Chem 101: SWIC-RBC

Dr. Clerc

 

The Golden Age of Science

Ch. 5 Supplementary Class Notes

 

A chronology of the atomic view of nature:

http://www.3rd1000.com/chronoatoms.htm

 

Timeline of Discovery: http://www.chemsoc.org/exemplarchem/entries/2004/dublin_fowler/timeline.html

 

Physics Time-Line 1900 to 1949:

http://www.cartage.org.lb/en/themes/Sciences/Physics/aboutphysics/physicstimeline/1900/1900.htm

 

 

 

(a)         Lord Kelvin (William Thomson) (1860-1890) - played a principal role in the great and final synthesis of 19th-century science (classical mechanics)

 

         "In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." [PLA, 1883-05-03]

 

         The famed physicist William Thomson (later Lord Kelvin) was a professor of natural philosophy at Glasgow University for some fifty years. Unable to meet his class one day, he posted a note on the door of his lecture room: "Professor Thomson," it said, "will not meet his classes today." As a joke, some of his mischievous students erased the "c," leaving a message reading: "Professor Thomson will not meet his lasses today." The following day when the pranksters assembled in anticipation of the effect of their joke, they were chagrined to find that the professor had outwitted them. The note was now found to read: "Professor Thomson will not meet his asses today." [Cyrus Northrup, University of Washington Address, November 2, 1908]

 

         "There is nothing new to be discovered in physics now, All that remains is more and more precise measurement."

 

         "I can state flatly that heavier than air flying machines are impossible."

 

         http://octopus.phy.bg.ac.yu/web_projects/giants/kelvin.html

         http://zapatopi.net/kelvin/quotes.html

(b)         Faraday (~1870): Battery experiments - "equivalent weights" of substances contain equal amounts of "electricity" (e.g. 1 gram H, 8 grams O, 108 grams Ag, …)

 

 

(c)          George Stoney (1874): Proposes a unit of "electricity" as an amount that is equivalent to one atom in a battery

 

         "And, finally, Nature presents us, in the phenomenon of electrolysis, with a single definite quantity of electricity which is independent of the particular bodies acted on. To make this clear I shall express `Faraday's Law' in the following terms, which, as I shall show, will give it precision, viz.:-- For each chemical bond which is ruptured within an electrolyte a certain quantity of electricity traverses the electrolyte which is the same in all cases. This definite quantity of electricity I shall call Er. If we make this our unit quantity of electricity, we shall probably have made a very important step in our study of molecular phenomena."

 

         http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html

 

 

(d)         George Stoney (1891): Suggests the word "electron" for this unit.

 

         “… an estimate was made of the actual amount of this most remarkable fundamental unit of electricity, for which I have since ventured to suggest the name electron.”

 

         http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html

 

 

(e)          Thomson (1897): Measures e/me, e = charge of the electron, me = mass of electron

 

         "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?"

 

         http://www.aip.org/history/electron/jj1897.htm

 

 

(f)           Planck (1900): Planck's black body law and Planck's constant marked a turning point in the history of physics. The importance of the discovery, with its far-reaching effect on classical physics, was not appreciated at first. However the evidence for its validity gradually became overwhelming as its application accounted for many discrepancies between observed phenomena and classical theory. Among these applications and developments may be mentioned Einstein's explanation of the photoelectric effect.

 

         http://nobelprize.org/physics/laureates/1918/planck-bio.html

 

 

(g)          Mulliken (1909): Measures e in his famous “oil drop experiment”. From the value of e, he calculates me = 9.1091 x 10-28 g. The negative value of e prompts implies that a balancing positive charge exists somewhere in the atom.

 

         "At the close of my sophomore year [...] my Greek professor [...] asked me to teach the course in elementary physics in the preparatory department during the next year. To my reply that I did not know any physics at all, his answer was, “Anyone who can do well in my Greek can teach physics.” “All right,” said I, “you will have to take the consequences, but I will try and see what I can do with it.” I at once purchased an Avery’s Elements of Physics, and spent the greater part of my summer vacation of 1889 at home … trying to master the subject. [...] I doubt if I have ever taught better in my life than in my first course in physics in 1889. I was so intensely interested in keeping my knowledge ahead of that of the class that they may have caught some of my own interest and enthusiasm.

         http://encyclopedia.thefreedictionary.com/Robert+A.+Millikan

 

 

(h)          Thomson (~1910): Proposes the “Jellium” model of the atom – in which a uniform positive charge fills the space of the atom, with the electrons occupying the same space. 

 

 

(i)            Rutherford (1911): Bombards gold and platinum foil with “-rays” (i.e. He2+ ions from radon or radium), and observes the angle of reflection. Most of the -rays pass through with little reflection, but others are reflected backwards at 180o. This result suggests a region of dense positive charge in the atom. This region becomes known as the “nucleus” of the atom.

 

 

(j)           Rutherford (1912): Proposes a “planetary” model of the atom.

         http://www.britannica.com/nobel/micro/514_59.html

 

 

(k)         Balmer (1885): derived a simple formula that expressed the known wavelengths () of the hydrogen spectrum in terms of two integers m and n.

 

http://www.colorado.edu/physics/2000/quantumzone/balmer.html

http://www.science.uwaterloo.ca/~cchieh/cact/c120/hspectra.html

 

(l)            Rydberg (1890): derived generalized formula that matched the form of the Balmer equation.

 

http://www2.kutl.kyushu-u.ac.jp/seminar/MicroWorld1_E/Part4_E/P42_E/atomic_spectra_E.htm

 

 

(m)       Bohr (1913): Proposes an “orbital” model of the atom. Electrons occupy discrete orbitals – not a continuous region of space. Each orbital has an associated energy. Electrons move to higher-energy orbitals when the atom absorbs light energy (photons). The energy of the absorbed light corresponds to the difference in energy between the two orbitals.

 

http://www.britannica.com/nobel/micro/76_6.html

 

 

(n)         Einstein (1913): Photoelectric effect explanation – light shining on a metallic surface causes electrons to be emitted from the surface. Einstein proposes that a certain threshold energy must be exceeded before electrons can be released.

 

http://www.ux1.eiu.edu/~cfadd/1160/Ch28QM/Photo.html

 

 

(o)         de Broglie (1923): The dualistic nature of matter – both particle and wave – was proposed in his Ph.D. thesis. This view is fundamental to our present view of matter known as “wave mechanics” or “quantum mechanics”.

 

http://theory.uwinnipeg.ca/physics/quant/node6.html

 

(p)         Pauli (1924): the exclusion principle: No two electrons in an atom can have identical quantum numbers. In other words, they cannot occupy the same orbital with the same spin.

 

http://www.chem.ufl.edu/~chm2040/Notes/Chapter_9/pauli.html (has Chem 101 examples)

 

(q)         Heisenberg (1926): the uncertainty principle "The more precisely the POSITION is determined, the less precisely the MOMENTUM is known".

 

http://www.aip.org/history/heisenberg/p01.htm

 

(r)          Schroedinger (1926): derivation of spectrum of hydrogen atom using the wave equation

 

http://www.bun.kyoto-u.ac.jp/phisci/Gallery/schroedinger_note.html

 

 

A profound result - a different world-view: wave-particle duality in nature.

 

 

Another important result: one equation – the Schroedinger (wave) equation - that when solved (where possible), can generate the physical properties of a material. Accurately solving the wave equation is now possible with modern computers, which may enable quantum mechanics to be used as a practical engineering tool.

http://scienceworld.wolfram.com/physics/SchroedingerEquation.html

http://scienceworld.wolfram.com/physics/HydrogenAtom.html

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/schr.html