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Just as a steno uses shorthand to save time in taking down notes from his officer, in the same manner a chemist, instead of writing full and lengthy names of elements, uses certain abbreviations called symbols. In the language of chemistry, a symbol represents one atom of an element and is usually the first letter of the name of the element. For example, O is the symbol for oxygen, N for nitrogen, sulphur for S and so on. When two or more elements begin with the same letter, another characteristic letter from its name is added in order to avoid confusion. Thus C stands for carbon, Ca for chlorine and Co for cobalt. It should be noted, however, that the first letter is always capital and the second is always small.
The abbreviations used for the lengthy names of elements are termed as symbols.
A symbol is not merely an abbreviation for the name of an element but has a quantitative significance as well. Thus O stands not only for the atom of oxygen but also 16 parts by weight of it, this weight being the atomic weight of the element.
Since atoms of elements combine to form molecules, it should be possible to represent the molecule in terms of symbols of the constituent atoms. This symbolic expression for a molecule is called Formula.
A molecule of atom of the element may contain one or more atoms of it, the number of atoms of the element present in the molecule being placed at the right lower corner of the symbol for that element. For example, H2 represents one molecule of hydrogen containing two atoms of it; I2 is the formula of iodine, O2 of oxygen, N2 of nitrogen and Cl2 that of chlorine and 4Cl2 represents four molecule of chlorine.
In case of compound, the molecule containing different atoms united in a certain fixed ratio is represented by placing symbol of the various elements present in it side by side indicating their number as explained above. Thus KNO3 represents one molecule of potassium nitrate containing one atom of potassium, one of nitrogen and three atoms of oxygen; 5CO2 represents five molecules of carbon dioxide, each containing one atom of carbon and two atoms of oxygen.
A formula has quantitative or weight significance also. Thus KNO3 represents 101 parts by weight of it containing 39 parts by weight of potassium, 14 parts by weight of nitrogen and 16×3 = 48 parts by weight of oxygen. The formula of some common compound is listed below.
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