They will then fall back, emitting a defined amount of energy as light. Absorption spectra result from electrons absorbing energy from electromagnetic radiation, and so effectively blocking it. Historically it is based on expt discovery. Describe the spectrum of atomic hydrogen and account for it, using Bohrs' model. This is shown in the syllabus, and can be easily derived from the above.
E is in joules, and m in kilograms.
The binding energy per nucleon is therefore, this divided by the number there are. This is often caused by the addition of neutrons to the atoms, causing it to become unstable and eventually break up. This breaking up may, in some cases, produce more neutrons, and so these can then go on to produce more fission reactions, creating a chain reaction which perpetuates itself. Fission is good because it provides a lot of energy form a source that is more viable long term than fossil fuels, and because it is relatively clean in terms of air pollution compared to fossil fuels. Nuclear fusion occurs when two smaller nuclei fuse together to form one bigger, and more stable nucleus, and produce lots of energy in the process.