Glossary: Key Terms in Atomic Structure

Term	Definition
Photon	A quantum (packet) of electromagnetic energy; E = hν
Work function (φ)	Minimum energy needed to eject an electron from a metal surface; φ = hν_{0}
Threshold frequency (ν_{0})	Minimum frequency of light required for photoelectric emission
Principal quantum number (n)	Integer (1,2,3,...) describing shell, size, and energy of electron
Azimuthal quantum number (l)	Integer (0 to n−1) describing subshell and orbital shape
Magnetic quantum number (mₗ)	Integer (−l to +l) describing orbital orientation in space
Spin quantum number (mₛ)	±½; describes intrinsic spin direction of electron
Orbital	Region of space where probability of finding electron is ≥ 90%; described by n, l, mₗ
Node	Point/region where wave function (and thus electron probability) = 0
Radial node	Spherical surface at specific distance from nucleus where ψ^{2} = 0
Angular node	Plane or cone through nucleus where ψ^{2} = 0
Degenerate orbitals	Orbitals with same energy (e.g., 2px, 2py, 2pz)
Aufbau principle	Electrons fill orbitals in order of increasing (n+l); lower n first if tie
Pauli exclusion principle	No two electrons in same atom have identical set of all four quantum numbers
Hund's rule	Electrons maximize unpaired spins in degenerate orbitals (maximum multiplicity)
Ground state	Lowest energy configuration of electrons in an atom
Excited state	Configuration where one or more electrons occupy higher-energy orbitals
Hydrogen-like ion	One-electron species (H, $He^{+}$ , $Li^{2+}$ , $Be^{3+}$ ...); Bohr model applies exactly
Wave function (ψ)	Mathematical function describing electron in quantum mechanics; ψ^{2} = probability density
de Broglie wavelength	λ = h/mv; wavelength associated with any moving particle
Stopping potential	Minimum negative voltage to stop photoelectrons; e $V_{0}$ = KE_max
Spectral series	Group of spectral lines with common lower level n_{1} (Lyman, Balmer, etc.)
Rydberg constant (R_H)	$1.097 \times 10^{7}$ $m^{-1}$ ; appears in Rydberg formula for hydrogen spectrum
Isoelectronic species	Atoms/ions having the same number of electrons (e.g., $Na^{+}$ , $Mg^{2+}$ , Ne)