Physical Science: Atomic and Nuclear Structure and Properties, Elementary Particles, and Nuclear Reactions
This domain covers atomic structure, nuclear decay and reactions, and the subatomic particles underlying them. Expect balanced nuclear equations and shell-filling logic.
Atomic structure
An atom has Z protons (which define the element), N neutrons, and electrons; mass number A = Z + N. Isotopes share Z and differ in N; ions differ in electrons. Electrons fill orbitals labeled s, p, d, f by the aufbau principle, Hund's rule (maximize unpaired spins), and Pauli exclusion (two electrons, opposite spins). Valence electrons govern chemistry. Excited atoms emit photons of energy E = hf.
Nuclear decay
- Alpha: emits a helium nucleus; A falls by 4, Z by 2.
- Beta-minus: a neutron becomes a proton; Z rises by 1, A unchanged; an electron and antineutrino are emitted.
- Positron (beta-plus): proton becomes neutron; Z falls by 1.
- Gamma: photon emission, no change in A or Z. Half-life is the time for half the nuclei to decay; activity is measured in becquerels (decays per second) or curies.
Reactions and binding energy
Mass defect converts to binding energy, E = mc². Fission splits heavy nuclei (U-235 chain reaction, critical mass); fusion joins light nuclei (stars) and releases more energy per nucleon. Both conserve charge and nucleon number.
Elementary particles
Matter particles are quarks and leptons. A proton is uud (2/3 + 2/3 − 1/3 = +1); a neutron is udd. Force carriers: photon (electromagnetic), gluon (strong), W and Z bosons (weak), and the hypothetical graviton. Hadrons include baryons (three quarks) and mesons (quark–antiquark).
Worked example
An 80 mg sample with a half-life of 5 days sits for 15 days. 15/5 = 3 half-lives, so remaining mass = 80 × (1/2)³ = 80/8 = 10 mg. Reasoning: each half-life halves the parent nuclei remaining, not the elapsed time or the decay rate constant.
Traps
Believing beta decay changes the mass number; forgetting gamma changes neither A nor Z; confusing fission (splitting) with fusion (joining); treating half-life as the lifetime of half the sample; forgetting electron capture lowers Z by 1 with A unchanged.