Atom:
☺ Atom is the smallest possible particle of the element that takes part in any chemical reaction but doesn’t exist in a free state in nature.
☺ All the characteristics of the element are present in it.

Molecule:
☺ The molecule is the smallest possible particle that doesn’t take part in any chemical reaction but exists in a free state in nature.

Fundamental Particles of Atom:
Electron:
☺ Invented by J.J. Thomson in 1897.
☺ It is a negatively charged particle.
☺ It rotates in various orbits around the nucleus.
☺ Mass of an Electron: 9.1 * 10^-31 Kg
☺ Charge of an Electron: -1.6 * 10^-19

Proton:
☺ Invented by Ernest Rutherford in 1919.
☺ It is a Positively charged stable particle.
☺ Mass of a Proton: 1.67 * 10^-27 Kg
☺ Charge of a Proton: +1.6 * 10^-19

Neutron:
☺ Invented by James Chadwick in 1932.
☺ It is a neutral unstable particle.
☺ Mass of a Neutron: 1.67 * 10^-27 Kg

Other Sub-Atomic Particles:

Positron:
☺ Positron is the antiparticle of the electron.
☺ Its mass is equal to the mass of an electron.
☺ It has an opposite charge of an electron i.e. positive charge.
☺ Positron was discovered by Carl D. Anderson in 1932.

Antiproton:
☺ Antiproton is the antiparticle of the proton.
☺ Its mass is equal to that of the proton.
☺ It has the opposite charge of a proton i.e. negative charge.
☺ Antiproton was discovered by Emilio Segre in 1955.

Neutrino and Antineutrino:
☺ Neutrino and Antineutrino are actually antiparticles of each other.
☺ It was predicted in 1930 by E. Pauli while explaining the emission of β particles from radioactive nuclei, but these particles were actually observed experimentally in 1956.

pi(π)- mesons:
☺ Pie-Mesons was firstly predicted by H. Yukawa in 1935, but those actually discovered in 1947.
☺ There are three types: positive π-meson, negative π-meson, neutral π-meson.

Quarks and Bosons:
☺ Quarks: the elementary particles from which other heavy subatomic particles like proton, neutron, etc. are formed.
☺ Bosons: A subatomic particle-like photon that has zero or integral spin and which follows Bose-Einstein statics is called a boson.