Lewis structure
/ LOO-iss /
Before writing equations, chemists like a quick sketch that shows where every outer electron lives. A Lewis structure is exactly that: a simple diagram in which dots stand for electrons and short lines stand for shared (bonding) pairs, so you can see at a glance how a molecule is wired together.
Precisely, a Lewis structure is a diagram of a molecule showing all of its valence (outer-shell) electrons: bonding pairs are drawn as lines connecting atoms, and lone (non-bonding) pairs are drawn as dots on individual atoms. They are built so that each atom typically reaches a stable count of eight outer electrons — the octet rule — with hydrogen content with two.
Lewis structures matter because they are the first step to predicting a molecule's shape (via VSEPR), its polarity, and where it is likely to react. The honest caveat is that they are a simplified bookkeeping picture: they treat electrons as fixed dots in fixed places, so they handle delocalized electrons poorly (needing resonance forms) and have well-known exceptions where the octet rule does not hold.
The Lewis structure of water draws the oxygen joined to two hydrogens by single lines (two bonding pairs) and carrying two pairs of dots (two lone pairs). That single picture immediately tells you oxygen has four electron groups, which is what VSEPR needs to predict water's bent shape.
A Lewis structure shows bonds as lines and lone pairs as dots.
The octet rule is a helpful guideline, not a law. Plenty of stable species break it: boron in BF₃ is happy with six electrons, sulfur in SF₆ holds twelve, and any molecule with an odd electron count (like NO) cannot give every atom an octet. Treat the octet as a starting expectation, not a guarantee.