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|Chapter Two: The Chemistry of Biology|
As mentioned in the previous section, molecules are made up of different types of atoms which join with one another through some sort of bond, usually either ionic or covalent. Molecules are often made of many different types of atoms; for example, a chemical called glucose contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. Scientists have adopted two ways of expressing what a particular molecule consists of: molecular formulas and structural formulas.
Molecular formulas are useful in that they are a very compact and simple way of writing complex molecules. As mentioned earlier, each element has a one or two letter symbol which is used to identify it. In a molecular formula, all of the symbols for the elements that are contained in the molecule are written, and to the right of each is a number which indicates how many atoms of that element are used. For example, glucose has six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. The symbols for carbon, hydrogen, and oxygen are C, H, and O respectively, so the molecular formula for glucose is C6H12O6. As another example, water has two hydrogen atoms and one oxygen atom, so its molecular formula is H2O. Notice that there is no "1" next to the O in the molecular formula for water. Scientists have decided that if there is just one atom of that element, then the number "1" does not need to be written next to the element in the molecular formula.
Sometimes a scientist may want to say that he or she has not one but several molecules of the same type. The method of writing this is very simple. To indicate that you have 8 molecules of water, you just write 8H2O. Notice that there are 16 total hydrogen atoms here, since each molecule of water has 2 and there are 8 molecules. Counting atoms like this is sometimes important when writing chemical reactions (which are discussed in the next section).
The only problem with molecular formulas is that they do not say how the atoms in the molecule are arranged; that is, which atom is bonded to which other atom. That's why scientists also use structural formulas. Structural formulas are actual sketches of the molecule, which indicate where each atom is placed. In a structural formula, each atom is represented by its symbol, and bonds are indicated by lines connecting the atoms. Occasionally, two molecules will have the exact same number of their types of atoms, even though their structure is different. In these cases, the molecular formulas for the molecules are identical, so only the structural formula can show the difference between them.