3RD PPT PHYS SCI.pdf

Discovery of
Structure of Atoms

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.

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”.

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.

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.

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

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

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

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.

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.

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

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.

The Discovery of Proton,
Electron and Neutrons

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.

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

Atomic model proposed by
different scientists

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).

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.

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.

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.

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.

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

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

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

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.

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