leptons, three of which have electrical charge and three of which don't. The best known charged lepton is the electron. The other two charged leptons are the muon and the tau, which are essentially electrons with a lot more mass. The charged leptons are all negative.
The other three leptons are the very elusive neutrinos. They have no electrical charge and little, if any, mass. There is one type of neutrino for every type of electrically charged lepton.
Leptons are independent lonely particles, they can exist without the companionship of other particles. Quarks, on the other hand, are only found in groups.
Because it has no charge and has negligible mass, the neutrino is extremely elusive; through the measurement of its recoil effect, research confirmed its peculiar properties. Conclusive proof of its existence was obtained in 1956 by the American physicists Frederick Reines and Clyde Lorrain Cowan, Jr…
The antiparticle of the neutrino is emitted in electron beta decay, whereas the neutrino is emitted in positron beta decay. Some physicists have theorized that in a rare form of radioactivity called double beta decay, two neutrinos may sometimes merge to form a particle called a majoron. Another type of high-energy neutrino, called the muon neutrino, is produced, along with a muon, in the decay of a pion (see Elementary Particles). When a pion decays, a neutral particle must be emitted in the direction opposite that of the muon in order to conserve momentum. The original assumption was that this particle was the neutrino that conserves momentum in beta decay. In 1962, however, researchers proved that the neutrino accompanying pion decay is different. A third type of neutrino, the tau neutrino (and its antiparticle), also exists.
Of great current interest is the possible existence of oscillating neutrinos, that is, the possibility that neutrinos can oscillate from one form to another. In 1996 physicists at Los Alamos National Laboratory detected the oscillation of a muon antineutrino into an electron antineutrino, suggesting that the neutrino had a rest mass of as yet undetermined value. This has profound implications for the physical sciences and cosmology. If the mass is greater than 25 electron volts (about 20,000 times lighter than an electron), the universe will not continue to expand but will eventually halt due to gravitation attractions and will begin to contract.
Quarks were first classified as three kinds: up, down, and strange. The proton, for example, is believed to be constituted of two up quarks and one down quark. Later theorists postulated the existence of a fourth quark; in 1974 the existence of this quark, named charm, was experimentally confirmed. Thereafter a fifth and sixth quark-called bottom and top, respectively-were hypothesized for theoretical reasons of symmetry. Experimental evidence for the existence of the bottom quark was obtained in 1977. The top quark eluded researchers until March 1995, when two teams of physicists at Fermi National Accelerator Laboratory (Fermilab) announced they had detected and measured the top quark.
Each kind of quark has its antiparticle, and each kind of quark or antiquark comes in three types of "colors," (see Color Charge & Confinement.) Quarks can be either red, blue, or green, while antiquarks can be either antired, antiblue, or antigreen. These quark and antiquark colors have nothing to do with the colors seen by the human eye. Rather, these colors represent a quantum property. When combining to form hadrons, quarks and antiquarks can only exist in certain color groupings. The hypothetical carrier of the force between quarks is called the gluon.
Quarks have the unusual characteristic of having fractional electric charge of either 2/3 or -1/3, unlike the -1 charge of an electron and the +1 charge of the proton. Quarks also carry another type of charge called color charge, which we will discuss later.
Quarks only exist in groups with other quarks. Individual quarks have fractional electric charges. However, these fractional charges are never directly observed because quarks never hang out alone; instead, quarks will form composite particles called hadrons. The sum of the quarks' electric charges in a hadron is always an integer number. While individual quarks carry color charge, hadrons are color-neutral.
On March 2, 1995, Fermilab announced the discovery of the top quark, the last of the six predicted quarks. The search began in 1977 when physicists found the fifth quark, bottom, at Fermilab. It took this long because the top quark was much more massive than was originally predicted, so it required a more powerful accelerator to create it.
Although the top quark decays too fast to be observed, it does leave behind particles that give record to its existence - a top quark "signature". The top quark can decay in more than one way. Since a top quark appears only once in several billion collisions, it was necessary to perform trillions of collisions.
Physicists still do not understand why the top is so massive. It is 40 times heavier than the next heaviest quark and about 35, 000 times heavier than the up and down quarks that make up most of the matter we see around us. In fact the question still remains why anything has mass at all. Physicists hope that the discovery of the top quark will give them insight to these questions.
There are two classes of hadrons:
Baryons:
Baryons are any hadron made of three quarks (qqq). For example, protons are 2 up quarks and 1 down quark (uud) and neutrons are 1 up and 2 down quarks (udd).
