Quantum and Atomic Physics Glossary

Dark lines in a continuous spectrum where atoms have absorbed photons, exciting electrons to higher energy levels.

Radioactive decay in which a nucleus emits a helium-4 particle ($^4_2$He), decreasing $A$ by 4 and $Z$ by 2.

Radioactive decay involving the conversion of a neutron to a proton ($\beta^-$) or proton to neutron ($\beta^+$), with emission of an electron/positron and neutrino.

A model of the hydrogen atom with quantized circular orbits and discrete energy levels: $E_n = -13.6/n^2$ eV.

The wavelength associated with a moving particle: $\lambda = h/p = h/(mv)$. Significant only for particles with very small mass.

Discrete bright lines produced when atoms emit photons during downward electron transitions. Each element has a unique emission spectrum.

A discrete, quantized energy that an electron can have in an atom. Transitions between levels produce or absorb photons.

Emission of a high-energy photon from an excited nucleus. Does not change the atomic number or mass number.

The time required for half of a radioactive sample to decay: $t_{1/2} = \ln 2 / \lambda$. A statistical property of large ensembles.

A fundamental quantum limit: $\Delta x \cdot \Delta p \geq \hbar/2$. Position and momentum cannot both be known precisely at the same time.

The ejection of electrons from a surface by light above a threshold frequency. Explained by Einstein's photon model: $KE_{\max} = hf - \phi$.

A quantum of electromagnetic radiation (light). A massless particle with energy $E = hf$ and momentum $p = h/\lambda$.

A fundamental constant: $h = 6.626 \times 10^{-34}$ J s. Relates energy to frequency ($E = hf$) and momentum to wavelength ($p = h/\lambda$).

The quantum principle that all entities exhibit both wave and particle properties depending on the experimental context.

The minimum energy ($\phi$) needed to free an electron from a metal surface. Depends on the material.