Atoms, Molecules, and Matter•Democritus ~ 450 B.C. – argued that by cutting up a piece of matter with sharpest knife imaginable, would eventually come to a piece that would not be cut any further:•Atom = (Greek word – indivisible)•Idea rejected by Plato and Aristotle – matter is continuous.•Due to their influence, Democritus Theory was left dormant for 20 centuries.

Pierre Gassendi (1592-1655)•Italian supported by Newton•1600’s: suggests the existence of atoms

John Dalton (1776-1844)•English chemist and school teacher•Developed several laws of chemistry using atomic theory •Atoms join to form molecules

Robert Brown (1773-1858)•Scottish botanist•1827: looked at pollen grains (suspended in water) under microscope – Browian Motion•Explained by Einstein (collision and momentum problem)

Dmitri Mendeleev (1834-1907)•Period table (only 63 elements)

–Period = rows–Groups = columns (elements that had similar properties = families)