Mesons:
Mesons contain one quark:
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All visible matter in the universe is made from the first generation of matter particles: up and down quarks, and electrons. Second and third generation particles are unstable, and decay into first generation particles. It is for this reason that all the stable matter in the universe is made from first generation particles.
The question then arises, if we almost never observe the higher generations of matter particles in our universe, why do they exist at all? Indeed, when the muon was discovered in 1936, the physicist I.I. Rabi asked,
The existence of spin was suggested by the Dutch-born American physicists Samuel Abraham Goudsmit and George Eugene Uhlenbeck in 1925. The two physicists noted that certain features of the atomic spectra could not be explained by the quantum theory in use at the time; by adding an additional quantum number-the spin of the electron-Goudsmit and Uhlenbeck were able to provide a more complete explanation of atomic spectra (see Spectrum). Soon the idea of spin was extended to all sub-atomic particles, including protons, neutrons, and antiparticles (see Antimatter). Groups of particles, such as an atomic nucleus, also have spin as a result of the spin of the protons and neutrons that make up the nucleus.
Quantum theory prescribes that spin angular momentum can only occur in certain discrete values. These discrete values are described in terms of integer or half-integer multiples of the fundamental angular momentum unit h/2p, where h is Plank's constant. In general usage, stating that a particle has spin 1/2 means that its spin angular momentum is 1/2 (h/2p). Fermions, which include protons, neutrons, and electrons, have odd half-integer spin (1/2, 3/2,…); bosons, such as photons, alpha particles, and mesons, have integer spin (0,1,…). Fermions obey the Pauli exclusion principle, while bosons do not.
Bosons are those particles that have an integer spin measured in the units of h-bar (spin = 0, 1, 2...).
The following are bosons:
The nucleus of an atom is a fermion or boson depending on whether the sum of the number of protons and neutrons is odd or even. This property explains the strange behavior of very cold helium, which is a superfluid (meaning it has no viscosity, among other things) because its nuclei are bosons and may pass through each other.
When a ray of light passes from one transparent medium, such as air, into another, such as glass or water, it is bent; upon reemerging into the air, it is bent again. This bending is called refraction; the amount of refraction depends on the wavelength of the light. Violet light, for example, is bent more than red light in passing from air to glass or from glass to air. A mixture of red and violet light is thus dispersed into the two colors when it passes through a wedge-shaped glass prism.
A device for producing and observing a spectrum visually is called a spectroscope; a device for observing and recording a spectrum photographically is called a spectrograph; a device for measuring the brightness of the various portions of spectra is called a spectrophotometer; and the science of using spectroscopes, spectrographs, and spectrophotometers to study spectra is called spectroscopy. For extremely accurate spectroscopic measurements, an interferometer is used. During the 19th century, scientists discovered that beyond the violet end of the spectrum, radiations could be detected that were invisible to the human eye but that had marked photochemical action; these radiations were termed ultraviolet. Similarly, beyond the red end of the spectrum, infrared radiations were detected that, although invisible, transmitted energy, as shown by their ability to raise the temperature of a thermometer. The definition of spectrum was then revised to include these invisible radiations, and has since been extended to include radio waves beyond the infrared, and X rays and gamma rays beyond the ultraviolet.
The term spectrum is often loosely applied today to any orderly array produced by analysis of a complex phenomenon. A complex sound such as noise, for example, may be analyzed into an audio spectrum of pure tones of various pitches. Similarly, a complex mixture of elements or isotopes of different atomic weights can be separated into an orderly sequence called a mass spectrum in order of their atomic weights.
Spectroscopy has not only provided an important and sensitive method of chemical analysis, but has also been the chief tool for discoveries in the apparently unrelated fields of astrophysics and atomic theory. In general, changes in motions of the outer electrons of atoms produce spectra in the visible, infrared, and ultraviolet regions. Changes in motions of the inner electrons of heavy atoms produce X-ray spectra. Changes in the configurations of the nucleus of an atom produce gamma-ray spectra. Changes in the configurations of molecules produce visible and infrared spectra. See Atom and Atomic Theory.
Different colors of light are similar in consisting of electromagnetic radiations that travel at a speed of approximately 300,000 km per sec (about 186,000 mi per sec). They differ in having varying frequencies and wavelengths, the frequency being equal to the speed of light divided by wavelength. Two rays of light having the same wavelength also have the same frequency and the same color. The wavelength of light is so small that it is conveniently expressed in nanometers, which are equal to one-billionth of a meter, or one-thousandth of a micrometer. The wavelength of violet light varies from about 400 to 450 mµ, and of red light from about 620 to 760 mµ, or from about 0.000016 to 0.000018 in. for violet, and from 0.000025 to 0.000030 in. for red.
