The word “Atom” was first used by a Greek philosopher known as DEMOCRITUS. He first named it “ATOMUS” meaning smallest particle that cannot be broken into pieces. The word “Atom” are made of 3 compartment namely:
1.Proton
2.Neutrons
3.Electrons

DALTON’S ATOMIC THEORY
Dalton said that an atom is indivisible and indestructible particle of an element that can take part in chemical reaction. Hence, further experiments shows that atom are divisible by the presence of proton, neutrons and electrons.

J.J Thompson Model
In 1897, Sir j.j Thompson show that a gas can conduct electricity when a potential difference of 500v was applied across a glass tube containing a gas at a very low pressure of about 0.0001atm. When the potential difference increase to about 1500, a bright green light appeared. Thompson was able to prove that the glow was due to some kind of ray which travel on a straight line from the cathode hence, he called it cathode ray.
N.B- Another name for cathode ray is ELECTRON. J.j Thompson discovered electron.
Xties of Cathode Rays
1. It is an electron
2. It is negatively charged
3. Cathode rays cast out shadow of an obstacle placed on its path
4. They are identical in the ratio of charge to mass
5. They produces mechanical motion.
N.B- Thompson concluded that cathode ray is an electron. Hence he calculated the charge to mass of electron to be 1.76 X 10∆11Ckg where C is columb.

In 1910, R.A Millican measured the charge of electron in his oil drop experiment while Rutherford performed alpha particles experiment. He calculated the charge to be -1.6 X 10∆-19.

DISCOVERY OF PROTON.
Atom is believed to be electrically neutral. Hence, there must exist inside the atom enough positive charge to balance the negative charge.
Thompson repeated his experiment in a discharge tube with a central cathode with a hole of which he noticed a reddish glow. The reddish glow is due to positive Ray.