### amplitude

height of a wave, measured as the distance from the point of equilibrium to a crest or a trough

### Aufbau principle

principle that states that electrons fill orbitals in order of increasing energy

### de Broglie hypothesis

idea that matter may behave as a wave

### de Broglie wavelength

wavelength ($\lambda$) of a particle having mass

*m*and traveling at velocity*v*using Planck's constant,*h*; given as $\lambda=\frac h{mv}$### degenerate orbital

one of two or more orbitals with the same energy

### electromagnetic radiation

wave of energy produced by the movement of particles through space

### electromagnetic spectrum

entire range of electromagnetic waves, defined by their energy, wavelengths, and frequencies

### electron configuration

orbital filling of electrons in an atom based on their quantum numbers in increasing energies

### electron orbital

area of an atom in which an electron has the greatest probability of being located. Each orbital can contain at most two electrons.

### electron shell

one or more electron subshells that have the same quantum number $n$

### electron subshell

group of electron energy levels with the same size and shape that have the same quantum numbers $n$ and $\ell$

### excited state

electron state that has a higher energy level than the lowest energy state (ground state)

### frequency

number of oscillations of a wave that occur in a given period of time, usually a second, measured in hertz (Hz)

### ground state

electron state that is lowest in energy

### Heisenberg uncertainty principle

principle that it is impossible to simultaneously measure the position and the momentum of a particle

### Hund's rule

rule that states that when filling degenerate orbitals (for example, the 3

*p*orbital), electrons must first singly occupy all the empty orbitals in the subshell before pairing within the same orbital### line spectrum

pattern of colors on a dark background produced by an element when it gives off light (emission) or a pattern of dark lines on a background of the electromagnetic spectrum produced by an element when it absorbs light (absorption)

### node

point on a standing wave that stays fixed and does not oscillate

### Pauli exclusion principle

principle that states that paired electrons may never have the same spin value, which means they cannot have the same four quantum numbers

### photoelectric effect

emission of electrons when light strikes a material

### photon

fundamental particle that has a fixed amount of energy stored as electromagnetic radiation

### Planck's constant (*h*)

$6.62607\times 10^{-34}\;\rm{J}\cdot\rm{s}$; describes the ratio of energy to frequency of a photon

### quantized

exists only as discrete values

### quantum mechanics

branch of science that deals with subatomic particles, their behaviors, and their interactions

### quantum number

one of four numbers, $n$, $\ell$, $m$, and $s$, that together describe the orbital state of subatomic electrons

### wave

periodic oscillation of energy or matter

### wave function

mathematical expression that gives information about measurable properties of a system, such as energy, momentum, and position

### wave-particle duality

having properties of both particles and waves

### wavelength

distance between two identical parts of a wave