fishhook arrow

If the ordinary curved arrow is a tool for moving electrons in pairs, the fishhook arrow is its single-electron cousin. It looks just like a curved arrow but with only half a head — one barb instead of two, like the hook on a fishing line. That little detail carries a big meaning: a fishhook arrow tracks the movement of just one electron, not a pair. You reach for it whenever electrons travel solo, which happens in the world of radicals.

Concretely, when a bond breaks evenly so that each atom walks away with one of the two shared electrons, you draw two fishhook arrows, one for each lone electron heading to each atom. This even split is called homolysis, and it produces radicals — atoms or molecules with a lonely unpaired electron. Chlorine gas under light, for instance, splits Cl-Cl into two chlorine radicals, each shown by a fishhook arrow peeling one electron off the bond. Radical reactions chain along with these single-electron steps, and every one of them is drawn with fishhooks rather than full arrows.

Getting the arrow type right is not a cosmetic choice — it tells the reader what kind of chemistry is happening. Full-headed arrows say ionic, electron-pair, polar chemistry (nucleophiles and electrophiles, charges and lone pairs). Fishhook arrows say radical, single-electron chemistry (homolysis, unpaired electrons, chain reactions). Mixing them up signals a misunderstanding of the mechanism itself, so the half-head versus full-head distinction is one of the first things to keep straight.