Atomic Structure

Atom

The smallest particle of an element is called an atom. An atom can take part in chemical combination and does not occur free in nature. The atom of the hydrogen is the smallest and lightest.
Example — Na, K, Ca, H etc.

Molecule

A molecule is the smallest particle of an element or compound that can have a stable and independent existence.
Example — O₂, N₂ Cl₂, P₄ , S₈ etc.

Mole

A mole is a collection of 6.023 x 10²³ particles. It means that

1 mole = 6.023 x 10²³
1 mole atom = 6.023 x 10²³ atoms
1 mole molecule = 6.023 x 10²³ molecules
1 mole ion = 6.023 x 10²³ ions

Avogadro's Number :
The number 6.023 x 10²³ is called Avogadro's Number.

Atomic Mass

It is the ratio of mass of one atom of the element to 1/12^th part of the mass of one atom of carbon-12.

Actual mass of 1 atom of an element = atomic mass in amu x 1.66 x 10^-24 g

Molecular Mass

It indicates how many times one molecule of a substance is heavier in comparison to 1/12^th mass of one atom of Carbon-12.