When Alpha or Beta particles pass another atom, they tend to pull electrons off it.

We then say that the atom is ionised.

If it has lost electrons, we call it a positive ion.
(Electrons have a negative charge, so losing electrons means the atom becomes positive).

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When an -particle or -particle ionises an atom, it slows the particle down. This is one reason that alpha particles have such a low penetrating power - they ionise other atoms strongly and thus get slowed down a great deal.