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Resonance structures are alternative electron arrangements for a single molecule; the real species is a resonance hybrid, not a set of forms that interconvert. Valid contributors preserve atom positions and σ bonds, maintain octets in typical cases, and conserve overall charge. Contributors with full octets and limited charge separation generally matter more, while charge is favored on electronegative atoms. Delocalization can stabilize conjugated alkenes and allylic intermediates, helping account for reaction behavior such as 1,2- versus 1,4-addition to conjugated dienes. It can also affect acidity, basicity, substituent effects, and UV–Vis absorption. Resonance is not the main explanation for terminal alkyne acidity, which chiefly reflects sp hybridization.