Matter and Atomic Physics
Matter and Atomic Physics
Matter and Atomic Physics is Section E. It tests the particle model of solids, liquids and gases, density, atomic structure, isotopes, radioactivity and half-life.
Core rules and concepts
- Kinetic model. Particles are in constant motion. In solids they vibrate about fixed positions; in liquids they move past each other; in gases they move rapidly and are far apart. Brownian motion supports the particle model.
- Density. ρ = m/V, in kg m⁻³ or g cm⁻³. An object floats if its average density is less than the fluid's.
- Atomic structure. A nucleus contains protons (charge +1, mass number 1) and neutrons (charge 0, mass number 1); electrons orbit in shells (charge −1, negligible mass). Proton number Z fixes the element; nucleon number A equals protons plus neutrons.
- Isotopes. Same Z, different A. They have identical chemical properties but different masses and possibly different nuclear stability.
- Radioactivity. Alpha decay emits ⁴₂He, reducing A by 4 and Z by 2. Beta-minus decay turns a neutron into a proton, increasing Z by 1 and leaving A unchanged. Gamma emission changes neither A nor Z. Ionising power is alpha > beta > gamma; penetration is gamma > beta > alpha.
- Half-life. The time for half the unstable nuclei, or half the activity, to decay. Activity is measured in becquerels, Bq.
Worked example
A radioactive sample has an initial activity of 800 Bq and a half-life of 5 days. Find its activity after 15 days. Reasoning: 15 days is 15/5 = 3 half-lives. After one half-life: 400 Bq; after two: 200 Bq; after three: 100 Bq. Equivalently, 800 × (1/2)³ = 100 Bq. The activity has fallen to one-eighth of its original value.
Traps that cost marks
- Writing a nuclear equation that does not conserve mass number and atomic number.
- Saying beta decay changes the mass number; it does not.
- Treating half-life as a linear decrease, or using the wrong number of half-lives.
- Confusing atomic number with mass number, or electrons with nucleons.
- Using density units inconsistently, such as g cm⁻³ with a volume in m³.