Physics (school) · Part 08
Atoms and radioactivity
What atoms are made of, why some decay, and how fission and fusion release energy.
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- Structure of the atomAn atom has a tiny nucleus of protons and neutrons, with electrons orbiting around it. Protons are positive, electrons negative and neutrons neutral.
- Atomic number and mass numberAtomic number is the number of protons. Mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons.
- Radioactive decayAn unstable nucleus randomly gives out radiation to become more stable. It cannot be predicted when a single nucleus will decay, and nothing such as heat or pressure changes the rate.
- Alpha, beta and gammaAlpha is a helium nucleus (2 protons, 2 neutrons), stopped by paper. Beta is a fast electron, stopped by thin aluminium. Gamma is an electromagnetic wave, reduced by thick lead or concrete.
- Half-lifeThe time taken for half of the unstable nuclei in a sample to decay. It is also the time for the count rate to fall to half.
- Ionising radiation and safetyAlpha, beta and gamma can knock electrons off atoms (ionisation), which can damage cells and cause cancer. Safe handling means limiting time, keeping distance and using shielding.
- Nuclear fissionA large nucleus, such as uranium-235, absorbs a neutron and splits into two smaller nuclei, releasing energy and more neutrons. These neutrons can split more nuclei in a chain reaction.
- Nuclear fusionTwo small nuclei, such as hydrogen, join to make a larger nucleus and release energy. It needs extremely high temperature and pressure, as inside stars.