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The structural formula of a chemical compound is a visual representation that shows how the atoms in that compound are bonded together. It is often more useful than the molecular formula, which indicates the atoms and their quantities but not their arrangement in space. For example, the molecular formula of water, H2O, shows that there are two hydrogen (H) atoms and one oxygen (O) atom in the molecule, but does not show how they are arranged or bonded. There are several different types of structural formula, including the electron dot diagram, the line-bond structural formula, the condensed structural formula, and the skeletal structural formula.
Atoms often bond to each other by sharing a pair of electrons, forming a covalent bond. They may also form a double bond, in which four electrons are shared instead of two. In some cases, not all electrons are shared in bonds between atoms in a compound. Such loose pairs of electrons are called lone pairs. The structural formula of a chemical compound visually represents each of these different components through a system of dots or lines.
In an electron dot diagram, all outer or valence electrons available for bonding are represented by dots around the atom’s chemical symbol. An oxygen atom has six electrons in its valence shell, so it would be represented by an O surrounded by six dots — two on the top and bottom, and one on each side. When oxygen bonds with two hydrogen molecules to form water, the electrons from hydrogen are shared in covalent bonds. Since each hydrogen has one valence electron, the new electron dot diagram appears as a central O surrounded by eight dots, two on each of the four sides, and an H to the left and to the right, signifying that these atoms are covalently bonded.
A line-bond structural formula replaces covalent bonds with straight lines, and double bonds with a set of parallel lines. Water’s line-bond formula would appear as H—O—H, with the lone pairs of electrons represented by dots, two above and two below the O. This type of formula is also called a Lewis structure. Sometimes lone pairs are omitted from the line-bond diagram for convenience.
Condensed structural formulas are useful for complicated molecules that would take up too much space if all bonds were shown. In this type of formula, certain atoms are grouped together, and only the essential bonds are drawn. Since atoms bond together in predictable ways based on their valence electrons, certain bonds can be omitted in describing the molecule’s structure.
The skeletal formula goes one step further and omits all carbon and hydrogen atoms, showing only the chemical symbols of other groups linked by a line to a zigzag line structure. This type of formula is often convenient in organic chemistry, where molecules containing repeating chains of carbon and hydrogen atoms are common. In a skeletal formula, each carbon is represented by an angle in the line. If two lines branch off from that angle, it is assumed that two hydrogen atoms are bonded to the carbon. The blank end of the line indicates a carbon bonded to three hydrogen atoms.