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ATOMS: Dalton and Beyond

A search for a simple theory of

matter Topic 7 – Spring 2006

Ted Georgian, Dept. of Biology

 

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The nature of science

Scientists are searching for explanations that are: 1. 2. 3.

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What is the world made of, at the most fundamental level?

www.ikros.net/mocs/ images/Castle2Clouds.jpg

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Early Greek atomists

Democritus (470 - 380 B.C.) www.livius.org/a/ 1/greeks/democritus.jpg

Leucippus (~480 - 420

B.C.)

http://cont1.edunet4u.net/cobac2/down/down05.ht

ml

• 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”

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But an alternate model won out

http://astsun.astro.virginia.edu/~jh8h/Foundations/chapter2.html

Aristotle (384 – 322 BC)Aristotle (384 – 322 BC) Based on observable characteristics of matter, such as ?

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The mechanical philosophy of the 1600s

Descartes, Boyle, Newton & others imagined a “clockwork” universe - perfectly predictable

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Would it work for chemistry as well?

Maybe chemistry would turn out to be as “simple” as Newtonian physics?

A few, simple objects following simple, general, and

precise laws.

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Start of the Modern Era of Chemistry

John Dalton’s Atomic Hypothesis (1808):

1. All matter is made up of indivisible atoms.

2. Compounds are composed of atoms in definite proportions.

3. Chemical change occurs when atoms are rearranged

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Dalton’s Atomic Model of Compounds

• explained observation of “constant proportions”

• used hypothesis (“Rule”) of greatest simplicity

• estimated relative atomic masses

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Meanwhile, many new elements being found

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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.

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DmitriMendeleev(1834-1907)

“Creator of the Periodic Table”

(but there were earlier attempts by others)

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Mendeleev’s early notes

for the Periodic Table

(1869)

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Characteristics of Mendeleev’s Table

• Organized 60+ known elements…

- by similar chemical properties in each vertical family (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)

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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

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Prediction of the properties of an unknown element below Silicon

Property Observed for Si

Predicted for eka-Si

Observed for Sn

Atomic mass

28 72 118

Density (g/cm2)

2.33 5.5 7.28

Formula of oxide

SiO2 Eka-SiO2 SnO2

Formula of

chloride

SiCl4 Eka-SiCl4 SnCl4

eka: “one beyond”

*

Observed for Ge

72.6

5.35

GeO2

GeCl4

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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)

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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):

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Cathode-Ray Tubes – ever seen one?

http://www.howstuffworks.com/tv4.htm

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Thomson’s conclusions

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

• “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."

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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

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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

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Thomson’s “plum pudding” atom model*

* Never had plum pudding? Think of a blueberry muffin.

• tiny “corpuscles” of negative charge

• surrounded by a sort of “cloud” of positive charge

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More pieces of the atom

Ernest Rutherford

• Thomson’s student

• Gold Leaf Experiment (1910-11) – actually conducted by Hans Geiger and undergraduate Ernest Marsden

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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

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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.

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like this:

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What actually happened:

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What’s going on?

Most alpha particles went straight through, andsome were deflected,

BUT

a few (1 in 20,000) reflected straight back to the source!

“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.”

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Rutherford’s Model of the Atom

• small, dense, positively-charged nucleus surrounded by “mostly empty” space in which the electrons must exist.

• positively charged particles called “protons”

• like tiny solar system

+

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The nucleus repels alpha particles

+

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How much of an atom is empty space?

In fact, if the nucleus of an atom were the size of Murphy Auditorium, the innermost electrons would be how far away?

A. DeLaRoche Hall?B. Francis Hall?C. Downtown Olean?D. NYC?

+

Most of it!

(click for the right answer)

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But wait – there’s more!

James Chadwick(1932)

• discovered a neutral (uncharged) particle in the nucleus.

• called it the “neutron”

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Atom “split” later that year

Atom “split” by John Cockcroft and

Ernest Walton, using a particle

accelerator, in late 1932

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Splitting the atom led to some very practical consequences

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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

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Why do 2 Group I atoms combine with 1 oxygen (R2O)?

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So: is this what atoms are like?

No! Calculations soon showed that a “Rutherford atom” would last less than one minute

Electrons would radiate away energy and spiral down into the nucleus.

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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

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Spectroscopes: seeing atomic light

Original 1859Bunsen-Kirchhoff spectroscope

Modern apparatus for viewing a“spectrum”

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Hydrogen’s emission “fingerprint”

The line-emission spectrum of hydrogen gas

Observation:

when heated with electricityhydrogen gives off light of specific wavelengths

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Niels Bohr(1885-1962)

Danish physicist

Bohr wondered why hydrogen emitted spectral

lines, and not just a continuous band of light

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Bohr’s Model of Atom (1913)

• Bohr assumed that electrons can orbit ONLY at 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

The first three allowed energy levels,at distances r1, r2, and r3 from nucleus.

r1

r2

r3

H's electron

H's nucleus containing 1 proton

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Electron orbits are distinct (“quantized”) in Bohr’s model

Trefil & Hazen. The Sciences: An integrated approach. 2nd ed. Fig. 7-6.

“Quantum leaps” from one level to

another

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But why should electrons behave this way?

“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

Louis de Broglie (1927)

Particle/Wave Duality of electrons

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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.

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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

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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!!

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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.”

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Here we go again!

By the 1950s hundreds of sub-atomic particles had been identified. Simplicity was getting lost again.

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Another attempt to simplify our model of matter

Murray Gell-Mann and George Zweig (1964) - proposed that protons and neutrons are made of

smaller particles they named quarks (aces)

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Gell-Mann & Zweig hypothesized 6 different quarks

• Quarks have fractional charges

• Then how do particles in the nucleus end up with +1 or zero charges?

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Protons & neutrons are not fundamental

Protons and neutrons are composed of UP and DOWN quarks, held together by gluon particles

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Fermi National Accelerator Lab: 6-km Tevatron ring and 3-km Main Injector *

• Chicago site for study of sub-subatomic particles

• Evidence for last quark (TOP) found in 1995

*contrast to world’s-largest machine: CERN 27-km LEP collider (1989-2000)

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So: are quarks fundamental?

Probably not: recent models of matter hypothesize 11-dimensional “strings” curled up inside of quarks …stay tuned for future developments.