CHE1001 Applied Chemistry
Lecture 2
Historical Perspective of Chemistry
• By late 1800 it was well
accepted that atom is the basic
constituent of matter.
• It was believed that nature was
understood. There were no big
discoveries to be made yet.
• Newtonian mechanics,
thermodynamics , statistical
mechanics were formulated.
• Maxwell equation was verified. It
proposed that propose that light
is an electromagnetic
phenomenon
J. J. Thompson
•Let’s talk that atom is not the
most basic constituent of matter.
•So, we are going to start here
with this gentle man named J. J.
Thompson.
•He was a British physicist and
Nobel Laureate in Physics,
credited with the discovery of the
electron, the first subatomic
particle to be discovered.
•He was interested in doing what
a discharge was.
Cathode rays
Cathode rays
• He constructed a partially evacuated glass tube called cathode
ray tube
• Applied high electrical voltage between the two electrodes
Findings1. Cathode rays travel from the negatively charged electrode to
the positively charged electrode
2. The particles that compose the cathode rays travel in straight
lines
3. These are independent of the composition of the material from
which they originate
4. These carry a negative electrical charge
Cathode rays Experiment
Findings
• There is a particle that is
less massive than
hydrogen.
• You can chop the hydrogen
atom up.
• The atom is not the most
basic constituent of matter.
Rutherford
Atomic
Model
Rutherford Experiment
Rutherford Atomic Model
Based on the above observations and conclusions, Rutherford
proposed the atomic structure of elements. According to the
Rutherford atomic model:
1. The positively charged particles and most of the mass of an atom
was concentrated in an extremely small volume. He called this
region of the atom as a nucleus.
2. Rutherford model proposed that the negatively charged electrons
surround the nucleus of an atom. He also claimed that the
electrons surrounding the nucleus revolve around it with very high
speed in circular paths. He named these circular paths as orbits.
3. Electrons being negatively charged and nucleus being a densely
concentrated mass of positively charged particles are held
together by a strong electrostatic force of attraction.
Rutherford Atomic Model
Failures
•Rutherford proposed that the electrons revolve around the nucleus in fixed
paths called orbits.
• Calculations have shown that as per the Rutherford model, an electron
would
collapse
in
the
nucleus
in
less
than
10-8
seconds.
So,
Rutherford model was not in accordance with Maxwell’s theory and could
not explain the stability of an atom.
•One of the drawbacks of the Rutherford model was also that he did not say
anything about the arrangement of electrons in an atom which made his
theory incomplete.
Classical Description of an Atom
Classical Description of an Atom
Classical Description of an Atom
Classical Description of an Atom
Failure Of Classical Mechanics
Assumptions made by Quantum
Mechanics
Atomic Absorption
Atomic Emission