Matter and Atomic Physics
States of matter and the particle model
Describe solids, liquids and gases in terms of particle arrangement, spacing and motion. The particle model explains density, compression, diffusion and Brownian motion. Density ρ = m/V, so convert grams to kilograms and cm³ to m³. A common trap is mixing mass and density, or forgetting that 1 cm³ = 1 × 10⁻⁶ m³. Pressure in gases arises from particle collisions with container walls; raising temperature increases particle speed and pressure.
Atoms and the nucleus
Know the structure of the atom: protons and neutrons in a nucleus, electrons in shells. Proton number defines the element; nucleon number is protons plus neutrons. Isotopes have the same proton number but different neutron numbers. Electrons occupy shells and can be excited to higher levels, emitting photons of specific energy when they fall back, producing line spectra. Ionisation removes or adds electrons, creating charged particles.
Radioactivity
Three emissions: alpha (helium nucleus, high ionising, low penetrating), beta (fast electron, moderate), gamma (electromagnetic wave, low ionising, high penetrating). Half-life is the time for half the nuclei to decay; after n half-lives the remaining fraction is (½)ⁿ. Radioactivity is random and spontaneous, unaffected by temperature or pressure. Handle safety: shielding, distance, time limitation.
Worked example
A sample has activity 800 Bq and half-life 5 days. After 20 days, n = 20/5 = 4 half-lives, so activity = 800 × (½)⁴ = 800/16 = 50 Bq. The number of atoms remaining follows the same fraction, but the sample's mass and chemical properties do not change appreciably.