Matter is made of atoms. The idea of the atom was first proposed by the Greek philosophers in the fifth century BC (BCE), but their theory was more philosophical than scientific. The first scientific theory of the atom was proposed by John Dalton. Some of Dalton's postulates were found incorrect by the later studies of J.J. Thomson, Rutherford, Neils Bohr and Schrodinger, and a new theory known as the modern atomic theory was put forward. This chapter covers atomic mass, molecular mass, the mole concept, percentage composition, Avogadro's hypothesis and vapour density.
1. Main Postulates of Modern Atomic Theory
- An atom is no longer indivisible (after the discovery of the electron, proton and neutron).
- Atoms of the same element may have different atomic mass (discovery of isotopes, e.g. Cl-35, Cl-37).
- Atoms of different elements may have the same atomic mass (discovery of isobars, e.g. Ar-40, Ca-40).
- Atoms of one element can be transmuted into atoms of other elements; the atom is no longer indestructible (discovery of artificial transmutation).
- Atoms may not always combine in a simple whole number ratio (e.g. Glucose C6H12O6, C:H:O = 6:12:6 or 1:2:1; Sucrose C12H22O11, C:H:O = 12:22:11).
- Atom is the smallest particle that takes part in a chemical reaction.
- The mass of an atom can be converted into energy (E = mc^2).