In the 5th cent. B.C. the Greek philosophers Democritus and Leucippus proposed that matter was made up of tiny, indivisible particles they called "atom", or in Greek ""a-tomos". The reason why they assumed this is because nothing can come from nothing. Democritus believed that all atoms were firm and solid, but they could not all be the same. If all atoms were identical, there would be no satisfactory explanation of how they could combine to form everything in different shapes. Democritus concluded atom with these ideal qualities:
             • invisible because of their extremely small size
             • indivisible as their name indicates
             • eternal because they are perfect
             • surrounded by an empty space (to explain their movement and changes in density)
             • having an infinite number of shapes (to explain the diversity observed in nature)
             Around 1803, John Dalton (1766-1844) developed the first useful atomic theory of matter. In the course of his studies on meteorology, Dalton concluded that evaporated water exists in air as an independent gas. Solid bodies can't occupy the same space at the same time, but obviously water and air could. Dalton reasoned that if the water and air were composed of discrete particles, evaporation might be viewed as a mixing of water particles with air particles. He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different. In 1808 Dalton's A New System of Chemical Philosophy was published. In this book he listed the atomic weights of a number of known elements relative to the weight of hydrogen. His weights were not entirely accurate but they form the ba
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