--- title: "Curly arrows as the language of mechanism" description: "Curly arrows show changes in electron placement. The tail identifies the electron source, the head marks the destination, and the arrowhead shows whether one electron or a pair is represented." 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Curly arrows as the language of mechanism Curly arrows show changes in electron placement. The tail identifies the electron source, the head marks the destination, and the arrowhead shows whether one electron or a pair is represented. 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 - Identifies lone pairs and bonds as electron-pair arrow sources and selects the receiving atom as the destination in a specified bond-cleavage step - Calculates an atom's formal charge from stated bonds and lone pairs, and checks supplied total charges for a complete step without electron exchange - Uses a one-barbed fishhook arrowhead for one electron and a two-barbed curved arrowhead for an electron pair - Identifies a ten-electron carbon product as invalid and selects the additional carbonyl-addition arrow needed to preserve carbon's octet ## Lesson: Arrows that say where electrons go An electron-pair curved arrow starts at a lone pair or bond pair. Its head marks the atom receiving a lone pair or the bond being formed. The head has two barbs at one end. A one-barbed fishhook represents one electron; it may start at an unpaired electron or at a bond whose pair splits. A bare H+ has no electrons to donate. **Example.** Formal charge = valence electrons - nonbonding electrons - half the bonding electrons. Single, double and triple bonds contain two, four and six bonding electrons. Carbon has four valence electrons, nitrogen five and oxygen six. Nitrogen with four single bonds and no lone pair has formal charge 5 - 0 - 4 = +1. Formal charge is bookkeeping, not the partial charge from unequal electron sharing. **Example.** Hydroxide reacting with chloromethane needs two pair arrows: oxygen lone pair to carbon, and C-Cl bond to chlorine. Together they give methanol and Cl-. Charge totals -1 on each side. Include all species: without electron exchange, total charge is conserved. A mismatch can come from an invalid step, missing species or incorrect charge labels. Matching totals alone does not prove a mechanism. Apply all arrows in a step together, then check electron counts. Carbon cannot exceed eight valence-shell electrons in these ordinary Lewis structures. Five full two-electron bonds would give ten. When a lone pair adds to a carbonyl carbon, move the C=O pi pair onto oxygen in the same step. Carbon then has four single bonds. These checks concern product structures, not partial bonds in transition states. Electron-flow arrows do not trace nuclei. Atoms can move and change bonds during reactions. Resonance contributors retain the nuclear framework and differ in electron placement. A straight two-ended arrow connects contributors; curved arrows show how to redraw their electrons. Contributors are not separate molecules switching forms. Each elementary reaction step has its own arrow set. **Recap.** Find each electron source and destination. Apply the whole arrow set, then check electron counts, formal charges and total charge. *Lesson, questions and flashcards written for this dataset.* ## 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) ## Optional connections - [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)