1. Main Menu Coliseum Telephone Invented Corpuscles “electrons”
Plum Pudding Model
80 ADE
1876
1897
1st Flight
Wright Brothers
1898
Saturnian Model
1903
1904
Planetary Model
Bohr Model
1911
Proton
Television Invented
1920
First Cyclotron
1927
Neutron
1931
Electron Cloud Model
Subatomic
Particles
1932
1932
1930’s - present
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2. 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
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”

3. 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

4. Hantaro Nagaoka Born 1865 - Died 1950 Japanese physicist
Proposed a “Saturnian Model” of the atom
- A flat ring of electrons that revolve around a positively charged particle

5. 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
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

6. 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.
alpha particle beam
gold foil
screen
alpha particle emitter
- The particles that bounced off must have been hitting objects with a very small volume but a very large mass.

7. 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.

8. 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

9. 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.
Bohr developed a model of the hydrogen atom that would explain its line spectrum.
657 nm
486 nm
434 nm
410 nm

10. 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.
Rydberg Constant x 10-2 nm-1
J.J. Balmer
Born Died 1898
Swiss school teacher

11. He came up with a simple formula for the energies of these orbits:
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 J

12. 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.

13. Energy of final orbit
Energy of initial orbit
Planck’s constant 6.63 x J. s
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:

14. H, He+, Li2+ 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+

15. Protons
Rutherford discovered that he could change nitrogen atoms into oxygen atoms by striking them with energetic alpha particles.
This process is known as transmutation.
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.

16. Neutrons
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.
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.
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.

17. James Chadwick Born 1891 - Died 1974 British physicist
Student and colleague of Rutherford
Knighted in 1945
Proved that the beryllium emissions contained a neutral component approximately equal to that of the proton. He called this neutral component a neutron.
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.
Chadwick is awarded the Nobel Prize in Physics for his discovery of the neutron.

18. 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.
De Broglie proposed that an electron in its orbit around the nucleus has associated with it a particular wavelength.
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

19. Werner Heisenberg Born 1901 - Died 1976 German physicist
Awarded the Nobel Prize in 1932 for the creation of quantum mechanics.
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.

20. 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.
Wave Equation
Time Independent Schrodinger Wave Equation
mass
Wave function
Planck’s constant
Total energy of the particle
Potential energy function of the particle

21. 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.

22. 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.

23. 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 Died 1981
Japanese physicist
Yukawa predicted the existence of the meson in 1935