Transcript 4 Timeline of Structure

Structure History
Coliseum
Telephone Invented
80 ADE
Corpuscles
“electrons”
1876
1st Flight
Plum Pudding
Model
1897
Wright Brothers
Saturnian Model
1904
1898
1903
Planetary Model
Bohr Model
1911
1913 - 1915
Proton
Television Invented
1920
First Cyclotron
1931
1927
Neutron
Electron Cloud
Model
1932
1932
Main Menu
1
Subatomic
Particles
1930’s - present
Structure History
J.J. Thomson
• Born 1856 - Died 1940
• British physicist
• Awarded the Nobel Prize in Physics in 1906
for his work on the conduction of electricity
by gases
•Knighted in 1908
1897 - Discovered “corpuscles”
- negatively charged particles which make up atoms
- The term “electron” was coined six years earlier by
George Stoney as a particle of charge
- The term electron later replaced “corpuscle”
2
Structure History
Plum-Pudding Model
1898 - Thomson developed his “plum-pudding” model
- Model is named after a
traditional English dessert
- The atom consisted of a
positively charged electric field
throughout which electrons were
suspended
- The atom consisted of mostly empty space with
its mass being due to the electrons
3
Structure History
Hantaro Nagaoka
• Born 1865 - Died 1950
• Japanese physicist
• 1904 - Proposed a “Saturnian
Model” of the atom
- A flat ring of electrons
that revolve around a
positively charged
particle
4
Structure History
Ernest Rutherford
• Born 1871 - Died 1937
• New Zealand chemist
• Awarded the Nobel Prize in Chemistry in
1908 for investigations into the
disintegration of the elements, and the
chemistry of radioactive substances
1909 - Gold Foil Experiment
- Experiments were performed by his students,
Hans Geiger and Earnest Marsden
- Shot alpha particles at a thin piece of gold foil
- He expected the alpha particles to pass straight
through the foil because of their large mass
5
Structure History
Contrary to Rutherford’s prediction, the results showed that
some of the particles were deflected to the side of the
foil while a small few bounced off the foil.
- He said that the alpha particles bouncing off the gold
foil was the equivalent of a cannon ball bouncing off a
piece of tissue paper.
- The particles that
bounced off must have
been hitting objects with
a very small volume but a
very large mass.
screen
gold foil
alpha particle beam
alpha particle emitter
6
Structure History
Planetary Model
Based on the results of this experiment, Rutherford proposed
his Planetary Model of the atom:
• The atom contains a tiny dense
center called the nucleus.
• The nucleus is essentially the
entire mass of the atom.
• The nucleus is positively charged.
• This positive charge balances out the
negative charge of the electrons making the
atom neutral.
• Electrons move around in the empty space surrounding
the nucleus much like the way the planets orbit the sun.
7
Structure History
Planetary Model
Electrons are held in their orbits by the electric force that
attracts negatively charged electrons to the positively
charged nucleus
One problem - This model of the atom is not stable when
applied to the rules of classical physics
- Classical electromagnetic theory says that any charged
particle that is not at rest or in uniform motion in a straight
line will emit energy as electromagnetic radiation
- Therefore the electrons in the planetary model will be
emitting electromagnetic radiation
- As they lose energy they will spiral towards the nucleus and
collide with it
8
Structure History
Niels Bohr
• Born 1885 - Died 1962
• Danish physicist
• Awarded the Nobel Prize in Physics in 1922
for the investigation of the structure of atoms
and the radiation emanating from them.
9
657 nm
486 nm
410 nm
434 nm
1913 - Bohr developed a model of the hydrogen atom
that would explain its line spectrum.
Structure History
1885 - J.J. Balmer developed a formula into which the
wavelengths of the four visible lines of the hydrogen
spectrum would fit. Balmer’s formula was modified five
years later by J.R. Rydberg and is now known as the
Balmer-Rydberg equation.
 1 1 
 R  2 - 2  ; n  3, 4, 5, 6
n
2 n 
1
Rydberg Constant
1.097 x 10-2 nm-1
J.J. Balmer
• Born 1825 - Died 1898
• Swiss school teacher
10
Structure History
• Bohr agreed with Rutherford’s idea that the electrons orbit
the nucleus. He assumed that the classical laws of physics
were inadequate to describe the nature of the atom. The
classical ideas state that a negative electron should spiral
into the positive nucleus.
• Bohr borrowed Planck’s idea that energies are quantized
and proposed that only orbits of certain radii,
corresponding to defined energies, are permitted.
- An electron orbiting in an allowed orbit will not radiate
energy and therefore will not spiral into the nucleus.
• He came up with a simple formula for the energies of
these orbits:
Rydberg Constant
2.18 x 10-18 J
 1 
En   - RH   2  ; n  1, 2,3, 4,...
n 
11
Structure History
• The integer n is called the principal quantum number
and each orbit corresponds to a different value of n.
- As the value of n increases, the radius of the
orbit increases.
• Bohr assumed that electrons could “quantum jump” from
one allowed orbit to another by absorbing or emitting
photons of light with specific frequencies.
• A photon is absorbed when an electron moves to an
orbit with a higher energy state and is emitted when an
electron moves to an orbit with a lower energy state.
• Bohr’s model states that only photons of specific frequencies
can be absorbed or emitted by the atom.
- These frequencies must correspond to the energy
difference between two orbits.
12
Structure History
Energy of
final orbit
Energy of
initial orbit
Planck’s constant
6.63 x 10-34 J. s
E  E f  Ei  h  f
Frequency(s-1)
Substituting the energy formula above into Bohr’s equation
gives the relationship between the frequency of the
absorbed or emitted light and the principal quantum
number of the two states.
Previous formula:
 1
hv    RH   2
n
 f

