Atomic and nuclear physics (AS91172)
Atomic and nuclear physics (AS91172)
This domain tests atomic structure, quantised energy levels, spectra, radioactive decay, half-life and nuclear equations. NCEA questions often ask you to justify a photon energy or balance a decay equation, so track both mass number and atomic number.
Core rules
- An atom is described by atomic number Z (protons) and mass number A (protons + neutrons). Isotopes have the same Z but different A.
- Energy levels are discrete. A photon is emitted when an electron falls to a lower level and absorbed when it moves higher: E = hf = hc/λ, with h = 6.63 × 10^-34 J s. Ionisation removes an electron completely.
- Emission spectra are bright lines at characteristic wavelengths; absorption spectra are dark lines on a continuous background.
- Alpha decay emits a helium nucleus (⁴₂He); beta-minus decay emits an electron and an antineutrino; gamma emission is a photon. In every nuclear equation, total A and total Z are conserved.
- Half-life T is the time for half the nuclei to decay: N = N0(1/2)^(t/T). Activity follows the same ratio. Decay constant λ = ln2/T.
- Gamma radiation has no mass or charge, so it does not change A or Z. Alpha is highly ionising but weakly penetrating; gamma is the reverse.
Worked example
An electron in hydrogen falls from n = 3 (E = -1.51 eV) to n = 2 (E = -3.40 eV). Photon energy = 1.89 eV = 1.89 × 1.60 × 10^-19 = 3.02 × 10^-19 J. Frequency f = E/h = 3.02 × 10^-19 / 6.63 × 10^-34 = 4.56 × 10^14 Hz. Wavelength λ = c/f = 3.00 × 10^8 / 4.56 × 10^14 = 6.58 × 10^-7 m = 658 nm.
Traps
Do not use A for atomic number. Balance nuclear equations by both A and Z. Half-life is not a linear rate. Photon energy must be in joules for E = hf unless h is given in eV s. Emission occurs when an electron drops to a lower level.