Glossary: Key Terms in Atoms & Nuclei

type: glossary | subtopic: Definitions

Term	Definition
Alpha particle	Helium-4 nucleus (2 protons, 2 neutrons). Symbol: ^{4}_{2}He. Emitted in alpha decay.
Atomic number (Z)	Number of protons in a nucleus. Determines the element.
Balmer series	Hydrogen spectral series with transitions to n=2. Visible region (656.3–364.6 nm).
Becquerel (Bq)	SI unit of radioactive activity. 1 Bq = 1 disintegration per second.
Binding energy (BE)	Energy required to separate a nucleus completely into free nucleons. BE = $\Delta m$ × 931.5 MeV.
Bohr radius (a_{0})	Radius of the first Bohr orbit of hydrogen: a_{0} = 0.529 Å = $5.29 \times 10^{-11}$ m.
Decay constant (λ)	Probability of decay per unit time for a single nucleus. [λ] = $s^{-1}$ .
Distance of closest approach	Distance at which a head-on alpha particle momentarily stops before bouncing back. d = 2k $Ze^{2}$ /KE.
Gamma ray	High-energy electromagnetic photon emitted during nuclear de-excitation. No mass or charge.
Half-life (t_{1}/_{2})	Time for half the nuclei to decay. t_{1}/_{2} = 0.693/λ.
Hydrogen-like atom	Single-electron atom or ion: H, $He^{+}$ , $Li^{2+}$ , $Be^{3+}$ . Bohr model applies exactly.
Isotopes	Nuclei with the same Z (same element) but different A (different neutron number).
Lyman series	Hydrogen spectral series with transitions to n=1. Ultraviolet (UV) region.
Mass defect ( $\Delta m$ )	Difference between sum of constituent nucleon masses and actual nuclear mass. Always positive for stable nuclei.
Mass number (A)	Total number of nucleons (protons + neutrons) in a nucleus.
Mean life (τ)	Average lifetime of a radioactive nucleus. τ = 1/λ = 1.443 t_{1}/_{2}.
Nuclear density	Mass per unit volume of nuclear matter. ≈ $2.3 \times 10^{17}$ kg/ $m^{3}$ (constant for all nuclei).
Paschen series	Hydrogen spectral series with transitions to n=3. Infrared (IR) region.
Positron	Antiparticle of the electron. Symbol: $e^{+}$ . Emitted in β^{+} decay.
Rydberg constant (R)	Constant in spectral formula: R = $1.097 \times 10^{7}$ $m^{-1}$ .
Spectral series	Set of spectral lines corresponding to electron transitions to a specific lower level.
Stationary state	Allowed electron orbit in Bohr model where electron does not radiate energy.