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Chemical Formulas
Chemical formulas are used to represent chemical substances (i.e. both elements and compounds). For elements, the formulas are the symbols of the elements times a number (written as a subscript). This number denotes the number of atoms required to make the element stable in its simplest quantity (i.e. as a molecule).
For compounds:
Compounds are made of more than one element, chemically combined. Their formulas consist of symbols of the respective elements which made them up.
The number of atoms of each element present in the combination is dependent on the combining power or valency of the element.
Notice that to make the simplest formula, the combining powers of the elements in combination are interchanged between them and expressed as
subscripts.
For example, water is made of hydrogen and oxygen. Hydrogen has valency or combining power of 1, while oxygen a valency of 2.
Therefore, interchanging the valencies of the elements to obtain their combining number of atoms gives the formula of water. I.e., H_{2}O.
The same principle goes for the formula of aluminium oxide. I.e. Al  trivalent, O  divalent. The formula is Al_{2}O_{3} (by interchanging the valencies of the two elements involved).
It also applies to situations where we have groups of atoms (called radicals) combining with each other or with atoms of metallic elements, or with atoms of nonmetallic elements.
That is, the formulas are obtained by interchanging the combining powers of both radicals or of the radical and the metallic or nonmetallic atoms. Examples of radicals, single elements, and their combining powers are given in the table below:
Tables showing elements and radicals and their
combining powers
Element 
Combining Power 
Calcium Ca^{2+} 
+2 
Copper(II)
Cu^{2+} 
+2 
Potassium K^{+} 
+1 
Sodium Na^{+} 
+1 
Iron(III) Fe^{3+} 
+3 
Magnesium Mg^{2+} 
+2 
Aluminium Al^{3+} 
+3 
Lead(II) Pb^{2+} 
+2 
Clorine Cl^{} 
1 
Oxygen O^{2} 
2 
Sulphur S^{2} 
2 
Radical 
Combining Power 
Carbonate(IV)
CO_{3}^{2} 
2 
Nitrate(V)
NO_{3}^{} 
1 
Sulphate(VI)
SO_{4}^{2} 
2 
Ammonium NH_{4}^{+} 
+1 
Hydroxyl OH^{} 
1 
Chromate(VI)
Cr_{2}O_{7}^{2} 
2 
To obtain the formula of calcium trioxonitrate(V) (formed from the combination of calcium and the nitrate(V) group):
Calcium has combining power of +2. Nitrate(V), NO_{3}^{} has combining power of 1.
Therefore, interchanging their combining powers gives the simplest formula, Ca(NO_{3})_{2}
The best way to be certain about the formula of an unknown compound is from experimental data
 Empirical Formula. This is so because some elements can combine to form more than one possible products.
