Download Early Atomic Theories and the Origins of Quantum Theory

UNIT1: STRUCTURE AND PROPERTIES OF MATTER
3.1a – EARLY ATOMIC THEORIES AND THE ORIGINS OF QUANTUM THEORY
EARLY DEVELOPMENTS
Aristotle
 Aristotle proposed that all matter is made of Earth, Air, Fire, or Water.
 Any combination of the four could produce all known matter
Democritus
 Democritus was the first to suggest that matter was composed of tiny
particles known as “atomos” - Greek for atom
John Dalton
Dalton proposed the 'Billiard Ball' model in which the atoms are like
billiard balls (solid spheres). He proposed the following theory:
 All matter is made up of tiny indivisible particles called atoms – we
have come to know the truth in this statement that atoms are made up
of
.
 All atoms of an element are identical and atoms of different elements
are different
 Atoms are rearranged to form new substances in chemical reactions,
but they are never created or destroyed or divided
J.J. Thompson and the Electron
 J.J. Thompson provided experimental
evidence for the existence of electrons.
 He accomplished this by building a device
called a
(an empty tube
with metal electrodes at each end).
 When the power turned on, a ray moved away
from the
electrode to the
electrode leading Thompson
to believe he was dealing with a
particle. Since atoms are neutral, he
reasoned that atoms should contain a
charge.
 As a result, he proposed the
: A positive charge with embedded
the
comes from the positive charge.
Robert Milikan
. The majority of
Main Discovery:
YOUTUBE VIDEOS
How did he do it?
https://www.youtube.com/watch?v=UFiPWv03f6g
Radioactivity, Becquerel and Rutherford:
From the experiments of Henri Becquerel and Marie and Pierre Curie, we have learned that
certain elements exhibit radioactivity – they give off a significant amount of
.
It was Rutherford who determined that radioactivity is he resulting energy comes from the
of atoms. He determined three different types of radioactive emissions:
Ernest Rutherford
Main Discovery
How did he do it?
James Chadwick
 James Chadwick worked with Rutherford to determine the masses of the nuclei of various
elements. They determined that the mass of the nuclei was not the same as the mass of
the protons themselves leading to another particle being discovered: the
.
REVIEW: ATOMS AND ISOTOPES
An atom consists of smaller particles called subatomic particles. In later chemistry, you will
learn that there are particles smaller than these.
Particle
Electron
Symbol
Charge
Location
Mass (kg)
Proton
Neutron
The nucleus is the centre of the atom and contains the protons and neutrons. It is electrons
that truly responsible for the chemical properties of atoms. The nuclear particles make up a
significant amount of the
of an atom, but the electron makes up a significant
amount of the
of an atom.
Also remember, that the atomic mass (A) is a sum of the protons and neutrons in an atom and
the atomic number (Z) is represented by the number of protons in an atom.
Isotopes are atoms with the same number of protons and a different number of
.
They have the same chemical properties. They are a heavier version of the atom.
A radioisotope is different, because these isotopes have a nucleus that is unstable and as a
result gives off
. There are many modern uses for radioactivity.
3.1b THE NATURE OF MATTER AND ENERGY
There have been many theories concerning the nature of light. We will look briefly at a few
of the proposals:
Scientist
Greek Philosophers
Theory
Christiaan Huygens
Isaac Newton
James Maxwell
Maxwell’s theory is what we used today – that light is composed of
waves with different wavelengths.
The Connection between Matter and Energy:
The Photoelectric Effect
Hertz came up with the Photoelectric effect in which matter
can absorb energy from
wavelengths and allow that matter to release
electrons. Hertz also determined that it was the
of light not the intensity of the light that determined
how many electrons would be released.
Max Planck and the Quantum Hypothesis


 When one heats a solid object at a very high
temperature, we know that they tend to radiate in
different colours. This is the same for any solid
object.
 Scientists used to think that if you increased
the temperature or intensity (brightness), you would
increase the energy that was being
or
by that object continuously.
 Instead, it was shown that the intensity had a
peak and then decreased after. Planck hypothesized
that matter could gain or lose energy.
Planck proposed that on a subatomic level of the atom, energy is absorbed or released
discontinuously in small packets of energy called
. The energies from
vibrating atoms were a multiple of these small packets of energy. There is only a
whole packet or nothing at all.
Planck explained that the reason energy appeared to be absorbed and liberated
continuously was because we were observing many trillions of these events
asynchronistically -
Albert Einstein and Photons
 Einstein hypothesized that electromagnetic
radiation could be seen as a stream of
(unit of light energy).
 He also hypothesized that if a photon collided
with an electron in a metal, an electron could
be emitted from that metal. This would be the
reason why some metals acquired a
when illuminated by light.
 When light strikes metal, there is a transfer of
energy between the
and the
. Some of the energy is
used to allow the electron to
from the atoms in the metal and the rest of
the energy is left over as
for the ejected electron.
 From this, we have determined that the kinetic energy of the electron released from the
matter depends on the
of light. A minimum frequency of light
is needed and this value is different for different metals.
A formula in physics is used to describe this phenomena:
h=
v=
Therefore, the lower the frequency of light, the
energy of the photon. What does this mean for electron?
Homework: Read Page 134-142 and complete questions 1-7, 9
the