 1 
    RH   2  E  - R  1 


n
n
H
 2
i
 

n 
 1
1 
1 
 RH   1
hv    RH   2  2 
v
 2


2
n

n n 
n
h


i 
f 
 f
 i
13
Structure History
 RH
v
 h
1 
 1
  2  2 
  ni n f 
nf > ni ; photon is absorbed
nf < ni ; photon is emitted
Important note!
- A calculated negative value only means that light is
being emitted during the transition. However, the
frequency of that photon of light is still positive.
Bohr’s model was important because it introduced the idea
of using quantized energy states for electrons in atoms.
However, his model is only accurate for atoms or ions with
a single electron.
H, He+, Li2+
14
Structure History
Protons
1919 - Rutherford discovered that he could change
nitrogen atoms into oxygen atoms by striking
them with energetic alpha particles.
This process is known as transmutation.
1920 - Rutherford and other physicists realized that upon
transmuting one atom into another a hydrogen
nuclei would be emitted.
• By comparing nuclear masses to charge, it was realized
that the positive charge of any nucleus could be
accounted for by an integer number of hydrogen nuclei.
• The hydrogen nucleus was said to be an elementary
particle and given the name proton by Rutherford.
• The term proton first appeared in print in 1920.
15
Structure History
Neutrons
1920 - Rutherford predicted the existence of a neutral
particle, with the approximate mass of a proton,
that could result from the capture of an electron by a
proton.
1930 - German physicists Walter Bothe and Herbert
Becker shot alpha particles at beryllium. The
beryllium emitted a neutral radiation which they
believed to be high energy gamma rays.
1932 - Irene and Frederick Joliet-Curie put a block of
paraffin wax in front of these beryllium rays and
observed high speed protons coming from the
paraffin. They knew gamma rays could eject
electrons from metals and assumed the same
phenomena was occurring with the protons in the
paraffin.
16
Structure History
James Chadwick
•Born 1891 - Died 1974
• British physicist
• Student and colleague of Rutherford
• Knighted in 1945
1932 - Proved that the beryllium emissions contained a neutral
component approximately equal to that of the proton.
He called this neutral component a neutron.
1933 - Franz Curie proves that the neutron are NOT the result
of the binding of an electron to a proton but an
elementary particle, like electrons and protons.
1935 - Chadwick is awarded the Nobel Prize in Physics for
his discovery of the neutron.
17
Structure History
Louis de Broglie
• Born 1892 - Died 1987
• French physicist
• Awarded the Nobel Prize in Physics in 1929
for discovery of the wave nature of electrons.
1924 - De Broglie proposed that an electron in its orbit around
the nucleus has associated with it a particular
wavelength.
1927 - Wave nature of the electron was experimentally proven
through electron diffraction by Davisson and Germer.
- An electron microscope uses the wave characteristics of
electrons in the same manner as an optical microscope (like
in school science labs) uses the wave behavior of light.
Light microscope magnify up to 1,000X
Electron microscope magnify up to 300,000X
18
Structure History
Werner Heisenberg
• Born 1901 - Died 1976
• German physicist
• Awarded the Nobel Prize in 1932 for the
creation of quantum mechanics.
1927 - Uncertainty Principle
• It is impossible to know both the exact momentum of an
electron and its exact location in space at the same time.
• Therefore, it is wrong to say that the electrons move in
defined orbits around the nucleus.
19
Structure History
Erwin Schrodinger
• Born 1887 - Died 1961
• Austrian physicist
• Awarded a share of the Nobel Prize in
Physics in 1933 for the development of his
wave equation.
1926 - Wave Equation
Time Independent Schrodinger Wave Equation
Planck’s
constant
h

2
mass
  2m
 2  E  U  x    x 
2
x
2
Total energy
of the particle
20
Wave
function
Potential energy
function of the
particle
Structure History
• Schrodinger’s wave equation includes both the
wavelike and particle-like behavior of the electron.
• The square of the wave function, , provides information
about the electron’s location when it is in an allowed
energy state.
• Heisenberg’s uncertainty principle states that if we know the
momentum of the electron, we cannot know the exact
location of the electron in its orbit.
• Therefore,  represents the probability that an electron
will be in a certain place at a given instant of time.
2
21
Structure History
Subatomic Particles
• Experiments, which began in the 1930’s, using particle
accelerators and other tools have shown the existence of
hundreds of different subatomic particles.
• Scientists were also able to prove that the proton and the
neutron are not elementary particles.
- They are composed of elementary particles called
quarks (up, down, charmed, strange, top and bottom).
- A proton consists of two up quarks and one down quark.
- A neutron consists of one up quark and two down quarks.
22
Structure History
• The proton and neutron belong to a family of particles
called baryons.
• Baryons, as well as another group of particles called
mesons, make up a larger family of particles known as
hadrons.
• The defining characteristic of all hadrons is that the are
made up of only quarks.
Hideki Yukawa
• Born 1907 - Died 1981
• Japanese physicist
• Yukawa predicted the existence of the
meson in 1935
23
Structure History
24