Lewis structure
/ LOO-iss /
A Lewis structure is the simplest possible map of a molecule: it shows which atoms are connected and where all the valence electrons live. Picture drawing the atoms' letters, putting a line for every shared pair of electrons (a bond) and a pair of dots for every non-bonding pair (a lone pair). With that one diagram you can see, at a glance, the skeleton of a molecule and where its reactive electrons sit. It was introduced by G. N. Lewis in 1916 and is still the first thing chemists sketch.
To build one you count up the total valence electrons from all the atoms, connect the atoms with single bonds, then distribute the leftover electrons as lone pairs to complete octets (eight around most atoms, two around hydrogen). If atoms are short of an octet, you convert lone pairs into extra bonds, making double or triple bonds. A line means two shared electrons; two dots means a lone pair. Checking formal charges then tells you which of several possible arrangements is most reasonable. The whole procedure is bookkeeping: no electron is created or lost, only placed.
Lewis structures matter because they are the working language of organic chemistry. Before you can talk about polarity, geometry, acidity or any reaction mechanism, you need to know where the electrons are, and the Lewis structure shows you. Its honest limit is that it is a flat, dots-and-lines cartoon: it does not show the true three-dimensional shape (that comes from hybridization and geometry), and it pretends electrons are neatly localized when, in resonance, they are actually spread out. It is a sketch, not a photograph — but an indispensable one.
Water's Lewis structure is H-O-H with two lone pairs of dots on the oxygen: 2 bonds use 4 electrons, 2 lone pairs use 4 more, accounting for all 8 of the available valence electrons (1 from each H, 6 from O).
Lines for bonds, dots for lone pairs — a complete inventory of a molecule's valence electrons.
A Lewis structure is a flat bookkeeping device, not a true 3D picture; for shape you need hybridization, and for delocalized electrons you need resonance.