Reaction mechanisms and curly arrows — a causal, temporal story about invisible entities

A mechanism is a claim about the sequence of elementary electron-pair (or single-electron) movements that convert reactants to products, drawn with arrows whose tail sits on an electron pair and whose head shows where that pair ends up. It is hard because it demands a causal, temporal story about invisible entities, and because the student must simultaneously track charge, geometry, and electron count. It is also the first time chemistry asks for a prediction with a reason rather than a recalled fact — and the first time a "wrong but plausible" answer is possible.