Atom is the smallest particle of element which might not be able to exist independently.

Molecule is the smallest particle of the substance which can exist independently. It can be subdivided as

1. Homoatomic molecules are molecules of same element and can be further divided as monoatomic, diatomic and polyatomic molecules depending upon number of atoms. eg: He, O₂, P₄ etc.
2. Heteroatomic molecules are molecules of compound. They can be diatomic and polyatomic. eg: H₂O, PCl₅, H₂SO₄, NO etc.

It is the unit of representing atomic masses. 1 a.m.u. = 1/ 12  th the mass of C-12.

MOLE

It is a unit which represents 6.023 × 10²³ particles. The number 6.023 × 10²³ is called Avogadro's number and is represented by N0 or NA. Avogadro's number of gas molecules occupy a volume of 22400 cm3 at N.T.P. Number of molecules in 1 cm³ of gas at NTP is Loschmidt N0. With value 2.688 × 10¹⁹.

"It is the number of times the atom of the element is heavier than H atom" was the first proposed definition. Later on oxygen was preferred as standard. In 1961 C-12 was chosen as standard and thus "the number of times the atom of an element is heavier than 12th part of C-12 is called atomic mass of the element.

AVERAGE ATOMIC MASS

It is the mass of each isotope determined separately and then combined in ratio of their occurrence. Suppose a and b are two isotopes of an element with their occurence ratio p : q then