Unlike molecular formulae or chemical names, structural formulas provide a representation of the molecular structure. Chemists nearly always describe a chemical reaction or synthesis using structural formulas rather than by chemical names, because the structural formulas allow the chemist to visualize the molecules and the changes that occur.
Many chemical compounds exist in different isomeric forms which have different structures yet the same overall chemical formula. A structural formula indicates the arrangements of atoms in a way that a chemical formula cannot.
In early organic chemistry publications, where use of graphics was severely limited, a text-based system arose to describe organic structures in a line of text. Although this system tends to break down with complex cyclic compounds, it remains a convenient way to represent simple structures such as ethanol:
Lewis structures are flat graphical formulas that show the atom connectivity, but lack information about the three-dimensional structure of molecules. This notation is mostly used for small linear molecules.
Several methods exist to picture the the three-dimensional arrangement of atoms in a chiral molecules.
Skeletal formulas are the standard notation for more complex organic molecules. Carbon (C) atoms are represented by the vertices (edges) and termini of line segments that are not marked with an atomic symbol. Each carbon atom is in turn assumed to bear enough hydrogen atoms to give the carbon atom four bonds. Hydrogen atoms on atoms other than carbon must be written explicitly.
Chirality in skeletal formulas is indicated by the Natta projection method. Solid or dashed wedged bonds represent bonds pointing above-the-plane or below-the-plane of the paper, respectively.