AQA GCSE Physics (8463) · Paper 1
☢️ Atomic Structure
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
The atom & its history
A tiny central nucleus, made of protons and neutrons, holds nearly all of an atom's mass, while electrons sit in energy levels around it. An atom has a radius of about 1×10⁻¹⁰ m, and its nucleus is over 10,000 times smaller than that. The atomic number is the number of protons in an atom, the mass number is the total number of protons and neutrons, and isotopes are atoms of the same element with different numbers of neutrons.
Electrons can move to a higher energy level by absorbing electromagnetic radiation, and they emit radiation when they fall back down to a lower level. If an atom loses an outer electron entirely, it becomes a positive ion.
The model of the atom has changed over time. The plum pudding model pictured electrons embedded in a positive 'dough'. This was disproved by the alpha scattering experiment: most alpha particles passed straight through a sheet of gold foil, showing that atoms are mostly empty space, while a few bounced back, showing that each atom has a tiny, dense, positive nucleus. Bohr then proposed that electrons orbit the nucleus at fixed distances, and Chadwick later discovered the neutron.
Radioactive decay
Some nuclei are unstable, and they release radiation in order to become more stable. This decay is completely random: it is impossible to say which nucleus will decay next, or when. Activity is the number of decays per second, measured in becquerels (Bq), and a Geiger–Müller tube is used to detect and count this radiation as a count rate.
When a nucleus decays, its mass number and atomic number change in specific ways. Alpha decay reduces the mass number by 4 and the atomic number by 2, so the new element moves two places back in the periodic table. Beta decay leaves the mass number unchanged but increases the atomic number by 1. Gamma decay changes neither the mass number nor the atomic number; it simply releases energy.
The half-life of an isotope is the time it takes for half of its unstable nuclei to decay, which is the same as the time it takes for its activity to halve. Half-life is fixed for a given isotope. To calculate decay, the activity halves once for every half-life that passes: a sample with an activity of 80 Bq and a half-life of 2 hours would fall to 40 Bq after 2 hours and 20 Bq after 4 hours.
Higher content: after n half-lives, 1/2ⁿ of the original nuclei remain, so the activity has fallen to 1/2ⁿ of its starting value.
Uses, fission & fusion (Triple) Separate science only
Background radiation is present everywhere, from sources such as radon gas released by rocks, cosmic rays, food, nuclear fallout and medical procedures. Radiation dose is measured in sieverts.
Medicine makes careful use of radiation. Tracers are gamma-emitting isotopes with short half-lives that are injected into the body and followed through organs using external detectors, and radiotherapy uses targeted doses of radiation to kill cancer cells. Every medical use weighs the benefit to the patient against the radiation dose received.