First scientific ideas

First scientific ideas: atoms and elements

More than 2,000 years had to pass before Dalton (1766-1844) laid the foundations of modern atomic theory:

• Pure substances are made of identical atoms, which are the basic and indivisible particles of matter.
• The atoms of different pure substances (chemical elements) combine with one another, in simple whole-number ratios, to form molecules.
• The atoms of different elements can combine in more than one proportion to form different compounds (law of multiple proportions).
• Chemical reactions are the result of the rearrangement of atoms.

In 1869 the Russian chemist Dmitri Mendeleev arranged the known chemical elements according to their chemical properties. To do so he built a table, the Periodic Table of the Elements, which has been the basis for the modern classification of all atoms by their atomic number, both natural and man-made.