3. • Hundred years ago, amidst glowing glass
tubes and the hum of electricity, the British
physicist J.J Thomson. was venturing into
the interior of the atom. At the Cavendish
Laboratory at Cambridge University,
Thomson was experimenting with currents
of electricity inside empty glass tubes. He
was investigating a long-standing puzzle
known as "cathode rays." His experiments
prompted him to make a bold proposal:
these mysterious rays are streams of
particles much smaller than atoms, they are
in fact minuscule pieces of atoms. He called
these particles "corpuscles," and suggested
that they might make up all of the matter in
atoms. It was startling to imagine a particle
residing inside the atom--most people
thought that the atom was indivisible, the
most fundamental unit of matter.
DISCOVERY OF ELECTRONS
J.J Thomson
Cathode ray tube
4. • During the 1880s and ’90s scientists searched
cathode rays for the carrier of the electrical
properties in matter. Their work culminated in the
discovery by English physicist J.J. Thomson of the
electron in 1897. The existence of the electron
showed that the 2,000-year-old conception of the
atom as a homogeneous particle was wrong and
that in fact the atom has a complex structure.
• Cathode-ray studies began in 1854 when Heinrich
Geissler, a glassblower and technical assistant to
the German physicist Julius Plücker, improved the
vacuum tube. Plücker discovered cathode rays in
1858 by sealing two electrodes inside the tube,
evacuating the air, and forcing electric current
between the electrodes. He found a green glow on
the wall of his glass tube and attributed it to rays
emanating from the cathode. In 1869, with better
vacuums, Plücker’s pupil Johann W. Hittorf saw a
shadow cast by an object placed in front of the
cathode. The shadow proved that the cathode rays
originated from the cathode. The English physicist
and chemist William Crookes investigated cathode
rays in 1879 and found that they were bent by a
magnetic field; the direction of deflection
suggested that they were negatively charged
particles. As the luminescence did not depend on
what gas had been in the vacuum or what metal the
electrodes were made of, he surmised that the rays
were a property of the electric current itself. As a
result of Crookes’s work, cathode rays were widely
studied, and the tubes came to be called Crookes
tubes
5. CANAL RAYS
Anode rays (or Canal rays) were
observed in experiments by a German
scientist, Eugen Goldstein, in 1886.
Goldstein used a gas discharge tube
which had a perforated cathode. A "ray"
is produced in the holes (canals) in the
cathode and travels in a direction
opposite to the "cathode rays," which are
streams of electrons. Goldstein called
these positive rays "Kanalstrahlen" -
canal rays because it looks like they are
passing through a canal. In 1907 a study
of how this "ray" was deflected in a
magnetic field, revealed that the particles
making up the ray were not all the same
mass. The lightest, formed when there
was a little hydrogen in the tube, was
calculated to be 1837 times as massive
as an electron. They were protons
6. Discovery of the Neutron
• It is remarkable that the neutron was not
discovered until 1932 when James
Chadwick used scattering data to
calculate the mass of this neutral particle.
Since the time of Rutherford it had been
known that the atomic mass number A of
nuclei is a bit more than twice the atomic
number Z for most atoms and that
essentially all the mass of the atom is
concentrated in the relatively tiny
nucleus. As of about 1930 it was
presumed that the fundamental particles
were protons and electrons, but that
required that somehow a number of
electrons were bound in the nucleus to
partially cancel the charge of A protons.
But by this time it was known from the
uncertainty principle and from "
particle-in-a-box" type confinement
calculations that there just wasn't enough
energy available to contain electrons in
the nucleus.
7. J. J. Thomson's raisin bread model
(plum pudding model)
• J. J. Thomson considered that the
structure of an atom is something
like a raisin bread, so that his
atomic model is sometimes called
the raisin bread model. He
assumed that the basic body of an
atom is a spherical object
containing N electrons confined in
homogeneous jellylike but relatively
massive positive charge distribution
whose total charge cancels that of
the N electrons. The schematic
drawing of this model is shown in
the following figure. Thomson's
model is sometimes dubbed a
plum pudding model.
