Chemical Reaction Arrows

Know Your Reaction Arrows

Chemical reaction formulas show the process of how one thing becomes another. Most often, this is written with the format:

Reactant → Products

Occasionally, you will see reaction formulas containing other types of arrows. This list shows the most common arrows and their meanings. 

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Right Arrow

Reaction Right Arrow
This shows the simple right arrow for chemical reaction formulas. Todd Helmenstine

The right arrow is the most common arrow in chemical reaction formulas.  The direction points the direction of the reaction. In this image reactants (R) become products (P). If the arrow was reversed, the products would become reactants.

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Double Arrow

Reaction Double Arrow
This shows the reversible reaction arrows. Todd Helmenstine

The double arrow denotes a reversible reaction. The reactants become products and the products can become reactants again using the same process.

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Equilibrium Arrow

Equilibrium Reaction Arrows
These are the arrows used to denote a chemical reaction at equilibrium. Todd Helmenstine

Two arrows with single barbs pointing in opposite direction show a reversible reaction when the reaction is at equilibrium.

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Staggered Equilibrium Arrows

Favored Equilibrium Arrows
These arrows show strong preferences in an equilibrium reaction. Todd Helmenstine

These arrows are used to show an equilibrium reaction where the longer arrow points to the side the reaction strongly favors.

The top reaction shows the products are strongly favored over the reactants. The bottom reaction shows reactants are strongly favored over the products.

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Single Double Arrow

Resonance Arrow
This arrow shows a resonance relationship between R and P. Todd Helmenstine

The single double arrow is used to show resonance between two molecules.

Typically, R will be a resonance isomer of P.

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Curved Arrow - Single Barb

Single Barbed Curved Arrow
This arrow shows the path of a single electron in a reaction. Todd Helmenstine

The curved arrow with a single barb on the arrowhead denotes the path of an electron in a reaction. The electron moves from the tail to the head.

Curved arrows are usually shown at individual atoms in a skeletal structure to show where the electron is moved from to in the product molecule.

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Curved Arrow - Double Barb

Curved Double Barb Arrow
This arrow shows the path of an electron pair. Todd Helmenstine

The curved arrow with two barbs denotes the path of an electron pair in a reaction. The electron pair moves from the tail to the head.

As with the single barbed curved arrow, the double barb curved arrow is often shown to move an electron pair from a particular atom in a structure to its destination in a product molecule.

Remember: One barb - one electron. Two barbs - two electrons.

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Dashed Arrow

Dashed Arrow
The dashed arrow shows unknown or theoretical reaction paths. Todd Helmenstine

The dashed arrow denotes unknown conditions or a theoretical reaction. R becomes P, but we don't know how. It is also used to ask the question: "How do we get from R to P?"

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Broken or Crossed Arrow

Broken Arrows
Broken arrows show a reaction that cannot occur. Todd Helmenstine

An arrow with either a centered double hash or cross shows a reaction cannot take place.

Broken arrows are also used to denote reactions that were tried, but did not work.