John Dalton developed atomic theory in 1808, proposing six main postulates: 1) Matter is made of extremely small indivisible particles called atoms, 2) Atoms of a given element are identical in mass and properties, 3) Atoms of different elements differ in mass and properties, 4) Atoms combine in simple whole number ratios to form chemical compounds, 5) In compounds the relative number and type of atoms is fixed, and 6) Atoms cannot be created, destroyed, or divided in chemical reactions. Dalton's theory did not account for isotopes or allotropes which have atoms of varying masses or properties within an element.