Absolutely Small - Michael D. Fayer [144]
light quanta—A single particle of light. A phonon.
lone pair—A pair of electrons in a molecule that occupies an atomic orbital but does not participate in bonding. Lone pair electrons are not shared between atoms.
molecular orbital—A wavefunction for a molecule composed of a combination of atomic orbitals (atomic wavefunctions) that span the molecule. Molecular orbitals can be bonding (bonding MO). Electrons in bonding MOs make the energy of the molecule lower. Molecular orbitals can be antibonding (antibonding MO). Electrons in antibonding MOs increase the energy of a molecule. To have a stable molecule, there must be more electrons in bonding MOs than in antibonding MOs.
momentum eigenstate—The state of a particle with perfectly defined momentum. A momentum eigenstate of a free particle, like a photon or electron, has a wavefunction that is delocalized over all space. The momentum can be known exactly but the position is completely uncertain. Momentum eigenstates can be superimposed (added together) to make a wave packet that has a more or less well-defined position.
nanometer—A unit of length that is one billionth of a meter, 10-9 m.
node—For a one-dimensional wave, a point where the amplitude of the wave is zero. For a three-dimensional wave, a node is a plane or other shaped surface where the wave amplitude is zero. The sign of the wavefunction changes when a node is crossed. In quantum mechanics, a node in a wavefunction describing a particle, such as an electron, is a place where the probability of finding the particle is zero.
optical transition—The change in state from one energy level to another in an atom or molecule caused by the absorption or emission of light.
orbital—Another name for the quantum mechanical wavefunction that describes an electron or pair of electrons in an atom or molecule. An atom has atomic orbitals, and a molecule has molecular orbitals.
particle in a box—A quantum mechanical problem in which a particle, such as an electron, is confined to a one-dimensional box with infinitely high and impenetrable walls. The energy levels of a particle in a box are quantized, that is, there are discreet energy levels. The particle in a box is the simplest quantum mechanical problem in which a particle is confined to a small region of space and has quantized energy levels.
Pauli Exclusion Principle—The principle that at most two electrons can be in an atomic or molecular orbital. If two electrons are in the same orbital, they must have opposite spins, that is, different electron quantum numbers s (one +1/2 and the other - 1/2). The Pauli Exclusion Principle is important in determining the structure of the Periodic Table of Elements and the properties of atoms and molecules.
phase—The position along one cycle of a wave. The peak of the wave (point of maximum positive amplitude) is taken to have a phase of 0 degrees (0°), then the first node (point where the amplitude is zero) is 90°. 90° is a quarter of a cycle of a wave. A phase of 180° is one half of a cycle. It is the point of maximum negative amplitude. Two waves of the same wavelength are said to be phase shifted if the peaks don’t line up.
photoelectric effect—The effect explained by Einstein in which a single particle of light, a photon, can eject a single electron from a piece of metal. Einstein’s explanation of the photoelectric effect showed that light is not a wave as described by classical electromagnetic theory.
photon—A particle of light.
Planck’s constant—The fundamental constant of quantum theory. It is designated by the letter h. It appears in many of the mathematical equations found in quantum mechanics. For example, E = hν says that the energy is the frequency ν (Greek letter nu) multiplied by Planck’s constant. Planck’s constant has the value h = 6.6 × 10-34 J s (Joule times seconds). Planck introduced the constant in 1900 in his explanation of black body radiation.
potential energy well—A region in space where energy is lowered because of some type of attractive