ABSTRACT
The periodic table of today has 118 elements, but the last more than 20 elements are extremely unstable and have therefore no environmental interest with the exception of their radioactivity. Ninety-one of the elements are naturally occurring, namely, the first 92 elements from hydrogen to uranium except element number 43, technetium. These 91 elements can be classified into four groups (the numbers before the elements indicate the number in the periodic table):
A. Elements that are present in biological material in relative high concentrations: (1) hydrogen, (6) carbon, (7) nitrogen, (8) oxygen, (11) sodium, (12) magnesium, (14) silica, (15) phosphorus, (16) sulfur, (17) chlorine, (18) potassium, (19) calcium, and (26) iron.
