Ionization of an atom
A charged particle (here an alpha particle) with a large amount of energy expels an electron from an individual atom lying on its path. The atom with the missing electron has become an ion, with its electron structure disturbed and its interactions with its neighbours disrupted. The energy transferred by the alpha particle is eventually released in the form of electromagnetic radiation or heat. The alpha particle, as opposed to what is shown in the diagram, can also liberate electrons at a distance thanks to its electric charge. Most particles ionize hundreds of thousands, if not million of atoms before stopping.
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