We have just seen, the first chemist to use the name ‘atom’ was John Dalton. Dalton used the word ‘atom’ to mean the smallest particle of an element. He then went on explaining how atoms could react together to form molecules; which he called ‘compound atoms’. Today we know what a molecule is. A molecule is an aggregate of two or more than two atoms of the same or different elements in a definite arrangement held together by chemical forces or chemical bonds. We can also define a molecule as smallest particle of an element or of a compound which can exist alone or freely under ordinary conditions and shows all properties of that substance (element or compound). A molecule will be diatomic if there are two atoms, for example, chlorine (Cl₂), carbon monoxide, CO; will be triatomic if there are three atoms, for example, water (H₂O) or carbon dioxide, (CO₂), will be tetratomic and pentatomic if there are four and five atoms respectively. In general, a molecule having atoms more than four will be called polyatomic. There are eight atoms in a molecule of sulphur and nine atoms in a molecule of ethyl alcohol and we write formulas as S₈ and C₂H₅OH respectively. Only a few years back, a form of carbon called buck-minsterfullerene having molecular formula, C⁶⁰ was discovered.