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Atoms, Ionization and the Types of Radiation
Ionization removes an electron; excitation only lifts it. Alpha particles stop in paper, beta in a few mm of plastic, and x-rays need lead.
Inside the atom
An atom has a small, dense nucleus surrounded by electrons in shells. The nucleus holds protons, which carry a positive charge, and neutrons, which carry none. The number of protons is the atomic number, Z. Electrons carry a negative charge and fill shells named K, L, M, N and so on, moving outward. Each shell holds at most 2n² electrons, where n is the shell number: K holds 2, L holds 8 and M holds 18. The outermost shell never holds more than 8. Binding energy is the energy holding an electron in its shell. It is highest in the K shell and rises with Z. In tungsten, Z 74, it is 69.5 keV for the K shell, about 12 keV for L and about 3 keV for M.