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• Ionization energy
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Ionization is the process in which one or more electrons are removed from an atom or molecule. The charged particle that results is called an ion. As an example, consider an atom of oxygen. An oxygen atom consists of a nucleus containing eight protons and eight neutrons. Each proton carries a single positive electrical charge, and each neutron is electrically neutral. The oxygen atom also contains eight electrons, each carrying a single negative electrical charge. With eight positive charges and eight negative charges, an oxygen atom is neutral.

Some of the electrons in an atom can be removed rather easily. If an oxygen atom should lose one electron, for example, it would then have eight positive charges and only seven negative charges. Overall the atom would have an electric charge of +1. If two electrons were to be...