ATOMS: Dalton and BeyondA search for a simple theory of matterTopic 7 – Spring 2006 Ted Georgian, Dept. of Biology
The nature of science Scientists are searching for explanations that are: 1. 2. 3.
What is the world made of, at the most fundamental level? www.ikros.net/mocs/ images/Castle2Clouds.jpg
Leucippus (~480 - 420 B.C.) http://cont1.edunet4u.net/cobac2/down/down05.html • All matter is made of tiny, indivisible particles called “atoms” • Change is caused by atoms moving through empty space (a “void”) • Atoms are therefore “fundamental” Early Greek atomists Democritus (470 - 380 B.C.) www.livius.org/a/ 1/greeks/democritus.jpg
But an alternate model won out http://astsun.astro.virginia.edu/~jh8h/Foundations/chapter2.html Based on observable characteristics of matter, such as ? Aristotle (384 – 322 BC)
Descartes, Boyle, Newton & others imagined a “clockwork” universe - perfectly predictable The mechanical philosophy of the 1600s
Maybe chemistry would turn out to be as “simple” as Newtonian physics? Would it work for chemistry as well? A few, simple objects following simple, general, and precise laws.
Start of the Modern Era of Chemistry John Dalton’s Atomic Hypothesis (1808): • 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” • used hypothesis (“Rule”) of greatest simplicity • estimated relative atomic masses
How to make sense of all these elements? Scientists like “a place for everything, and everything in its place.” And no more places and things than necessary.
Dmitri Mendeleev (1834-1907) “Creator of the Periodic Table” (but there were earlier attempts by others)
Mendeleev’s early notes for the Periodic Table (1869)
Characteristics of Mendeleev’s Table • Organized 60+ known elements… - by similar chemical propertiesin eachverticalfamily(group) - by roughly increasing atomic weight within each horizontal row • Used to predict existence of new elements (of 10, found 7; other 3 do not exist)
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
Prediction of the properties of an unknown element below Silicon * eka: “one beyond”
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: a new type of matter is discovered J. J. Thomson (1897) • experimented with “cathode rays” • decided that they are charges of electricity carried by particles of matter Schematic of actual 1897 apparatus (vacuum inside):
Cathode-Ray Tubes – ever seen one? http://www.howstuffworks.com/tv4.htm
Thomson’s conclusions • “We have, in the cathode rays, matter in a new state...a state in which all matter...is of one and the same kind; this matter being the substance from which all the chemical elements are built up." but... • “What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?” - J. J. Thomson http://www.aip.org/history.electron/jjrays.htm
How big are “electrons”? • Thomson calculated the mass-to-charge ratio for cathode ray particles: it was over 1000 times smaller than of a charged hydrogen atom • This fact suggested: - either cathode rays carried a huge charge, - or they had very small mass
Answer: very, very small • Robert Millikan measured the charge of a cathode ray particle in 1910. • From that & Thompson’s mass-to-charge ratio, he could calculate the mass: ~1800 times lighter than a hydrogen atom
tiny “corpuscles” of negative charge • surrounded by a sort of “cloud” of positive charge Thomson’s “plum pudding” atom model* * Never had plum pudding? Think of a blueberry muffin.
More pieces of the atom Ernest Rutherford • Thomson’s student • Gold Leaf Experiment (1910-11) – actually conducted by Hans Geiger and undergraduate Ernest Marsden
The gold leaf experiment • fired positively-charged alpha particles at very thin gold foil – they caused flashes of light when they hit the screen • counted flashes and measured the angle of deflection
Gold leaf 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.
Most alpha particles went straight through, and someweredeflected, BUT a few(1 in 20,000)reflectedstraight back to the source! What’s going on? “It was quite the most incredible event that has ever happened to me. It was almost as incredible as if you had fired a fifteen inch shell at a piece of tissue paper and it came back and hit you.”
small, dense, positively-chargednucleus surrounded by “mostly empty” space • in which the electrons must exist. • positively charged particles called “protons” • like tiny solar system + Rutherford’s Model of the Atom
Most of it! • In fact, if the nucleus of an atom were the size of Murphy Auditorium, the innermost electrons would be how far away? • DeLaRoche Hall? • Francis Hall? • Downtown Olean? • NYC? + How much of an atom is empty space? (click for the right answer)
But wait – there’s more! James Chadwick (1932) • discovered a neutral (uncharged) particle in the nucleus. • called it the “neutron”
Atom “split” later that year Atom “split” by John Cockcroft and Ernest Walton, using a particle accelerator, in late 1932
Now we understand why the periodic table works • The order of the elements is determined by their atomic number(= the number of protons) • The atomic mass of the elements is determined by the number of protons and neutrons. • The chemical properties of the elements are determined by the number of electrons in their outer (valence) shells
So: is this what atoms are like? No! Calculations soon showed that a “Rutherford atom” would last less than one minuteElectrons would radiate away energy and spiral down into the nucleus.
A new understanding of the atom from spectroscopy When elements are heated, they give off light of a particular wavelength (or color) Sodium Potassium Lithium
Spectroscopes: seeing atomic light Original 1859 Bunsen- Kirchhoff spectroscope Modern apparatus for viewing a “spectrum”
Hydrogen’s emission “fingerprint” Observation: when heated with electricity hydrogen gives off light of specific wavelengths The line-emission spectrum of hydrogen gas
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) • Bohr assumed that electrons can orbit ONLYat certain distances from nucleus • this model permits electrons to exist for a long time without giving off radiation • Bohr’s model enabled him to predict the number and wavelength of hydrogen’s emission lines
Electron orbits are distinct (“quantized”) in Bohr’s model “Quantum leaps” from one level to another Trefil & Hazen. The Sciences: An integrated approach. 2nd ed. Fig. 7-6.
But why should electrons behave this way? Louis de Broglie(1927) Particle/Wave Duality of electrons “Thus I arrived at the following general idea …: 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.” http://www.spaceandmotion.com/Physics-Louis-de-Broglie.htm
Electrons as waves Only at certain distances from the nucleus would an electron complete an integer number of wavelengths in its orbit When de Broglie did the mathematics, he could predict exactly the distances that Bohr had assumed for the hydrogen atom.
Then, suddenly, trouble for the mechanistic approach Werner Heisenberg(1927) The “Uncertainty Principle” • There’s an upper limit to how precisely an electron’s position and momentum can be known • The more precisely one is known, the less precisely the other can be known
Electrons move in “probability clouds”, not circular orbits • The exact path of an electron can’t be predicted! • If we know the electron is in a given atom, its velocity is uncertain by ~16 million mph!!
Newtonian certainty cannot be obtained in the subatomic world “I cannot believe that God plays dice with the universe.” “Albert, stop telling God what to do.”