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Electron configuration describes how electrons occupy available atomic states. This arrangement is central to periodic trends and helps explain why elements in the same group often react in related ways. The outer electrons are especially useful when you want to anticipate chemical behaviour. 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. Chemical bonding describes how atoms are held together through interactions involving electrons and nuclei. Ionic, covalent and metallic models highlight different limiting behaviours rather than three completely isolated worlds. The useful question is not only “what bond is this?” but “what does this model help predict?”
Primary references
IUPAC Gold Book · NIST · BIPM