John Dalton performed experiments in 1800 that showed matter is made of indivisible particles called atoms. This led him to formulate Dalton's Atomic Theory which stated that atoms are identical for each element and compounds form when different types of atoms combine. While initially controversial, the theory became widely accepted after discoveries of subatomic particles like the electron and proton in the 1900s. These discoveries included J.J. Thomson finding the electron using cathode ray tubes in 1897 and identifying it as an atom's negatively charged component 2000 times smaller than hydrogen, and Rutherford proposing the nuclear model of the atom with a small, positively charged nucleus surrounded by orbiting electrons.

1. Discovery of
Structure of Atoms

2. Dalton’s Atomic Theory
The idea of atomic theory
was revived by John Dalton
(1766-1844) 2,000 years
after Democritus first
proposed about it.

3. In 1800, John Dalton performed a series of
experiments showing the matter consists of lumpy
particles called “atoms”.
His Findings led to the formulation of what is commonly
known as “Dalton’s Atomic Theory”.

4. Dalton formulated his theory based on Antoine
Lavoiser’s (1743-1794) law of conservation of mass and
Joseph Proust’s (1754-1826) law of definite proportion.
Law of Conservation of
Mass
Law of Definite Proportion
The total mass of substances
before and after a complete
reaction are equal.
Elements combine in a fixed
proportion to form a
compound.

5. The following are the major postulates of
Dalton’s Atomic theory:
1.Matter is composed of small
indivisible particles called atoms.
2.Atoms of the same element are
identical.
3.Compounds are formed by a
combination of two or more different
kinds of atoms.
4.A chemical reaction is
a rearrangement of atoms.

6. The formulation of Dalton’s atomic
theory marked the start of the modern
era of chemistry.

7. However, during his time, (1800s), some
scientists still did not believe that there
was an indivisible atom found in all
matter.

8. In the 1900s, the theory became
widely accepted upon discovery of
subatomic particles.

9. Let us look at the Cathode Ray Tube experiment
The CRT by J.J Thomson
is a glass tube with two
electrodes: the positive
electrode called anode
and the negative
electrode called
cathode.

10. Once you apply a sufficiently high voltage, a stream
of ELECTRONS flows towards the "cathode"; this is
what you call a cathode ray. Inside the tube there
exists near perfect vacuum, so there are barely any
other gas molecules, ions etc.
Compare it to a lightning.

11. JJ Thomson decide to apply a magnetic field in his
experiment.

12. J.J. Thomson was able to have a ratio
of the mass of the bent particles
He found out that it was 2000x lighter
than a Hydrogen atom. And that was
the time the electron was discovered.

13. The Discovery of Proton,
Electron and Neutrons

14. In 1886, Eugen Goldstein (1850-1930) discovered the
positively charged subatomic particle.
However, it was Ernest Rutherford (1871-1937) who coined
the term proton for the positively charged particle in an atom.
Then using CRT experiment, J.J. Thomson (1856-1940)
discovered that an atom is also composed of negatively
charged particles which he named electrons.

15. In 1932, James Chadwick (1891-1974)
discovered the neutral particles, which he
called neutrons, in the nucleus of an atom.

16. Atomic model proposed by
different scientists

17. J.J. Thomson’s Plum Pudding Model
In 1904, he proposed the “plum
pudding” model of divisible atom.
According to him, atoms consist
of a large sphere of uniform
positive charge embedded with
smaller negatively charged
particles called electrons
(corpuscles).

18. Ernest Rutherford’s Nuclear Model
Based on this model, the atom has a
very small positively charged
nucleus that contains most of the
mass of the atom.
The electrons move around the
nucleus
The protons found inside the nucleus
are positively charged.

19. The number of protons is equal to the
number of electrons.
Later he postulated the existence of a
neutral particle in the nucleus to make up
for he calculated mass deficiency in the
atom studied.

20. Niels Bohr’s Planetary model
In his model (1.) the
electrons move around
the nucleus in fixed orbit.
 An electron in a
particular orbit has
constant energy.

21. 2. An electron can absorb energy and move to a
higher energy orbit of larger radius;
3. An excited electron can fall back to its original
orbit by emitting energy as radiation;
4. Electrons can only exist in certain discrete
energy levels.

22. James Chadwick Model
According to him, the nucleus
of an atom contains neutrons,
electrically neutral particles
with a mass similar to that of
proton.

23. Quantum Mechanical Model
In this model, the
electrons are considered
to be moving around the
nucleus- but not in a fixed
orbit.

24. Their exact location is not known, it
is only their probable location that is
being considered.

25. This location is around the
nucleus, in an arbitrary cloud.
This “electron cloud” is that
region around the nucleus
where electrons are most
likely to be found.