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An ion is an atom or molecular entity carrying a net electric charge because its numbers of protons and electrons are not balanced. Losing electrons gives a positive charge; gaining electrons gives a negative charge. This simple bookkeeping is behind salts, electrochemistry and much of solution chemistry. Valence electrons are the electrons most directly involved in bonding and chemical reactivity, although the exact boundary between valence and core can depend on context. For main-group elements they provide a simple route to periodic similarities; transition metals require more care. Ionization energy is the energy required to remove an electron from a gaseous atom or ion in a specified state. Successive ionization energies refer to successive electron removals. Its periodic pattern is powerful but not perfectly smooth because subshell structure, pairing and shielding matter.
Primary references
IUPAC Gold Book · NIST · BIPM