The basic unit of electrical current, or flow, is the statampere, which is defined as a current of 1 electrostatic unit per second. The statvolt, the basic unit of electromotive force, or potential difference, is the difference in potential that exists between two points when 1 erg of work is required to force 1 electrostatic unit of electricity between those two points.
The mathematical relationships between the electrostatic and electromagnetic units are as follows: 1 esu equals 3.3356 × 10-11 abcoulombs; 1 statampere equals 3.3356 × 10-11 abamperes; and one statvolt equals 29,979,245,800 abvolts. This last figure is exactly equal to the velocity of light through a vacuum, which is expressed in centimeters per second, as predicted by the electromagnetic-wave theory developed by British physicist James Clerk Maxwell.
In all the SI electrical units, the conventional prefixes of the metric system are used to indicate fractions and multiples of the basic units. Thus, a micromicrofarad is a trillionth of a farad, a microampere is a millionth of an ampere, a millivolt is a thousandth of a volt, a millihenry is a thousandth of a henry, a kilowatt is 1000 watts, and a megohm is 1 million ohms.
The existence of the positron was first suggested in 1928 by the British physicist Paul Adrien Maurice Dirac as a necessary consequence of his quantum-mechanical theory of electron motion. In 1932 the American physicist Carl David Anderson experimentally confirmed the existence of the positron. See Atom and Atomic Theory; Elementary Particles.
Baryons always contain three quarks, and at any time may also contain some gluons and quark-antiquark pairs. A proton = uud and a neutron = udd.
Each quark in a baryon rapidly exchanges color charges with other quarks in that baryon. However, baryon (and all hadrons) have no net color charge because the different color charges cancel each other out.
They have spins of 1/2, 3/2, ... so they are fermions.
For each baryon there is an antimatter baryon (antibaryon) made of the 3 corresponding antiquarks.
The antiproton, the antiparticle of the proton, is also called a negative proton. It differs from the proton in having a negative charge and not being a constituent of atomic nuclei. The antiproton is stable in a vacuum and does not decay spontaneously. When an antiproton collides with a proton or a neutron, however, the two particles are transformed into mesons, which have an extremely short half-life. Although physicists had postulated the existence of this elementary particle since the 1930s, the antiproton was positively identified for the first time in 1955 at the University of California Lawrence Berkeley National Laboratory.
Because protons are essential parts of ordinary matter, they are obviously stable. Particle physicists are nevertheless interested in learning whether protons eventually decay after all, on a time scale of many billions of billions of years. This interest derives from current attempts at grand unification theories that would combine all four fundamental interactions of matter in a single scheme. Many of these attempts call for the ultimate instability of the proton, so research groups at a number of accelerator facilities are conducting tests to detect such decays. By the end of the 1980s no clear evidence had yet been found; possible results thus far can also be interpreted in other ways.
The antiparticle of a neutron, known as an antineutron, has the same mass, spin, and beta-decay constant. These particles are sometimes the result of the collisions of antiprotons with protons, and they possess a magnetic moment equal and opposite to that of the neutron. According to current particle theory, the neutron and the antineutron-and other nuclear particles-are themselves composed of quarks (see Quark).
First used in Europe in the 1930s, neutron radiography has been employed widely in the U.S. since the 1950s for examining the fuel and other components of nuclear reactors. More recently it has been used in examining explosive devices and the components of space vehicles. Beams of neutrons are widely used now in the physical and biological sciences and in technology, and neutron activation analysis is an important tool in such diverse fields as paleontology, archaeology, and art history.
Cohesion in liquids is reflected in the surface tension caused by the unbalanced inward pull on the surface molecules, and also in the transformation of a liquid into a solid state when the molecules are brought sufficiently close together. Cohesion in solids depends on the pattern of distribution of atoms, molecules, and ions, which in turn depends on the state of equilibrium (or lack of it) of the atomic particles. In many organic compounds, which form molecular crystals, for example, the atoms are bound strongly into molecules, but the molecules are bound weakly to each other.
A meson is a color-neutral object, since its quark and antiquark have opposite color charges. Thus, a meson can be found in isolation. All mesons are unstable.
Since a meson has an integer spin, it is a boson. The spin is made of the spins of the quarks, plus a contribution from their motion around each other. For example notice that a pion and a rho have the same quarks, but different spins and masses.