1. An electron is an indivisible quantity of charge that orbits the nucleus of an atom.
2. Thermionic emission is the emission of electrons from the surface of a hot metal.
3. The work function of a metal is the minimum energy needed to remove the loosest electrons from the surface of that metal.
4. X-rays are electromagnetic radiation of extremely high frequency (short wavelength).
5. The photoelectric effect is the emission of electrons from the surface of a metal due to electromagnetic radiation of a suitable frequency hitting the metal.
6. A photon is a bundle of light energy.
7. The threshold frequency is the minimum frequency needed to cause photo- electric emission.
8. Protons are positively charged nucleons, with the same size charge as the electron.
9. Neutrons are electrically neutral nucleons; they have slightly more mass than protons.
10. Atomic number is the number of protons in the nucleus of an atom. 11. Mass number is the number of protons plus nucleons in the nucleus of an atom. 12. Isotopes are atoms of the same atomic number but different atomic mass.
13. Radioactivity is the spontaneous emission of radiation due to the natural disinte- gration of an unstable nucleus.
14. Ionisation is the charging of a neutral atom or molecule when it loses or gains one or more electrons.
15. Penetration is the distance the radiation travels through a medium.
16. A radioisotope is an unstable isotope of an element that decays spontaneously, emitting radiation.
17. Alpha emission is the emission of a helium nucleus from a large atom. 18. Beta emission is the emission of an electron when a neutron decays into a proton. 19. Gamma emission is the emission of high energy photons from an unstable nucleus. 20. The activity of a radioactive source is the rate of emissions from that source.
21. The half-life of a radioactive isotope is the time taken for half of the nuclei in a given sample to decay.
22. Nuclear fission is the splitting of a large nucleus into two smaller nuclei, of roughly equal mass, with emission of neutrons and release of energy.
23. Nuclear fusion is the combining of two small nuclei to form a single larger nucleus, with the release of energy.
24. Chain reactions are reactions where at least one neutron is released during each fission reaction.
25. Critical mass is the minimum amount of the fissile material needed in order for chain reactions to occur.
