A diagram of several simple Lewis structures of molecules and atoms.

In chemistry, a Lewis structure is a 2D representation of a molecule on paper. It is used primarily to show the approximate relative positions of each atom in relation to the other atoms in the molecule, the topology of the covalent bonds linking them, and the allocation of electrons to the atoms in the molecule.

There are a number of simplified Lewis structures. In organic chemistry, rather than drawing every single atomic symbol, simple lines are used. Essentially, at every "bend" in the line, there is assumed to be a saturated carbon (CH2), and the end of every line is a saturated carbon (CH3). This often saves quite a bit of time. For a few examples: propane, which is a simple three carboin chain, would be a simple zig-zag line, with two line segments; cyclohexane, which is a hexagonal ring of carbons, would be drawn simply as a hexagon.

A number of heuristic rules (based mostly on the octet rule) can then be applied to work out the distribution of charge across the structure. This does not always produce correct results, but works in enough cases to make Lewis structures a useful tool in chemistry.

It was devised by and named after G. N. Lewis.