---
title: "Curly Arrows as the Language of Mechanism"
description: "A curly arrow is a claim about where a pair of electrons moves, not decoration on a scheme. Getting the tail, the head and the count right is what separates a mechanism that can be checked from a stor"
canonical: https://lightmysky.com/learn/science/curly-arrows-as-the-language-of-mechanism-mt_C0bq2Mu8HT
source: https://lightmysky.com/learn/science/curly-arrows-as-the-language-of-mechanism-mt_C0bq2Mu8HT.md
retrieved: 2026-09-12
---

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# Curly Arrows as the Language of Mechanism

A curly arrow is a claim about where a pair of electrons moves, not decoration on a scheme. Getting the tail, the head and the count right is what separates a mechanism that can be checked from a story.

Subject: Science · Area: Chemistry · Ages 18 to 19
Page: https://lightmysky.com/learn/science/curly-arrows-as-the-language-of-mechanism-mt_C0bq2Mu8HT

## Ready when they can

- Starts every arrow at an electron source, a lone pair or a bond, and ends it where the pair arrives
- Assigns formal charges before and after a step and checks that charge balances
- Uses a single-headed arrow for radical steps and a double-headed arrow for pairs, without mixing them
- Spots and corrects an arrow that would give an atom more bonds than it can have

## Lesson: Arrows that say where electrons go

A curly arrow is a claim about where electrons move, not decoration. Its tail sits exactly on the electron source: a lone pair or a bond. Its head lands exactly where the pair arrives: a new bond or a new lone pair. Read every arrow as one sentence with a subject and a destination, and vague arrows will embarrass you fast.

Check every step with formal charges, the bookkeeping number found by taking valence electrons and subtracting assigned ones. Add the charges before the step and after it. The two totals must match, since electrons only move and never appear or vanish. A mismatch convicts one of your arrows.

**Example.** Mind the two head shapes and the bonding cap. A double headed arrow moves a whole pair, while a single headed fishhook moves one electron in a radical step, and mixing them up changes the story. No arrow may hand an atom more bonds than it can hold: carbon stops at four. An arrow that would build a five bond carbon is illegal, so redraw it to a legal destination.

**Tip.** Arrows move electrons only, never atoms, which is also why resonance arrows only shuffle pairs between fixed atoms. Give each elementary step its own arrow set and check tails, heads, charges, and octets in that order. Fluency in this grammar is fluency in mechanism.

**Recap.** Tails at electron sources, heads at destinations, charges balanced, octets respected.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Hybridisation, Conjugation and Delocalisation in Organic Molecules](https://lightmysky.com/learn/science/hybridisation-conjugation-and-delocalisation-in-organic-molecules-mt_Anv0MSQ4hq)
- [Nucleophilic Substitution in Haloalkanes](https://lightmysky.com/learn/science/nucleophilic-substitution-in-haloalkanes-mt_QG-nKlnVvW)

## Opens up

- [pKa as a Predictive Tool in Organic Chemistry](https://lightmysky.com/learn/science/pka-as-a-predictive-tool-in-organic-chemistry-mt_fI03mNIC_m)