Dynamic equilibrium. Two straight arrows pointing in opposite directions. The two surrounding symbols, A and B, represent one or more chemical compounds that are physically or chemically distinguishable. The atoms that appear on one side must also appear on the other so that the equilibrium is balanced (chemical reactions do not create or destroy atoms).

The equilibrium double arrow emphasizes the fact that the experimental conditions that allow A to change into B, also allow the backward transformation of B into A. (Organic chemists tend to draw equilibrium symbols only when both the forward and backward transformations are fast.)

If change can occur at a practical rate in both directions, equilibrium will usually be achieved during the experiment. This means the concentrations of A and B have reached a steady state and net change no longer occurs. (Note: although net change does not occur at equilibrium, large numbers of individual molecules continue to change. Net change is prevented because the number of molecules changing in one direction is precisely matched by the number of molecules changing in the opposite direction.)

Equilibrium favors reactants. Equilibrium arrows, but the arrow leading from reactants to products is shorter.

The short arrow implies the flow of reactants to products is relatively difficult, so equilibrium is reached when there are more reactants than products.

Equilibrium favors products. Equilibrium arrows, but the arrow leading from products to reactants is shorter.

The short arrow implies the flow of products to reactants is relatively difficult, so equilibrium is reached when there are more products than reactants.

Chemical reaction. One straight arrow pointing from reactants (A) to products (B). A single arrow emphasizes one direction of chemical change (A into B).

Reaction arrows are typically used to draw reaction mechanisms and chemical syntheses because they emphasize, "this is how you get from A to B."

Organic chemists are quite "free" (sloppy) in the way they use reaction arrows. Rarely do they draw a balanced reaction. Less important reactants may be drawn above or below the reaction arrow. Noteworthy experimental conditions (solvent, catalysts, reaction time and temperature) may also be drawn above or below the reaction arrow. The products from several reactions may even be listed together after a single reaction arrow.

Resonance. One straight double-headed arrow pointing between two formulas, A and B. These formulas share all of the same atoms, but have different electron patterns.

A and B represent different electron patterns (or resonance forms) for the same molecule. Therefore, A and B must contain the same atoms, and the atoms must be in the same spatial locations. The molecule's actual electron pattern is neither A nor B, but a superposition of the two.

Do not confuse the double-headed resonance arrow with the equilibrium double arrow. The former connects two drawings of a single molecule, while the latter connects two different and distinguishable compounds.

Curved arrow. One double-barbed curved arrow that starts at a "mobile" electron pair and points to the electron pair's "destination."

The curved arrow is used to draw reaction mechanisms by focusing attention on which electron pairs change and how. Read more...

Fishhook. One single-barbed curved arrow that starts at a "mobile" electron and points to the electron's "destination."

Fishhooks are used to draw reaction mechanisms involving free radicals, atoms or molecules that contain at least one unpaired electron. The free radical can be either a reactant or a product.