D2.2 Line Structures

Think about the importance of structures of covalent molecular substances. Hydrocarbons or closely related molecules are good examples because there are numerous different hydrocarbon molecules and many hydrocarbons have lots of atoms. Knowing which atoms are bonded to which (connectivity) and the different ways they are bonded (geometry) enables predictions about the physical and chemical properties of the corresponding compounds.

Drawing the molecules on paper is a great way to think about a molecule’s connectivity and geometry, but drawing full Lewis structures are not always an easy task. For example, molecules of hydrocarbons in motor oil contain 16-20 carbon atoms and more than twice as many hydrogen atoms. Drawing full Lewis structures for such large molecules take a lot of time, and the drawing itself can be overly complicated and thus less meaningful/useful.

Therefore, for most molecules containing >10 atoms, a simpler kind of drawing, called a line structure, is often used. In a line structure (also called a skeletal structure) for a hydrocarbon molecule, only the C–C bonds are shown; element symbols and C–H bonds are omitted. Carbon atoms are represented by the end of a line or a juncture between two lines. Figure: Lewis Structure and Line Structure shows an example line structure. Decide where each C atom and each H atom is in the line structure; then count the number of C and H atoms to verify that the line structure represents the same molecule as the full Lewis structure.

Line structures are much easier to draw, and the general molecular geometry of the carbon chain or chains is clearer. However, it is also much easier to make mistakes when drawing and reading line structures: an extra line is a whole extra -CH_n group and it is also easier to miss some of the implied C-H bonds.

In line structures, atoms other than carbon and hydrogen are represented by their elemental symbols, and any H atoms bonded to them are shown explicitly (for example, see molecule C in the exercise above). In line structures, lone pairs on atoms are often omitted as well. Because C-H bonds and lone pairs are not shown, it is necessary to include the formal charge on any atom that has a nonzero formal charge when drawing a line structure. Otherwise, there would be a miscount of bonds or electrons.