A reaction mechanism explains a chemical transformation as a sequence of bond-making and bond-breaking steps. In polar mechanisms, curved arrows track the movement of electron pairs during each step.

A full curved arrow has two important parts:

• its tail begins where an electron pair currently resides, such as a lone pair or covalent bond;
• its head points to the atom or bond where that electron pair will reside after the step.

Curved arrows therefore move electrons, not atoms. Atomic positions and connectivity change as a consequence of the new electron placement.

A common bond-forming pattern is nucleophile-to-electrophile donation. If a nucleophile has a lone pair,

$$\mathrm{:Nu^-+E^+\rightarrow Nu-E},$$

the curved arrow starts at the nucleophile's lone pair and ends at the electrophilic atom.

Bond breaking is represented by starting an arrow in the bond being broken. If both bonding electrons remain with one atom,

$$\mathrm{R-Br\rightarrow R^+ + Br^-},$$

the electron-pair arrow goes from the R-Br bond to Br.

Example: proton transfer

For hydroxide reacting with HBr,

$$\mathrm{OH^-+H-Br\rightarrow H_2O+Br^-},$$

two electron movements occur simultaneously:

1. an oxygen lone pair moves toward H, forming the new O-H bond;
2. the H-Br bonding pair moves onto Br, breaking H-Br and producing $\mathrm{Br^-}$.

These arrows preserve both charge and the octet around second-row atoms.

Reading charges after an arrow

If a neutral atom donates one of its lone pairs to make an additional bond without losing another bond, it generally becomes more positively charged. If an atom receives both electrons from a broken bond, it generally becomes more negatively charged. Formal-charge bookkeeping provides a check on the proposed step.

Curved arrows also appear when drawing resonance contributors, but the meanings must not be confused. Resonance arrows between structures describe alternative electron-bookkeeping representations of one molecule; mechanistic curved arrows within a structure describe electron redistribution associated with a reaction step.

A valid curved-arrow proposal must obey three basic constraints:

• every arrow begins at electrons that actually exist;
• the resulting bonding and formal charges must conserve total charge and respect ordinary valence rules;
• the electron flow should connect an electron-rich source to a plausible electron-poor destination.

Curved-arrow notation is therefore not decorative notation or a mnemonic for products. It is a compact conservation-based language for explaining how changes in bonding follow from movement of electron pairs.