Lewis Structure
Definition
A diagram showing how atoms are bonded and where the electrons sit: dots for lone pairs, lines for bonds. It is the standard sketch of a molecule.
Technical definition
A Lewis structure is a two-dimensional diagram representing a molecule's valence electrons as dots (lone pairs) and lines (bonding pairs) around element symbols, constructed to satisfy the octet rule where possible and showing formal charges and resonance when needed.
Example
Water's Lewis structure shows oxygen with two single bonds to hydrogen and two lone pairs. The bent shape of the molecule starts here.
Common confusion
Lewis structures do not show true 3D shape. Water looks linear in its Lewis structure but is actually bent; use VSEPR theory to get the real geometry.