Radiation interacting with matter

The nucleus of an atom is surrounded by electrons. These are arranged energy levels at different distances from the nucleus:

  • the lowest energy level is closest to the nucleus
  • electrons usually occupy the lowest energy levels available to them

The arrangement of electrons differs from element to element, and is responsible for the chemical properties of each element. However, the arrangements may change when electromagnetic radiation is absorbed or emitted.

Exciting electrons

An inner electron in a low energy level can rise to a higher energy level. We say that it becomes ‘excited’. An electron can be excited when its absorbs energy from electromagnetic radiation.

Different changes in energy level need different frequencies of electromagnetic radiation. The absorption of radiation can be analysed by an ‘absorption spectrum’. The different black lines show the frequencies absorbed by electrons in atoms.

This graphic shows the absorption spectrum of visible light for hydrogen.The absorption spectrum of visible light for hydrogen

An excited electron can fall to a lower energy level. When this happens, energy is lost as electromagnetic radiation. Different changes in energy level cause the emission of different frequencies. The emission of radiation can be analysed by an ‘emission spectrum’. The different coloured lines show the frequencies emitted by electrons in atoms.

This graphic shows the emission spectrum of visible light for hydrogen. It is a black bar with 5 thin, vertical stripes of colour.The emission spectrum of visible light for hydrogen

Demonstrating excitation

Changes in atoms and nuclei can produce and absorb radiation over a wide range of frequencies. For example, tonic water (a type of colourless fizzy drink) contains a substance called quinine. This glows when ultraviolet light from a ‘black light’ or from the Sun shines through it.

Fluorescent lamps contain mercury vapour. An electric current excites electrons in mercury atoms. These emit ultraviolet light when the excited electrons return to lower energy levels. A substance coated on the inside of the glass absorbs the ultraviolet light and emits visible light.

Ionisation

Atoms are electrically neutral because:

  • protons have a +1 charge and electrons have a -1 charge
  • the number of protons equals the number of electrons

However, an outer electron may absorb enough electromagnetic radiation to leave the atom entirely. When this happens, the atom becomes a positively charged ion. The process is called 'ionisation' - the atom has been ionised.

Molecules can be ionised if one or more of their atoms become ionised. Radiation that can ionise atoms and molecules is called ionising radiation. Alpha, beta and gamma radiations are examples of ionising radiation.