concept of the atom democritus 400 b.c.e., greece democritus
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Multiple Proportions
O/C
2.39
1.27
O/N
0.58
1.27
2.39
Oxides of Carbon %C %O
28 72
44 56
Carbonic Acid (1801)
Carbonous Acid (1789)
Oxides of Nitrogen %N %O
63.30 36.70
44.05 55.95
29.50 70.50
Nitrous Oxide (1810)
Nitrous Gas (1810)
Nitric Acid (1810)
%err
4
5
%err
2
11
5
(1)
(~2)
(~4)
(~2)
(1)
Rel.
Rel.
integral values consistent with simple atomic ratios
Experimental Evidence for the Existence of AtomsJohn Dalton (1766-1844 ), British
Put the pieces together to form the
first atomic theory!If elements combined in whole-number ratios,
there must be atoms!
(matter is made of discrete particles!)
• Was the first to create a table of atomic weights
• He was a quaker school teacher at age 12
• He was color-blind; never married
• More than 40 thousand people marched in his funeral procession.
Dalton
Dalton’s tableof elements and their combinations
Dalton’s Atomic Theory
1. Elements are composed of atoms solid, tiny, hard, unbreakable, spheres
2. All atoms of a given element are identicalall carbon atoms have the same chemical and
physical properties
3. Atoms of a given element are different from those of any other element
carbon atoms have different chemical and physical properties than sulfur atoms
Dalton’s Atomic Theory
4. Atoms of one element combine with atoms of other elements to form compounds (in simple ratios).
Law of Constant Composition all samples of a compound contain the same proportions
(by mass) of the elements
5. Atoms are indivisible in a chemical process. Reactions only cause a rearrangement of atoms.
Law of Conservation of Mass:all atoms present at beginning are present at the end atoms are not created or destroyed, just rearranged
atoms of one element cannot change into atoms of another element
cannot turn Lead into Gold by a chemical reaction
Discovery of the electron
“Could anything at first sight seem more impractical than a body
which is so small that its mass is an insignificant fraction of the
mass of an atom of hydrogen?"-- J.J. Thomson.
Joseph John Thomson (1856-1940), British
Used the cathode ray tube (modified by Crookes, 1850s) to do experiments that led to the idea of electrons.http://www.chem.uiuc.edu/clcwebsite/cathode.html
Electrons are negatively charged particles that exist in an atom
Thomson won the Nobel Prize in Physics, 1906 for evidence of electrons as particles.
Ironically, his son won the Nobel in 1937 for evidence of electrons as waves (it has been accepted since then that electrons have both wave and particle properties).
Plum Pudding Model of the AtomAtoms (matter) is electrically neutral.
If electrons are negatively charged, there mustbe positive charges to balance the electrons(later, protons were discovered by Goldstein).
Lord Kelvin(William Thomson, no relation to JJ)
JJ Thomson
Discovery of the nucleus
“All science is either physics or stamp collecting”
-Ernest Rutherford
Ernest Rutherford (1871-1937), British
His famous gold foil experiment proved theexistence of the nucleus in atoms and that 1) The nucleus was composed of positive
charge (protons). 2) Atoms are mostly empty space
Born and raised on a farm in New Zealand
Won scholarship to study at Cambridge Became a student of JJ Thomson
Performed extensive research in radioactivity
Nobel Prize in 1908 in Chemistry
Was a great mentor—several of his students also won Nobel Prizes
Bohr’s Model
This model involves the idea of quantization (developed by Planck):
Energy levels (orbits) increase in quantized amounts of energy (discrete—not continuous—amounts of energy)
This means the light emitted from excited electrons are not continuous—only certain colors are seen (only QUANTA of energy can be absorbed and released)
"We are tracing the description of natural phenomena back to combinations of pure numbers which far transcends the boldest dreams of the Pythagoreans." -- Bohr
Wave-Particle Duality
Bohr’s model did not explain why electrons do not spiral into the nucleus. (It makes the assumption that this does not occur).
It was accepted by physicists that light seemed to behave as both waves and as particles (called photons). This idea is called “Wave-Particle Duality.”
Louis de Broglie asked “If light can be wave-like and particle-like, why can’t matter also be wave-like and particle-like?”
This led to the concept that
electrons can behave as waves and as particles.
Prince Louis de Brogliefirst got a history degree,then went into science.Won the Nobel Prizein physics (1929).His work helped shapequantum mechanics.
Electrons as waves
Electrons behave like standing (stationary) waves (not traveling waves, like light)
Stationary waves are like the waves (vibrations) from plucking a guitar string.
There are only certain areas where the waves (electrons) are allowed. The 3-D space where electrons might be is described by orbitals.
This is onekind of orbitalcalled a p-orbital
OrbitalsWave functions determine the sizes and shapes of orbitals
Four common orbital types are: s, p, d, f (theoretically, there are an infinite number of orbital types: g, h, I, j, k, etc…)
Let’s look at the difference between Bohr’s atom and Schrödinger’s:
http://www.colorado.edu/physics/2000/quantumzone/schroedinger.html
Joseph Louis Gay-Lussac
(1809)
Oxidation of Sugar, etc. with NaClO3
Cleans up Lavoisier's Mass Balance
1.9989 volumes of hydrogen per
1.0000 volumes of oxygen Water gives
3.08163 volumes of hydrogen per
1 volume of nitrogen Ammonia gives
Alternative to Dalton's Law of Greatest Simplicityhttp://www.bibliotheque.polytechnique.fr/associations/gaylussac/pages/AssofrGL.html
1804 - 7,016 m(1778-1850)