QUANTUM - Smallest unit of some quantity. Plural: quanta.
QUANTUM MECHANICS - Theory which describes the behavior of the very small, such as molecules, atoms, and subatomic particles. Based upon the idea that physical quantities are discrete rather than continuous, and come in discrete bundles of "quanta," this theory has proven spectacularly successful at explaining experimental data. Gravity, however, cannot yet be made to fit within the theory.
QUANTUM NUMBERS - Numbers specifying the exact state of an electron in an atom (principal, angular, magnetic, and spin quantum numbers). The principal quantum number (n = 1, 2, 3...) indicates the distance between the electron and the nucleus. The average distance increases with n, and hence quantum states with different principal quantum numbers are said to belong to different shells. In spectrographic notation the shells are labeled K (n = 1), L (n = 2), M (n = 3), N (n = 4), etc. The maximum number of electrons in a shell is 2n2, thus a shell with n = 2 may hold a maximum of 8 electrons.
The azimuthal quantum number (l = 0, 1 ... n-1) (also known as the angular quantum number or orbital quantum number) specifies the shape of an atomic orbital and strongly influences chemical bonds and bond angles. In spectrographic notation, l = 0 (spherical shape) is called an s orbital; l = 1 (dumbbell shape) is a p orbital; l = 2 (cloverleaf shape) is a d orbital; and l = 3 is an f orbital (complex shape).
The magnetic quantum number (ml = -l, -l+1 ... 0 ... l-1, l) specifies orientation in space. There is only one way in which a sphere (l = 0) can be oriented in space. Orbitals that have polar (l = 1) or cloverleaf (l = 2) shapes, however, can point in different directions.
To distinguish between the two electrons in each orbital, we need a fourth quantum number: the spin quantum number (ms = -½ or +½) was found experimentally from spectroscopy. In essence, one can imagine that electrons can spin either clockwise or counterclockwise. Electron shells of different levels (n) may have overlapping energies. Particles in the nucleus of an atom may also be specified using a similar set of quantum numbers.
QUANTUM TUNNELING - Quantum-mechanical effect of transitioning through a classically-forbidden energy state. It can be generalized to other types of classically-forbidden transitions as well. Also called "barrier penetration" or "tunneling," quantum tunneling is the means by which many fusion reactions take place in stars. This effect is due to the Heisenberg Uncertainty Principle; even if particles or nuclei lack the energy to pass an energy barrier there is a very small probability that they will pass through the barrier. However, this process happens with very small probability, and the Coulomb barrier represents a strong hindrance to nuclear reactions in stars.
QUARKS - Six fundamental particles with fractional charge that combine in pairs to produces mesons and in threes to produce baryons. Each quark type is called a "flavor": up (+2/3), down (-1/3), charm (+2/3), strange (-1/3), top (+2/3), and bottom (-1/3). Quarks are tiny (<10-19 m) and only exist inside hadrons because they are confined by the strong (or color charge) force fields. Therefore, it is not possible to measure their mass by isolating them. Examples of quark combinations are the proton (uud) and neutron (ddu).