Introduction
A key concept in chemistry is linking a chemical formula written
on a page to the way a molecule actually fills space and vice versa. In this exercise you will determine the formula for a variety of molecules. This will involve tallying the number of atoms of each kind in the molecule. Before beginning this exercise you will need to understand the possible types of formulas and when they are used.
Types of Formulas
As a sample exercise, consider the organic molecule given below. An
organic compound will contain carbon and hydrogen atoms and possibly one or more atoms of oxygen, nitrogen, sulfur, or phosphorus. To view the molecule’s shape and the atoms in each molecule most easily, the ball-and-stick representation shown here will be used throughout this exercise.
Molecular Formula
To determine the molecular formula for a compound such as this you must first find how many atoms of each type are in the molecule. This molecule contains two dark gray carbon atoms, one dark blue nitrogen atom, and seven light gray hydrogen atoms (a total of five bonded to the two carbon atoms and two bonded to the nitrogen atom). Since this is an organic compound the next step is to write the symbols for the elements in the order CH followed by other element symbols in their alphabetical order. In this case, CHN. Finally, the number of atoms of each kind in the molecule is
denoted as a subscript to the right of the element’s symbol. The molecular formula for this compound is C2H7N.
Why isn’t the compound’s molecular formula C2H7N1?
Leaving out the subscript "1" for nitrogen implies that there is only one nitrogen atom in this compound.
Structural Formula
To picture how the atoms in the molecule are bonded together, a structural formula is used. This formula does not depict the shape of the molecule. For the above molecule the structural formula is:
Condensed Formula
Another way for writing out a molecule’s formula is the condensed formula. This method, unlike the molecular formula, emphasizes groups of atoms in the molecule. In the figure below, boxes have been drawn around the groups of atoms as a visual aid. The condensed formula for the molecule is CH3CH2NH2.
Empirical Formula
Some experimental methods like elemental analysis solve for the simplest ratio of atoms in a molecule, the empirical formula. If you are given the empirical formula CH2, its molecular formula could be C2H4, C3H6, or any multiple of CH2. While the empirical formula gives the simplest possible ratio, the molecular formula contains the total number of atoms in the molecule.
A very important example of this occurs with the empirical formula CH2O. The molecular formula for formaldehyde is CH2O. This is also the empirical formula for acetic acid (molecular formula: C2H4O2). In addition, glucose and fructose also have CH2O as their empirical formula, but have C6H12O6 as their molecular formulas. Since they have the same molecular formulas, to tell glucose and fructose apart requires knowledge of their structural formulas.
