ATOMS: Dalton and Beyond A search for a simple theory of matter Topic 7 – Spring 2005 Ted Georgian, Dept. of Biology
The nature of science
Models in science Scientists use their experimental results (and their imaginations) to create modelsA model is a representation of a complex natural system that permits us to understand its behavior.
Prior Examples? Remember the models of planetary motion? What were they trying to explain?
But an alternate model won out http://astsun.astro.virginia.edu/~jh8h/Foundations/chapter2.html
The mechanical philosophy of the 1600s
Would it work for chemistry as well?
Start of the Modern Era of Atoms John Dalton’s Atomic Hypothesis (1803):All matter is made up of indivisible atoms. Compounds are composed of atoms in definite proportions. Chemical change occurs when atoms are rearranged
Dalton’s Atomic Model of Compounds explained observation of “constant proportions” as based on atomic composition of compounds estimated relative atomic masses, based on his hypothesized structures
Meanwhile, many new elements being found
How to make sense of all these elements?
DmitriMendeleev(1834-1907)“Creator of the Periodic Table”(but there were earlier attempts by Dobereiner and Newlands, and Meyer probably formulated the periodic idea at same time as Mendeleev)
Mendeleev’s early notes for the Periodic Table (1869)
Mendeleev’s table, as originally published Formatted sideways compared to modern table? instead of a name: element was predicted to exist but not known yet
Characteristics of Mendeleev’s Table Organized 60+ known elements… - by similar properties in each vertical family (group) - by roughly increasing atomic weight within each horizontal row (moved 17 elements based on properties rather than weight)Used to predict existence of new elements (of 10, found 7; other 3 do not exist)
Prediction of the properties of an unknown Group 4 element below Silicon
An attempt to simplify the elements William Prout (1815)hypothesized that the hydrogen atom is fundamental all other elements made up of hydrogen atoms his hypothesis was rejected by the 1830s (for ex. chlorine atom had mass 35.4 times that of hydrogen)
News flash: atoms aren’t fundamental J. J. Thomson (1897)experimented with “cathode rays” “and then... made a bold speculative leap. Cathode rays are not only material particles, he suggested, but in fact the building blocks of the atom: they are the long-sought basic unit of all matter in the universe.” (http://www.aip.org/history/electron/jjrays.htm)
Cathode-Ray Tubes – ever seen one?
Thomson’s conclusions “I can see no escape from the conclusion that [cathode rays] are charges of electricity carried by particles of matter.”but...“What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision? - J. J. Thomson
Thomson’s “plum pudding” atom model*
If electrons exist, how big are they? Thomson calculated the mass-to-charge ratio for cathode ray particles: it was over 1000 times smaller than for a charged hydrogen atom This fact suggested: - either cathode rays carried a huge charge, - or they had very small mass Robert Millikan measured the charge of a cathode ray particle in 1910. From that he could calculate the mass: ~1800 times lighter than a hydrogen atom
More pieces of the atom Ernest Rutherford(1871-1937)nuclear physicist,Thomson’s student,New Zealander teachingGold Leaf Experiment
Rutherford’s Experiments (1910-11) (done by undergrad Ernest Marsden/physicist Hans Geiger) Fired beam of positively-charged alpha particles at very thin gold foil.Alpha particles caused flashes of light when they hit the zinc sulfide screen
Rutherford’s Experiment: prediction By Thomson’s model, mass and + charge of gold atom are too dispersed to deflect the positively-charged alpha particles,so... particles should shoot straight through the gold atoms.
Rutherford’s experiment: what actually happened
Rutherford’s Model of the Atom
The Nucleus Repels Alpha Particles
How much of an atom is empty space?
But wait – there’s more! James Chadwick(1932) Discovered a neutral(uncharged) particle in the nucleus. Called it the “neutron”Atom “split” by John Cockcroft and Ernest Walton, using a particle accelerator, in late 1932
Atom “split” later that year Atom “split” by John Cockcroft and Ernest Walton, using a particle accelerator, in late 1932
Splitting the atom led to some very practical consequences
Properties of Subatomic Particles
Now we understand why the elements come in periods of 8
Why do 2 Group I atoms combine with 1 oxygen (R2O)?
Modern Periodic Table Organization Elements are NOW placed in order of increasing atomic number (# of + protons). - Why? Gives absolute order... atomic weights not characteristic (different-mass atoms called isotopes exist!) A relationship between nuclear charge and arrangement of elements in the Table was finally discovered in 1914 (Henry Moseley). In 1860s, Mendeleev could NOT have predicted a relationship to subatomic particles!
So: is this what atoms are like?
A new understanding of the atom
Spectroscopes: Seeing Atomic Light
Spectroscopy can identify elements on distant stars
Hydrogen’s Emission “Fingerprint”
Niels Bohr(1885-1962)Danish physicist Bohr wondered why hydrogen emitted spectral lines, and not just a continuous band of light
Bohr’s Model of Atom (1913) Circling electron maintains orbit ONLY at specific distances from nucleusOnly way electron could exist for long time without giving off radiationBohr’s model enabled him to predict the number and wavelength of hydrogen’s emission lines
Electron orbits are distinct (“quantized”) in Bohr’s model Trefil & Hazen. The Sciences: An integrated approach. 2nd ed. Fig. 7-6.
But why should electrons behave this way? Thus I arrived at the following general idea which has guided my researches: for matter, just as much as for radiation, in particular light, we must introduce at one and the same time the corpuscle concept and the wave concept. In other words, in both cases we must assume the existence of corpuscles accompanied by waves. De Broglies Nobel Prize speech, 1927. http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm
Electrons can be thought of as standing waves …
Electrons as waves Only at certain distances from the nucleus would the electron complete an integer number of wavelengths in its movement around the nucleus
The position of electrons can’t be predicted precisely Werner Heisenberg (1927) The “Uncertainty Principle”
Electrons move in “probability clouds”, not circular orbits The exact path of an electron can’t be predicted If we know the electron is somewhere in the atom, it’s velocity is uncertain by ~7,300 km/s (~ 16 million mph)!
Newtonian certainty cannot be obtained in the subatomic world “I cannot believe that God plays dice with the universe.”
Here we go again! By the 1950s hundreds of sub-atomic particles had been identified. Simplicity was getting lost again.
Another attempt to simplify our model of matter Murray Gell-Mann and George Zweig (1964) - proposed protons and neutrons are made of smaller particles they named quarks (aces)
Protons & neutrons are not fundamental Gell-Mann & Zweig hypothesized 6 different quarks with fractional charge (UP quark has +2/3 charge, DOWN quark has –1/3)Protons and neutrons are composed of UP and DOWN quarks, held together by gluon particles
Fermi National Accelerator Lab: 6-km Tevatron ring and 3-km Main Injector * Chicago site for study of sub-subatomic particlesEvidence for last quark (TOP) found in 1995
So: are quarks fundamental? Probably not: recent models of matter hypothesize 11-dimensional “strings” curled up inside of quarks.
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