This document summarizes the history of atomic structure and the discovery of subatomic particles. It discusses: 1) Early ideas about atoms from Maharshi Kanada and Democritus. 2) John Dalton's atomic theory from 1808 that atoms are indivisible and combine to form molecules. 3) Discovery of the electron from cathode ray experiments by Crookes and Thomson, showing electrons are negatively charged and part of atoms. 4) Realization that atoms must also contain positively charged particles, leading to the discovery of the proton by Goldstein and verification by Thomson.

1. Atomic Structure
By Santosh Sir

4. Idea of smallest unit of Matter
The idea of smallest unit of matter was first given by Maharshi
Kanada in the 6th Century B.C in india.
According to him matter consisted of
indestructible minute particles called
PARAMANU. Paramanu combines with paramanu
to form ANU.

5. The Greek word Atomos means Indivisible
Greek philosopher Democritus called the
paramanu is the atom which is indivisible.

6. Atomic theory by John Dalton
The first scientific theory about the structure of
matter was given by John Dalton in 1808. This
theory was based on experimental facts.
The main postulates of Dalton’s theory are :
1) Matter consist of small and indivisible particles called atom.
2) Atoms can neither be created nor be destroyed.
3) The atoms of an elements are alike .but differ from other atoms of other
elements.
4) Atoms combine to form molecule .
5) Atoms of different elements combine to form compound.
6) Atoms are smallest unit of matter that can take part in chemical reaction

7. Definition of an Atom
• Atom is the smallest particle of an element
that exhibits all the properties of that
elements.
• It may or may not exist independently.
Definition of a Molecule
A group of atoms bonded together,
representing the smallest fundamental unit
of a chemical compound that can take part
in a chemical reaction.

8. Discovery of the subatomic particles
Electron Proton Neutron

9. Discovery of electrons
Discovery of cathode rays :
William Crook carried the experiment in low
atmospheric pressure(0.01mm of Hg) with
high voltage (10,000 volts) in a discharged
tube. He noted that gas which is taken
inside the discharged tube became good
conductor of electricity and begin to flow
from cathode to anode in the form of rays.
Since these rays flow from cathode to
anode, it is called cathode rays

10. Joseph John Thomson studied the characteristics of cathode rays by
conducting the same experiment and he concluded that :
1. Cathode rays consists of –ve charged particles , which are
electrons
2. These are integral part of atom.
3. Electron has both mass and electric charge
Properties of ELECTRONS :
1. Electron from all sources are alike.
2. The mass of electron is 1/1837 of the hydrogen atom.
3. The magnitude of charge of electron is -1.602 x 10 -19

11. Discovery of PROTON
Since negatively charged particles electrons are present in all atoms
and atom as a whole is electrical neutral, it indicates that there must be
some positively charged particle present in the atom which neutralizes
the negative charge on electron.
This realization led to the discovery of proton.

12. The presence of positively charged particles in an atom was shown by
Eugen Goldstein in 1886. He repeated the same discharge tube
method in which he used perforated cathode.

14. by J.J Thomson

18. BOHR-BURY SCHEME

20. Presented by Santosh Sir