Energy changes and frequency

1. Atoms and nuclei can both absorb and emit electromagnetic radiation when their energy changes.
2. In an atom, the energy changes when an electron moves between energy levels.
3. In a nucleus, the energy changes when the nucleus rearranges itself, for example after a radioactive decay.
4. The size of the energy change decides the frequency of the radiation that is emitted or absorbed.
5. A small energy change gives radiation of low frequency, and a large energy change gives radiation of high frequency.
6. Different changes involve very different amounts of energy, so together they produce radiation across a wide range of frequencies in the electromagnetic spectrum.
7. In order of increasing frequency, the spectrum runs from radio waves through microwaves, infrared, visible light, ultraviolet and X-rays to gamma rays.

Radiation from electrons

1. Changes involving the outer electrons of an atom involve fairly small amounts of energy.
2. These changes emit or absorb infrared, visible light or ultraviolet radiation.
3. The coloured light from a neon sign and the orange glow of a sodium street lamp come from outer electrons falling to lower energy levels.
4. Inside a fluorescent tube, mercury atoms emit ultraviolet radiation, which a coating on the glass absorbs and re-emits as visible light.
5. Changes involving the inner electrons of large atoms, which are held very tightly close to the nucleus, involve much larger amounts of energy.
6. These inner-electron changes emit X-rays, which is how an X-ray tube produces X-rays when fast electrons strike a metal target such as tungsten.

Radiation from nuclei

1. The energy changes inside a nucleus are far larger than those involving electrons.
2. A nucleus that is left with excess energy loses it by emitting a gamma ray, the highest-frequency radiation in the spectrum.
3. Gamma rays therefore come from changes in the nucleus, while X-rays come from changes in the electrons.
4. X-rays and gamma rays can have overlapping frequencies, so they are told apart by where they come from rather than by their frequency alone.

Absorbing radiation

1. Atoms and nuclei can absorb radiation as well as emit it, when the radiation supplies the right amount of energy for a change.
2. An atom absorbs radiation of the frequencies that match the energy gaps between its electron levels, which are the same frequencies it emits.
3. Cool sodium vapour absorbs the same yellow-orange light that hot sodium vapour emits.
4. Dense materials such as bone contain atoms whose inner electrons absorb X-rays strongly, which is why bones show up clearly on an X-ray image.
5. Nuclei can also absorb gamma rays of the right energy, which moves the nucleus into a higher energy state.