Alkanes are a family of organic molecules that are part of a group of familes called hydrocarbons. Hydrocarbons are molecules that are made up of only carbon (C) and hydrogen (H). The thing that charaterizes the alkanes is that all of the carbon atoms have four single bonds to them. this means that every carbon in an alkane has tetrahedral geometry. This is illustrated by the Jmol model for the alkane 2-methylpentane shown below.

A three dimensional model of the alkane 2-methylpentane. The grey spheres represent the carbons, while the white spheres represent the hydrogens. Click on and rotate the molecule with your mouse and observe that all 6 of the carbons in this molecule have four bonds to them and, consequently, all have a tetrahedral shape.

Alkanes that have only a straight chain of carbon atoms are called normal alkanes. By this definition, the 2-methylpentane molecule shown above is not a normal alkane, but rather a branched-chain alkane. Since alkanes contain only carbon and hydrogen, they are nonpolar. This means that the only noncovalent interaction that exists between alkanes molecules is the London dispersion force. Raymond describes in Section 4.4 how this leads to an increasing attractions between alkanes molecules as the molecules get larger. This is reflected in the melting points and boiling points, which also increase with the size of a molecule.

Because it is possible to have more than one alkane that share the same molecular formula, each with a different structures, the names need to be based on the structural formulas and not the molecular formulas. For example, the 2-methylpentane molecule shown above has a molecular formula of C₆H₁₄. (Count the atoms if you do not believe me.) Below are shown four other molecules that share this same molecular formula.

The molecule on the left, n-hexane is a normal alkane, while the other three molecule branched chain alkanes. Organic molecules that share the same molecular formula, which have their atoms connected differently, are called constitutional isomers. The five molecules shown above are constitutional isomers of one another. The names used to label each of these molecules, describes their structural formulas.