Isotopes, alpha and beta particles
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 practice questions, 5 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Atoms that change identity
- Isotopes
- Alpha and beta particles
Read the transcript of the teaching part
Last time
Before we start: last time, in Decay chains and the N-Z graph, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Atoms that change identity
Some nuclei are unstable. Left alone, they eventually change, firing out a particle, and becoming a different element. No heating, no chemistry, no outside push. The change happens at random, and it follows strict conservation rules that let us write down every decay.
Isotopes
Isotopes of an element have the same number of protons, but different numbers of neutrons. They have the same chemical behaviour, because that depends on the electrons. But they have different masses and may differ in stability. Tritium is hydrogen-3: one proton and two neutrons, with one electron in the neutral atom.
Alpha and beta particles
An alpha particle is a helium nucleus: two protons and two neutrons. Its mass is 4 u and its charge is plus two e. A beta-minus particle is an electron emitted from the nucleus. Its mass is about 0.0005 u, and its charge is minus e. A beta-plus particle is a positron: the same mass, charge plus e. Alpha has a short range in air and is easily stopped. Beta travels about a metre. Both are charged, so electric fields deflect them. Gamma is not deflected.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Isotopes.
The whole chapter: Particle Physics, Particle Physics.
Chapter 11.2: Radioactive decay
- Isotopes, alpha and beta particles 6 min
- Alpha decay, energy and beam current 7 min
- Decay chains and the N-Z graph 7 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.