A structural formula is a chemical formula that gives you a more complete picture of a compound than its molecular formula can. While a molecular formula, such as H2O, shows the types of atoms in a substance and the number of each kind of atom, a structural formula also gives information about how the atoms are connected together. Some complex types of structural formulas can even give you a picture of how the atoms of the molecule are arranged in space. Structural formulas are most often used to represent molecular rather than ionic compounds.
There are several different ways to represent compounds in structural formulas, depending on how much detail needs to be shown about the molecule under consideration. We will look at complete structural formulas, condensed formulas, line formulas, and three-dimensional formulas.
Complete structural formulas show all the atoms in a molecule, the types of bonds connecting them, and how they are connected with each other. For a simple molecule like water, H2 O, the molecular formula, becomes H—O—H, the structural formula. This structural formula shows that in a water molecule, the oxygen atom is the central atom, and it is connected by single covalent bonds to the hydrogen atoms. Carbon dioxide, CO2, can be represented structurally as O=C=O. This structural formula tells you that in this case the carbon atom is the central one, and the oxygen atoms are joined by double covalent bonds to the carbon atom.
For small molecules like these, the amount of new information in a structural formula is not great, but structures become more important when we study larger molecules. Let’s look at the molecular formula C2H6O. With some knowledge of valences for the three kinds of atoms involved, we can arrange these atoms in a complete structural formula as shown below.
This is the formula of ethanol, which is known for its intoxicant and antiseptic properties, and is also being used in reformulated gasoline. It is a liquid with a boiling point of 172°F (78°C). However, we can also produce another structural formula that satisfies all the bonding requirements for the atoms involved but shows a completely different molecule.
This molecule is methyl ether. It is a gas at room temperature and has very different chemical properties from ethanol.
Chemical formula —A way to show the number and kind of atoms combined together in a single pure substance.
Compound —A pure substance that consists of two or more elements, in specific proportions, joined by chemical bonds. The properties of the compound may differ greatly from those of the elements it is made from.
Covalent compound —A chemical compound which uses shared electrons to form bonds between atoms. The atoms do not become electrically charged as in ionic compounds.
Ionic compound —A compound consisting of positive ions (usually, metal ions) and negative ions (nonmetal ions) held together by electrostatic attraction.
Molecule —A chemical compound held together by covalent bonds.
Valence —The combining power of an atom, or how many bonds it can make with other atoms. For the examples used in this article, carbon atoms can make four bonds, oxygen atoms can make two bonds, and hydrogen atoms can make one bond.
After you become familiar with the rules for writing complete structural formulas, you’ll find yourself taking shortcuts and using condensed structural formulas. You still need to show the complete molecule, but the inactive parts can be shown more sketchily. Thus the two formulas above look like this when written in condensed form:
Even condensed formulas take up a lot of space and a lot of time to write. They can be transformed still further by the shorthand of line formulas, which show the main bonds of the molecule instead of the individual atoms, and only show a particular atom if it is different from carbon or hydrogen, or if it is involved in a reaction under consideration. Our examples of condensed formulas look like this when represented by line formulas.
At each unmarked vertex of the lines, there is a carbon atom with enough hydrogen atoms to satisfy its valence of four. There is also a carbon atom with its accompanying hydrogen atoms at the end of any bond line that doesn’t show some other atom. Compare the condensed formulas of these three compounds with the condensed formulas in order to find the atoms implied in the line formulas.
All of these structural formulas show you a flat molecule on a flat piece of paper. However, most carbon-containing molecules are three-dimensional; some of the atoms stick forward toward you from the carbon chain, and some project to the rear of the molecule. Chemists have devised special ways to show these forward- and backward-projecting atoms in order to understand how three-dimensional molecules behave. These three-dimensional structural formulas are often used when complex molecules are studied.
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G. Lynn Carlson