Formulas for Molecular Compounds

Molecules consist of two or more atoms bonded to one another through "covalent" bonds. Identification of these molecules is through their molecular formulas. There is a hierarchy of formulas:

Empirical formula

Consider the benzene molecule which has 6 carbon and 6 hydrogen atoms. Therefore the ratio of carbon to hydrogen atoms is 6 to 6 which we can simplify to 1 to 1 (i.e., 1:1). The empirical formula expresses this most simple ratio, i.e. C1H1 (or CH). The empirical formula expresses the most simple ratio of atoms in the molecule. Examples:

Molecular formula

Molecular formulas go one step beyond the empirical formula in that they express not only the correct ratio but the correct number of atoms in the molecule. In the case of benzene the molecular formula would be C6H6. other Examples:

Notice that sometimes the empirical and molecular formulas are the same. This will happen when the molecular formula also expresses the most simple ratio of atoms.

Structural formula

The structural formula not only has the correct number of atoms but includes the bonding structure of the molecule (i.e., which atoms are bonded together). Examples:

3D Structural formula

 Molecules are three dimensional (3D) objects. The 3D structural formula is an attempt to convey the 3D geometry of the molecule. Examples:

The triangular bonds depict atoms coming out of the plane and the dashed bonds depict atoms going in back of the plane.

Space filling formula

 The space filling formula includes the relative sizes of the atoms. The molecule acetaldehyde (C2H4O) below is an example. Gray spheres are carbon atoms, yellow spheres are hydrogens and the red sphere is oxygen.


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C101 Class Notes
Prof. N. De Leon