Lecture notes: CHEM103 Fall 2008 – September 11
Quiz Tuesday: key ideas to be comfortable with…
activities of science
macroscale vs. atomic scale
chemical calculations – rules for reporting results
Outline for the day:
a. Democritus’ atomistic philosophy
· Contrast to Plato/Aristotle in ancient Greek thought
· From alchemy to chemistry in the “age of enlightenment”
· Three critical experiments in discovering the structure of the atom
…and some historical context
Greek scientific “philosophy” largely a mental and observational exercise – no experimentation:
…but generated TWO radically different WORLDVIEWS: atomistic VS. continuous
4th century B.C.
Democritus (after Leucuppus)
Nature is: “atomos” separated by void
1) infinite in number
2) many varieties (shapes & sizes)
3) in constant, random motion & collision
4) collisions lead to deflection endlessly
5) or to combination of particles to form substances which we can perceive
3rd century B.C.
(read excerpts of Epicurus…
…and writings by Lucretius 95 – 55 B.C.)
4th century B.C.
Plato (after Empedocles)
Nature is: “four primordial substances” & “aether”
matter is continuous, NOT divisible
”elements” are representative of types of matter
3rd century B.C.
tetrahedron (4) = fire
octahedron (8) = air
icosahedron (20) = water
hexahedron or cube (6) = earth
dodecahedron (12) = “quintessence” – later “aether”
QUESTION: Which of these two scientific models dominated western thought for most of the next TWO MILLENNIA???
ALCHEMY: roots in ancient Egypt, with later contributions from the region of Arabia
…similar beliefs arose in China, Tibet, & India.
Keys to alchemy dealt with the purification and mixing of metals (used for currency and industry)
AND the prolonging of life (the “philosopher’s stone” originally for eternal life)
AND EVENTUALLY, the conversion of “base metals” into gold (Pb2Au)
Its central theme was the purification or perfection of matter (including humans) –
but the approach was more experimental than philosophical!!!
It went wrong in that it viewed chemistry like cooking: add a little bit of oxygen like adding a bit of cayenne pepper or cinnamon to a recipe!
more correctly: add another element, and make something completely different - unrelated!
Carbon and oxygen form the basis of life: combined 1:1, carbon monoxide is toxic!
Sodium and chlorine are both dangerously reactive: combined they make table salt!
Hydrogen is extremely flammable & oxygen promotes combustion: combined, water puts out fires!
Further reading on alchemy: http://www.chm.bris.ac.uk/webprojects2002/crabb/history.html
Skip a bit… (almost 2000 years!)
1624: Etienne de Clave arrested for heresy for holding a debate on atomistic philosophy with a group of French intellectuals.
Robert Boyle (1627 – 1691)
(The Sceptical Chemyst – 1661) http://oldsite.library.upenn.edu/etext/collections/science/boyle/chymist/
1) argues against 4 Platonic elements
2) but still believes in transmutation
3) matter consists of atoms and clusters of atoms in motion
4) every phenomenon was the result of collisions of particles in motion
3) all substances are elemental unless divisible
Note: Daniel Bernoulli publishes “Hydrodynamique” in 1738, using atomistic philosophy (kinetic theory of gases) to explain Boyle’s Law (P vs. V)
John Dalton (1766 – 1844)
DALTON’S EXPERIMENTAL “RESULTS”
1 g carbon
1.33 gram oxygen
1 gram carbon
2.66 gram oxygen
Note: follows Lavoisier’s work with hydrogen and oxygen (1 mass of H to 8 masses of O) in 1783.
Dalton “assigns” mass values to each element, with hydrogen arbitrarily at 1.
(A New System of Chemical Philosophy -- 1808)
1) each element made up of atoms
2) which cannot be created, destroyed, divided or converted
3) all atoms of each type have the same properties
4) atoms of different types have different properties
5) “reactions” involve reorganizing atoms: joining, separating or rearranging them
6) these combine in small, whole-number ratios to form compounds; multiple combinations are possible
P.S. Plato’s 5th element, “quintessence,” later, Aristotle’s idea of the “aether” persists until well into the 19th century!
THREE CRITICAL EXPERIMENTS IN UNDERSTANDING THE NATURE OF THE ATOM:
KEY: HOW DOES THE NATURE OF SUBATOMIC PARTICLES
DETERMINE THE STUFF THEY BUILD (ATOMS)
AND THE MANNER IN WHICH ATOMS INTERACT???
0. WORK LEADING UP TO EXPERIMENT 1.
Heinrich Geissler develops mercury vacuum pump (1855)
William Crookes (1850’s) builds a “Crookes’ tube”
Eugen Goldstein (early 1876 - 1886) describes “cathode rays” and “canal rays”
(Canal rays opposite in charge to cathode rays, but MUCH greater in mass.)
J.J. Thomson (1897)
“We have in the cathode rays matter in a new state… in which the subdivision of matter is carried much further than in the ordinary gaseous state…; this matter being the substance from which the chemical elements are built up.”
(J.J. Thomson (1897). "Cathode Rays," Philosophical Magazine 44, 295.)
THIS IS REJECTED BY HIS PEERS – but get’s mass/charge ratio correct!
"At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my  lecture at the Royal Institution that he thought I had been `pulling their legs.' "
(J.J. Thomson (1936). Recollections and Reflections. G. Bell and Sons: London. p. 341.)
Note: despite winning the Nobel Prize in Physics in 1906, he DOES NOT accept the terminology “electron” until 1913, instead using the general term, “corpuscle.”
Thomson’s “Plum Pudding” model of the atom
Note: At this time there was great rivalry between German and British researchers. As concerning the nature of the cathode ray, the Germans tended to the explanation that cathode rays were a wave (like light), whereas the British tended to believe that the cathode ray was a particle. As events unfold over the next few decades, both will be proven correct.
In fact, J.J. Thomson will be awarded the Nobel Prize in Physics in 1906 for proving the electron is a particle and his son, George Paget Thomson, will be awarded the Nobel Prize in Physics in 1937 for showing that the electron is a wave.
0. WORK LEADING UP TO EXPERIMENT 2.
1911 Nobel Prize in Physics: for various sources for subatomic particles
Marie Curie (Nobel #2 – in Chemistry! – in 1935)
a: heavy w/ positive charge
b: light w/ negative charge
g: no mass, no charge