- amplitude
- extent of the displacement caused by a wave (for sinusoidal waves, it is one-half the difference from the peak height to the trough depth, and the intensity is proportional to the square of the amplitude)

- angular momentum quantum number (
*l*) - quantum number distinguishing the different shapes of orbitals; it is also a measure of the orbital angular momentum

- atomic orbital
- mathematical function that describes the behavior of an electron in an atom (also called the wavefunction), it can be used to find the probability of locating an electron in a specific region around the nucleus, as well as other dynamical variables

- Aufbau principle
- procedure in which the electron configuration of the elements is determined by “building” them in order of atomic numbers, adding one proton to the nucleus and one electron to the proper subshell at a time

- blackbody
- idealized perfect absorber of all incident electromagnetic radiation; such bodies emit electromagnetic radiation in characteristic continuous spectra called blackbody radiation

- Bohr’s model of the hydrogen atom
- structural model in which an electron moves around the nucleus only in circular orbits, each with a specific allowed radius; the orbiting electron does not normally emit electromagnetic radiation, but does so when changing from one orbit to another.

- continuous spectrum
- electromagnetic radiation given off in an unbroken series of wavelengths (e.g., white light from the sun)

- core electron
- electron in an atom that occupies the orbitals of the inner shells

- covalent radius
- one-half the distance between the nuclei of two identical atoms when they are joined by a covalent bond

*d*orbital- region of space with high electron density that is either four lobed or contains a dumbbell and torus shape; describes orbitals with
*l*= 2. An electron in this orbital is called a*d*electron

- degenerate orbitals
- orbitals that have the same energy

- effective nuclear charge
- charge that leads to the Coulomb force exerted by the nucleus on an electron, calculated as the nuclear charge minus shielding

- electromagnetic radiation
- energy transmitted by waves that have an electric-field component and a magnetic-field component

- electromagnetic spectrum
- range of energies that electromagnetic radiation can comprise, including radio, microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays; since electromagnetic radiation energy is proportional to the frequency and inversely proportional to the wavelength, the spectrum can also be specified by ranges of frequencies or wavelengths

- electron affinity
- energy required to add an electron to a gaseous atom to form an anion

- electron configuration
- electronic structure of an atom in its ground state given as a listing of the orbitals occupied by the electrons

- electron density
- a measure of the probability of locating an electron in a particular region of space, it is equal to the squared absolute value of the wave function
*ψ*

- excited state
- state having an energy greater than the ground-state energy

*f*orbital- multilobed region of space with high electron density, describes orbitals with
*l*= 3. An electron in this orbital is called an*f*electron

- frequency (
*ν*) - number of wave cycles (peaks or troughs) that pass a specified point in space per unit time

- ground state
- state in which the electrons in an atom, ion, or molecule have the lowest energy possible

- Heisenberg uncertainty principle
- rule stating that it is impossible to exactly determine both certain conjugate dynamical properties such as the momentum and the position of a particle at the same time. The uncertainty principle is a consequence of quantum particles exhibiting wave–particle duality

- hertz (Hz)
- the unit of frequency, which is the number of cycles per second, s
^{−1}

- Hund’s rule
- every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin

- intensity
- property of wave-propagated energy related to the amplitude of the wave, such as brightness of light or loudness of sound

- interference pattern
- pattern typically consisting of alternating bright and dark fringes; it results from constructive and destructive interference of waves

- ionization energy
- energy required to remove an electron from a gaseous atom or ion. The associated number (e.g., second ionization energy) corresponds to the charge of the ion produced (X
^{2+})

- isoelectronic
- group of ions or atoms that have identical electron configurations

- line spectrum
- electromagnetic radiation emitted at discrete wavelengths by a specific atom (or atoms) in an excited state

- magnetic quantum number (
*m*)_{l} - quantum number signifying the orientation of an atomic orbital around the nucleus; orbitals having different values of
*m*but the same subshell value of_{l}*l*have the same energy (are degenerate), but this degeneracy can be removed by application of an external magnetic field

- node
- any point of a standing wave with zero amplitude

- orbital diagram
- pictorial representation of the electron configuration showing each orbital as a box and each electron as an arrow

*p*orbital- dumbbell-shaped region of space with high electron density, describes orbitals with
*l*= 1. An electron in this orbital is called a*p*electron

- Pauli exclusion principle
- specifies that no two electrons in an atom can have the same value for all four quantum numbers

- photon
- smallest possible packet of electromagnetic radiation, a particle of light

- principal quantum number (
*n*) - quantum number specifying the shell an electron occupies in an atom

- quantization
- occurring only in specific discrete values, not continuous

- quantum mechanics
- field of study that includes quantization of energy, wave-particle duality, and the Heisenberg uncertainty principle to describe matter

- quantum number
- integer number having only specific allowed values and used to characterize the arrangement of electrons in an atom

*s*orbital- spherical region of space with high electron density, describes orbitals with
*l*= 0. An electron in this orbital is called an*s*electron

- shell
- set of orbitals with the same principal quantum number,
*n*

- spin quantum number (
*m*)_{s} - number specifying the electron spin direction, either $+\phantom{\rule{0.2em}{0ex}}\frac{1}{2}$ or $-\phantom{\rule{0.2em}{0ex}}\frac{1}{2}$

- standing wave
- (also, stationary wave) localized wave phenomenon characterized by discrete wavelengths determined by the boundary conditions used to generate the waves; standing waves are inherently quantized

- subshell
- set of orbitals in an atom with the same values of
*n*and*l*

- valence electrons
- electrons in the outermost or valence shell (highest value of
*n*) of a ground-state atom; determine how an element reacts

- valence shell
- outermost shell of electrons in a ground-state atom; for main group elements, the orbitals with the highest
*n*level (*s*and*p*subshells) are in the valence shell, while for transition metals, the highest energy*s*and*d*subshells make up the valence shell and for inner transition elements, the highest*s*,*d,*and*f*subshells are included

- wave
- oscillation that can transport energy from one point to another in space

- wave-particle duality
- term used to describe the fact that elementary particles including matter exhibit properties of both particles (including localized position, momentum) and waves (including nonlocalization, wavelength, frequency)

- wavefunction (
*ψ*) - mathematical description of an atomic orbital that describes the shape of the orbital; it can be used to calculate the probability of finding the electron at any given location in the orbital, as well as dynamical variables such as the energy and the angular momentum

- wavelength (
*λ*) - distance between two consecutive peaks or troughs in a wave