The nuclear atom
An atom is mostly empty space: a tiny, dense, positive nucleus of protons and neutrons, with electrons around it. The proton number says which element it is; the nucleon number counts protons plus neutrons. Isotopes have the same but different numbers of neutrons. Rutherford's alpha-scattering experiment revealed the nucleus.
≈ 25 minRadioactive emissions and decay
Unstable nuclei decay randomly and spontaneously, giving out alpha particles (helium nuclei), beta particles (fast electrons) or gamma rays (electromagnetic waves). Alpha is the most ionising but is stopped by paper; gamma is the least ionising and most penetrating. Decay equations balance both the nucleon numbers and the proton numbers.
≈ 30 minHalf-life, uses and safety
The half-life of an isotope is the time for half its nuclei to decay, so the activity halves every half-life. After half-lives, remains. Each use of radioactivity needs the right emission and the right half-life. Because ionising radiation damages cells, sources are handled with tongs, shielded and kept at a distance.
≈ 30 minFission and fusion
In fission, a heavy nucleus such as uranium-235 absorbs a neutron and splits into two smaller nuclei, releasing energy and two or three neutrons that can split more nuclei: a chain reaction. In fusion, light nuclei join to make a heavier one, releasing even more energy per kilogram; it powers the Sun. Both convert a little mass into a lot of energy.
≈ 30 min