Atomic and Nuclear Physics
Atomic and Nuclear Physics on ORT Physics tests the photon model, discrete energy levels, the photoelectric effect, radioactive decay and binding energy. You need to handle powers of ten and unit conversions confidently.
Core rules
- Photon energy E = hf = hc/λ, with h = 6.63 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m/s. Convert to electronvolts using 1 eV = 1.60 × 10⁻¹⁹ J.
- Spectral lines arise from transitions between discrete energy levels. Emission photon energy equals the energy difference; absorption lines correspond to upward transitions.
- Photoelectric effect: hf = φ + Ek(max). Below the threshold frequency no emission occurs; above it, intensity changes the rate, not the maximum kinetic energy.
- de Broglie wavelength: λ = h/p.
- Nuclear notation: A is mass number, Z is atomic number. Alpha decay reduces A by 4 and Z by 2. Beta-minus decay converts a neutron to a proton, raising Z by 1 and leaving A unchanged, with an antineutrino emitted. Gamma emission changes neither A nor Z.
- Half-life: N = N₀(1/2)^(t/t½); activity A = λN.
- Mass defect Δm gives binding energy E = Δmc².
Worked example
Find the energy of a 500 nm photon. Convert 500 nm = 5.00 × 10⁻⁷ m. Then E = hc/λ = (6.63 × 10⁻³⁴ × 3.00 × 10⁸)/(5.00 × 10⁻⁷) = 3.98 × 10⁻¹⁹ J. Dividing by 1.60 × 10⁻¹⁹ gives 2.49 eV. This photon could excite an electron across a gap of at most about 2.5 eV, but not across a 3.0 eV gap.
Traps
- Substituting nanometres into E = hc/λ without converting to metres.
- Confusing Z with A when balancing a nuclear equation.
- Claiming gamma decay changes the element.
- Treating half-life as a linear decrease rather than repeated halving.
- Omitting the antineutrino in beta-minus decay.