8. RUTHERFORD’S ALPHA
SCATTERING EXPERIMENT
• In the years 1909-1911 Ernest Ruthefordand
his students - Hans Geiger (1882-1945) and
Ernest Marsden conducted some
experiments to search the problem of alpha
particles scattering by the thin gold-leaf.
Rutheford knew that the particles contain the
2e charge. The experiment caused the
creation of the new model of atom - the
"planetary" model.
Rutheford suggested to hit the gold-leaf
(picture no. 1) with fast alpha particles from
the source 214Po. (The source R was in the
lead lining F). The particles felt from the
source on the gold-leaf E and were observed
by the microscope M. The whole experiment
was in the metal lining A and was covered
with the glass plate P. The instrument was
attached to the footing B. The gold leaf was
about 5*10-7 meter thick. The scientist knew
that reckoning the scattering angle could say
much about the structure of atoms of the
gold-leaf.
9. • Rutheford made a theoretical analysis of
angles of scattering in accordance with
Thomson's theory of atom and in accordance
with his own theory. He assumed that atom
consisted of positive charged nucleus and
negative charged electrons circling around
the nucleus. Then his theoretic calculations
he compared with the experiment result.
Alpha particles going through atom created
in accordance with the "plum cake" model
wouldn't be strong abberated because the
electric field in that atom wouldn't be strong.
In the model created by Rutheford the field is
much stronger near to the nucleus, so some
of alpha particles are much more abberated.
The other going in the far distance to the
nucleus are almost not at all abberated. The
probability that any alpha particle will hit the
nucleus is small but there is such a chance.
10. PLANETARY MODEL OF ATOM BY
BOHR
• The Bohr Model is probably familar as
the "planetary model" of the atom
illustrated in the adjacent figure that,
for example, is used as a symbol for
atomic energy (a bit of a misnomer,
since the energy in "atomic energy" is
actually the energy of the nucleus,
rather than the entire atom). In the
Bohr Model the neutrons and protons
(symbolized by red and blue balls in
the adjacent image) occupy a dense
central region called the nucleus, and
the electrons orbit the nucleus much
like planets orbiting the Sun (but the
orbits are not confined to a plane as is
approximately true in the Solar
System). The adjacent image is not to
scale since in the realistic case the
radius of the nucleus is about 100,000
times smaller than the radius of the
entire atom, and as far as we can tell
electrons are point particles without a
physical extent
• This similarity between a planetary
model and the Bohr Model of the
atom ultimately arises because the
attractive gravitational force in a
solar system and the attractive
Coulomb (electrical) force between
the positively charged nucleus and
the negatively charged electrons in
an atom are mathematically of the
same form. (The form is the same,
but the intrinsic strength of the
Coulomb interaction is much larger
than that of the gravitational
interaction; in addition, there are
positive and negative electrical
charges so the Coulomb interaction
can be either attractive or repulsive,
but gravitation is always attractive in
our present Universe.)
11.
12. QUANTUM MECHANICAL MODEL
• According to the Principles of Quantum Mechanics electrons are
distributed around the nucleus in "probability regions". These
probability regions are called "atomic orbitals". According to
Quantum Mechanics, these orbitals are mathematically defined
and are described by a uniquely different math function for each
electron in the atom called an "eigen function" and a differential
equation generated by the following equation:
• H(eigen function) = Energy ( eigen function)
• The H in the above equation stands for a mathematical operator
called the Hamiltonian. We should be familiar with math operators
since we have been dealing with them since grade school. The
addition operator has to operate upon two numbers one that
appears on its left and the other on its right. For example the
addition operator operates upon the number 4 and 3 and the result
of that operation as all knows would be 7. We have subtraction,
multiplier, division,common log, natural log,exponentiation,etc. The
Hamiltonian operator is kinda like these but much more complex.
The result of the Hamiltonian operator operating on the eigen
function of an electron is to generate a differential equation.
Differential equations often have more than one root or solution
which is not new to those who have had a first year algebra course
where quadratic equations are studied. However, one property of
differential equations that might be new to you is the fact that
differential equations cannot be solved exactly. We must use
approximation methods to extract any roots out of the equation,
and those roots or solutions will be approximate solutions.
