Britannica Sources used in this answer

What do the lines in a structural formula represent?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

In a structural formula, the lines represent chemical bonds between atoms. Specifically, these lines indicate the number of electron pairs shared between atoms in a covalent bond:12

Single line: Represents a single bond, where two atoms share one pair of electrons.31
Two lines: Represent a double bond, where two atoms share two pairs of electrons.12
Three lines: Represent a triple bond, where two atoms share three pairs of electrons.12

These formulas are essential for identifying the specific arrangement of atoms and bonds within a molecule, providing structural information that simple molecular formulas—which only list the types and numbers of atoms—do not convey.24