Atomic mass

• Dalton’s Atomic theory proposed the idea of atomic mass which explained the law of constant proportion so well that scientists were prompted to measure the atomic mass of an atom.
• The mass of an atom of an element is called its atomic mass.
• In 1961, IUPAC have accepted atomic mass unit (u) to express atomic and molecular mass of elements and compounds.
• The atomic mass unit is defined as the quantity of mass = 1/12 of mass of an atom carbon-12.

1 amu or u =1/12* mass of an atom of carbon-12

1u = 1.66 * 10^-27kg

Atomic existence

• Most of the atoms of elements are not able to exist independently.
• Atoms from molecules and ions and these molecules or ions aggregate in large numbers to form matter.