Chemistry (NTSE/Olympiad)  

3. Atoms & Molecules

Chemical Formulae

A compound is represented in the abbreviated form by chemical formula.
The chemical formula of a compound represents the composition of a molecule of the compound in terms of the symbols of the elements present in it.
Example. Water is a compound made up of 2 atoms of hydrogen element and 1 atom of oxygen element, so the formula of water is written as H2O. In the formula H2O, the subscript 2 indicates 2 atoms of hydrogen. In the formula of water, oxygen O is written without a subscript and it indicates 1 atoms of oxygen.
Formulae of elements
The chemical formula of an element is a statement of the composition of its molecule in which symbol tells us the element and the subscript tells us how many atoms are present in one molecule. One molecule of hydrogen element contains two atoms of hydrogen, therefore, the formula of hydrogen is H2.
Example. The formula H2 indicates that one molecule of hydrogen element contains 2 atoms of hydrogen. 2 H represents 2 separate atoms of hydrogen; H2 represents 1 molecule of hydrogen and 2H2 represents 2 molecules of hydrogen.
Formulae of compounds
The chemical formula of a compound is a statement of its composition in which the chemical symbols tell us which elements are present and the subscripts tell us how many atoms of each element are present in one molecule of the compound.
Example. Water is a compound whose molecule contains 2 atoms of hydrogen and 1 atom of oxygen. So, the formula of water is H2O.

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