Lecture notes: CHEM103 Fall 2008 – September 16

Quiz #1

Outline for the day:

· Review:

a. Thomson’s atomic model

b. Three radioactive particles

· Rutherford’s “Gold Foil” experiment

· Milliken’s “Oil Drop” experiment

· Summary of the “modern” atomic understanding

0. WORK LEADING UP TO EXPERIMENT 2.

1911 Nobel Prize in Physics: for various sources for subatomic particles

Henri Becquerel

Pierre Curie

Marie Curie (Nobel #2 – in Chemistry! – in 1935)

Rutherford (a student of J.J. Thomson) receives Nobel Prize in Chemistry in 1908

a: heavy w/ positive charge

b: light w/ negative charge

g: no mass, no charge

2. NUCLEUS:

Later, Rutherford – together with Geiger & Marsden (experimental work in 1909; published in 1911),

were trying to better understand the nature of the a-particle…

(Ironically, generally considered his most important work, but AFTER his Nobel Prize!)

http://micro.magnet.fsu.edu/electromag/java/rutherford/

http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Rutherford-Model.html

“Then I remember two or three days later Geiger coming to me in great excitement and saying ‘We have been able to get some of the particles coming backwards…’ It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”

"Geiger and Marsden observed the surprising fact that about one in eight thousand a particles incident on a heavy metal like gold is so deflected by its encounters with the molecules that it emerges again on the side of the incidence. Such a result brings to mind the enormous intensity of the electric field surrounding or within the atom."

“Plum Pudding” model is superseded by new data… old model inadequate!

Foreshadowing: In March 1912, 27-year-old Niels Bohr (awarded a Ph.D. in May 1911, the same month of Rutherford's classic paper) will arrive in Rutherford's laboratory, having just spent a bit more than 6 months in J.J. Thomson's laboratory.

Source: http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Rutherford-Model.html

Later on, Rutherford will succeed J.J. Thomson as the director of the Cavendish Lab at Cambridge University…

3. ELECTRON CHARGE (AND MASS)

Background work:

Wilhelm Roentgen (1895) – the very 1^st Nobel Prize in Physics (1901)

for the discovery of X-Rays…

Robert Millikan (1909)

Other researchers had measured the mass/charge ratio of the electron;

Some had also estimated the absolute charge (including J.J. Thomson)

Millikan was simply a better experimentalist and got better results!

http://www.hesston.edu/academic/FACULTY/NELSONK/PhysicsResearch/Millikan/millikan.html

“Millikan’s Experimental Data”

drop #	charge on drop	number of electrons on drop
1	-1.28 x 10^-18	8
2	-4.81 x 10^-19	3
3	-1.12 x 10^-18	7
4	-3.20 x 10^-19	2
5	-1.76 x 10^-18	11

Millikan gets the charge of an electron very close to present value! (-1.6 x 10 ^-19 Coulomb)

and wins the 1923 Nobel prize in Physics!

(together with the mass/charge ratio already measured, this gives both quantities…)

Final Note: in "The Discovery of Subatomic Particles" by Steven Weinberg there appears a footnote on p. 97. It reads:

“. . . there appeared a remarkable posthumous memoir that throws some doubt on Millikan's leading role in these experiments. Harvey Fletcher (1884-1981), who was a graduate student at the University of Chicago, at Millikan's suggestion worked on the measurement of electronic charge for his doctoral thesis, and co-authored some of the early papers on this subject with Millikan. Fletcher left a manuscript with a friend with instructions that it be published after his death; the manuscript was published in Physics Today, June 1982, page 43. In it, Fletcher claims that he was the first to do the experiment with oil drops, was the first to measure charges on single droplets, and may have been the first to suggest the use of oil. According to Fletcher, he had expected to be co-author with Millikan on the crucial first article announcing the measurement of the electronic charge, but was talked out of this by Millikan.”

4. THE NEUTRON

In 1920, Rutherford postulates that there are heavy particles in the nucleus with NO CHARGE.

Note – why is this a problem???

Finally, after many, many years of trying to identify this “extra” mass in the nucleus,

he assigns his student, James Chadwick, to perform some additional experiments

Note – how can we tell the difference between these?

In 1932, he discovers the neutron, winning the 1935 Nobel prize!

ONE FINAL WEB REFERENCE:

http://nobelprize.org/index.html

SUMMARY: WHAT MAKES UP AN ATOM?

– THE MODERN UNDERSTANDING

SUB-ATOMIC PARTICLES

	mass (g)	mass (amu)	charge (C)	charge (eV)	abbreviation
proton	1.672622 x 10^-24	1.007276	1.6022 x 10^-19	+1	p^+
neutron	1.674927 x 10^-24	1.008665	0	0	n
electron	9.109383 x 10^-28	0.0005485799	-1.6022 x 10^-19	-1	e^-

http://physics.nist.gov/Constants/index.html

Mass in grams is terribly inconvenient: conversion between mass units: grams and amu.

1 amu = 1.660 538 782 x 10^-24 g

Charge in Coulombs is inconvenient: conversion between charge units: Coulombs & electron volts.

1 eV = 1.602 176 487 x 10^-19 C

HOW CAN WE APPLY THIS UNDERSTANDING?

Review: mass spectrometer – how DOES it work?