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Basic Chemistry
Elements are pure substances that cannot be broken down or decomposed by a chemical change. Familiar
examples are iron, gold, carbon and oxygen. Oxygen constitutes nearly one-half of the total quantity of the
elements in the atmosphere and the earth’s crust. Table 2-1 provides a breakdown of the elements in the
atmosphere and in the earth’s crust.
Table 1 - Percentages of Elements in the Atmosphere and the Earth’s Crust
Oxygen
49.20%
Chlorine
0.19%
Silicon
25.67
Phosphorus
0.11
Aluminium
7.50
Manganese
0.09
Iron
4.71
Carbon
0.08
Calcium
3.39
Sulphur
0.06
Sodium
2.63
Barium
0.04
Potassium
2.40
Nitrogen
0.03
Magnesium
1.93
Fluorine
0.03
Hydrogen
0.87
Strontium
0.02
Titanium
0.58
All others
0.47
th
Reference: College Chemistry. 5 Edition. 1976. Nerbergall, et.al D.C. Health & Company.
Each element differs from any other in weight, size, and chemical properties. Table 2-2 shows a list of other
elements of interest in the discussion of water chemistry. Each element is assigned a one-or two-letter symbol
that is straight forward, such as “O” for oxygen. Others are based on the Latin name for the element, for
example, sodium’s symbol is Na from the Latin word for sodium, natrium.
Table 2 - Common Elements in Water Chemistry
Element
Symbol
Element
Symbol
Antimony
Sb
Manganese
Mn
Arsenic
As
Mercury
Hg
Beryllium
Be
Molybdenum
Mo
Boron
B
Nickel
Ni
Bromine
Br
Radium
Ra
Cadmium
Cd
Selenium
Se
Chromium
Cr
Silver
Ag
Copper
Cu
Sodium
Na
Hydrogen
H
Thallium
Tl
Iodine
I
Uranium
U
Iron
Fe
Vanadium
V
Lead
Pb
Zinc
Zn
Compounds are substances which are composed of two or more different elements and can be decomposed by
chemical changes. Elements in combination are different from elements in the free, or un-combined, state. For
example, white crystalline sugar is a compound consisting of the element carbon, which is usually a black solid
when free, and the two other elements, hydrogen and oxygen, which are colourless gases when un-combined. If
heated sufficiently in the absence of air, sugar decomposes to carbon and water.
Water, a compound, can be decomposed by an electric current into its’ two constituent elements, hydrogen and
oxygen.
Table salt, also a compound, can be broken down by an electric current into sodium and chlorine.
Whereas there are 118 known elements, there are hundreds of thousands of chemical compounds representing
different combinations of these elements. Each compound possesses definite chemical and physical properties
by which chemists can distinguish it from all other compounds.
For Further Information:
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Correct as at 17th October, 2012.