Download This Week Final Exam Marks on the Web

This Week
March 28 – 30
Tutorial and Test 4, Chapters 27, 28
Final Exam
Thursday, April 20, 1:30 – 4:30 pm
Frank Kennedy Brown Gym
The Whole Course
Formula Sheet Provided
Marks on the Web
Check your marks and inform your TA or Dr. Kunkel of errors
Monday, March 27, 2006
1
Chapter 30: The Nature of the Atom
• Rutherford scattering and the nuclear atom – the nucleus is really
small!
• Atomic line spectra – wavelengths characteristic of each element
• Bohr model of the hydrogen atom – quantization of angular momentum ➙ quantization of energy
• x-rays
• The laser, holograms
• Omit 30.5, 6 – quantum mechanical picture of H-atom, Pauli
Exclusion Principle, periodic table
Monday, March 27, 2006
2
Very schematic picture of an atom
Size of the nucleus ≈ 10-15 – 10-14 m
Size of the atom ≈ 10-10 m
In the normal state, the atom is
electrically neutral – equal amounts of
+ and – charges.
Was thought that the + and – charges
occupied the whole volume of the
atom – a positively-charged “pudding”
with negatively-charged electron
“plums” embedded in it (“plum
pudding” model). This was ruled out
by Rutherford following experiments
by Geiger+Marsden...
Monday, March 27, 2006
(charge +Ze)
3
Geiger+Marsden: Scattering of alpha particles
Alpha (α) particles – nuclei of
4He atom, emitted by some
radioactive nuclei.
α-particles scattered from a thin
gold foil and were observed on
a screen. Far more were
scattered at large angle than
would be possible with the
weak electric field inside a
“plum pudding” atom.
Rutherford: the positive charges
of the atom had to be confined
in a very small volume – the
nucleus.
⇒ planetary model of the atom with electrons
in orbit around nucleus
Monday, March 27, 2006
4
Prob. 30.1: The nucleus of the hydrogen atom has a radius of about
10-15 m. The electron is normally at a distance of about 5.3×10-11 m
from the nucleus.
Assuming the hydrogen atom is a sphere of radius 5.3×10-11 m, find:
a) the volume of the atom,
b) the percentage of the volume of the atom that is occupied by the
nucleus.
Prob. 30.3: What is the ratio of the density of the hydrogen nucleus to
the density of the complete hydrogen atom?
Monday, March 27, 2006
5
Prob. 30.6: There are Z protons in the nucleus of an atom, where Z is the
atomic number of the element. An α particle (nucleus of He atom) carries
a charge +2e.
In a scattering experiment, an α particle, heading directly toward a
nucleus in a metal foil, will come to a halt when all the particle’s kinetic
energy is converted to electric potential energy.
How close will an α particle with kinetic energy of 5×10-13 J come to a
gold nucleus (Z = 79)?
Monday, March 27, 2006
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Problem with a Planetary Model of the Atom
• Electrons orbit the nucleus.
• The electrons suffer centripetal acceleration in their orbits.
• Any accelerated charge should radiate electromagnetic energy.
⇒ The electrons should lose energy and spiral into the nucleus in very
little time.
⇒ A planetary atom should not be stable!
⇒ Classical theory does not explain the structure of the atom.
⇒ Small systems, such as atoms, must behave differently from
large.
30.1
Monday, March 27, 2006
7
Atomic Line Spectra
Pass a current through low pressure gas. Light is emitted at particular
wavelengths that are characteristic of the chemical element.
The spectrum of sunlight is continuous – blackbody radiation for ~6000 K. In
addition, dark (Fraunhofer) lines are visible. They are due to absorption of
light by chemical elements in the “atmosphere” of the sun. This was how
helium was discovered.
Monday, March 27, 2006
8
Fraunhofer absorption lines
from the sun
http://www.harmsy.freeuk.com/fraunhofer.html
1 Å (Angstrom) = 0.1 nm
• Fraunhofer lines – chemical elements emit and
absorb light at the same wavelengths
• Models of the atom need to explain this
Monday, March 27, 2006
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