What a learner can do afterwards
- 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
1 · Read
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.
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.
Subtracting zero leaves 5. Subtract four assigned bonding electrons to reach +1.
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.
Find each electron source and destination. Apply the whole arrow set, then check electron counts, formal charges and total charge.
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More to read
Assigning electrons for formal charge
Formal charge uses an equal division of each bond's electrons. Assign all nonbonding electrons to their atom and half the bonding electrons to each bonded atom. Subtract this assigned total from the neutral atom's valence-electron count.
Carbon begins with four valence electrons, nitrogen five and oxygen six. For nitrogen with three single bonds and one lone pair, the assigned total is three bonding electrons plus two nonbonding electrons. Its formal charge is 5 - 3 - 2 = 0. With four single bonds and no lone pair, nitrogen instead has formal charge 5 - 4 = +1.
Add the formal charges to recover the charge of the whole species. Then compare the total charge of all species before and after a reaction step. When no electrons cross the system boundary, those totals must agree. Agreement is a required check, not proof of a valid mechanism. Formal charge also differs from partial charge: it does not measure unequal electron sharing in a polar bond.
Apply the arrows together
Several curved arrows can belong to one elementary step. Draw the result of the complete set before checking electron counts. Reading just the incoming arrow can miss a bond that breaks or changes order in the same step.
In carbonyl addition, a nucleophile's pair forms a bond to the carbonyl carbon. A second pair arrow places the C=O pi electrons on oxygen. The carbonyl bond becomes single while the new bond forms. The resulting carbon has four single bonds, rather than five bond-order units.
Keep elementary steps separate when drawing a longer mechanism. An intermediate produced by one step can react in a later step. The electron arrows describe proposed bond changes; they do not establish the reaction rate. A rate-determining-step argument needs kinetic evidence and attention to reversible steps and conditions. Matching an arrow convention is therefore different from testing a complete mechanism against experiment.
Two electron pairs in a proton transfer
Ammonia can accept a proton from HCl. Identify two electron sources before drawing the step: the nitrogen lone pair and the H-Cl bond pair. Draw one pair arrow from nitrogen's lone pair towards hydrogen to form an N-H bond. Draw another from the H-Cl bond onto chlorine.
The resulting species are ammonium and chloride. Nitrogen has four single bonds and no lone pair, giving formal charge +1. Chloride has charge -1. Their charges add to zero, matching the two neutral starting molecules.
Hydrogen changes its bonding partner in this reaction. The curved arrows still represent electrons rather than the path of the hydrogen nucleus. A bare H+ cannot donate a pair because it has no electrons. For bond formation to H+, the arrow must begin at the pair supplied by the other reacting species.
Resonance symbols and curved arrows
A straight arrow with a head at each end connects resonance contributors. It identifies a relationship between Lewis drawings. A curved electron-pair arrow has a different job: its tail marks a pair, and its two-barbed head marks the pair's new placement in a drawing.
For an amide contributor, use the nitrogen lone pair to draw a C=N bond and place the C=O pi pair on oxygen. Applying both changes gives positive nitrogen and negative oxygen while preserving carbon's octet and the total charge.
The nuclear framework remains the same between these contributors. The curved arrows show how to redraw the electrons, not an elementary reaction that the molecule undergoes. The actual amide has one electronic structure with partial C-N double-bond character. Distinguishing these symbols prevents a resonance relationship from being mistaken for a sequence of reacting species.
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