Atoms of elements that have the same atomic number, but different mass
numbers are called isotopes. This difference is do to a difference in number of
neutrons.
Each chemical element is identified by a one or two-letter symbol that corre-
sponds to the first letter or the first two letters in its name. For example, the sym-
bol C is used for carbon. Some chemical elements have English names while
others have Latin names. It is for this reason that symbols for some chemical
elements seem strange at first glance. Take sodium, for example. You would
think its symbol should be S, but that’s the symbol for sulfur. The symbol for
sodium is Na—the first two letters of its Latin name, natrium.
There are 92 natural chemical elements and others that scientists synthesized
(created). All of these are organized into a table called the Periodic Table (see
“A Dinner Table of Elements: The Periodic Table”). The six most abundant
chemical elements in living things are carbon, oxygen, hydrogen, nitrogen,
phosphorus and calcium. The rest are also important (see Table 2-1) and are
found in trace amounts.(^26) CHAPTER 2 Chemical Elements of Microorganisms
Atomic Approximate
Element Symbol Number Atomic Weight
Calcium Ca 20 40
Carbon C 6 12
Chlorine Cl 17 35
Hydrogen H 1 1
Iodine I 53 127
Iron Fe 26 56
Magnesium Mg 12 24
Nitrogen N 7 14
Oxygen O 8 16
Phosphorus P 15 31
Potassium K 19 39
Sodium Na 11 23
Sulfur S 16 32
Table 2-1. Chemical Elements Commonly Found in All Living Things