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ATOMS: Dalton and Beyond

A search for a simple theory of

matter Topic 7 – Spring 2005

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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Models in science

Scientists use their experimental results (and their imaginations)

to create models

A model is a representation of a complex natural system that permits us to understand its behavior.

These models may be mental, mathematical, or even physical.

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Prior Examples?

Remember the models of planetary motion? What were they trying to explain?

(These animated gifs were obtained from Dr. Stephen J. Daunt's Astronomy 161 web site at

The University of Tennesee, Knoxville.)

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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

• There is only one type of matter, found in tiny, indivisible particles called “atoms”

• All change is caused by atoms moving through empty space

• 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)

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The mechanical philosophy of the 1600s

• Descartes, Boyle and Newton

• A “clockwork” universe

• Simplicity, generality, and precise predictions

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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 Atoms

John Dalton’s Atomic Hypothesis (1803):

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” as based on atomic composition of compounds

• used “Rule of greatest simplicity” to guess at the atomic structure of compounds

• estimated relative atomic masses, based on his hypothesized structures

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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 Dobereiner and Newlands, and Meyer probably formulated the periodic idea at same time as

Mendeleev)

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Mendeleev’s early notes

for the Periodic Table

(1869)

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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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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)

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Prediction of the properties of an unknown Group 4 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: 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)

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

• “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

• “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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Thomson’s “plum pudding” atom model*

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

Cathode rays (electrons) are...

• tiny “corpuscles”

of negative charge

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

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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

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More pieces of the atom

Ernest Rutherford(1871-1937)

nuclear physicist,Thomson’s student,

New Zealander teaching in Great Britain

Gold Leaf Experiment

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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

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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.

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Rutherford’s Experiment: prediction

Alpha particles will pass through like this …

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Rutherford’s experiment: 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 AtomExpt. Interpretation:

• gold atom has 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 a marble, the innermost electrons would be how far away?

• One-half inch• Six inches• Eighteen inches• One-half mile

+

(click for the right answer)

Most of it!

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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

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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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Properties of Subatomic Particles

Property

Particle

Mass (amu),

Mass (g)

Relative

Charge

Electron 0.00055

9.110 x 10-28

- 1

Proton 1.00728

1.673 x 10-24

+ 1

Neutron 1.00866

1.675 x 10-24

0

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Now we understand why the elements come in periods of 8

• 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. A given element can have different number of neutrons, and therefore different atomic masses.

• 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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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!

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So: is this what atoms are like?

No! Electrons moving through the electrical field generated by the protons in the nucleus would radiate away energy and spiral down into the nucleus

Calculations soon showed that a “Rutherford atom” would last less that one minute.

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A new understanding of the atom

It had long been know that when chemical elements are heated, they gave off light of a particular wavelength (or color)

Sodium Potassium Lithium

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Spectroscopes: Seeing Atomic Light

Original 1859Bunsen-Kirchhoff spectroscope

Typical setup for viewing aline-emissionspectrum

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Spectroscopy can identify elements on distant stars

Helium was discovered in a spectrograph of the Sun in 1868 and not on Earth until 27 years later.

Hydrogen

Helium

http://heasarc.gsfc.nasa.gov/docs/xmm_lc/edu/lessons/student-worksheet-spectragraph2.html

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Hydrogen’s Emission “Fingerprint”

The line-emission spectrum of hydrogen gas (the bands visible to humans)

Observation:

when hit with electricityhydrogen gives off light of specific wavelengths, NOT continuous range!

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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)

• Circling electron maintains orbit ONLY at specific distances from nucleus

• Only way electron could exist for 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.

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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

Louis de Broglie (1927)

Particle/Wave Duality of electrons

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Electrons can be thought of as standing waves …

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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

When the mathematics was worked out, these distances agreed exactly with those assumed by Bohr for the hydrogen atom.

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The position of electrons can’t be predicted precisely

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 somewhere in the atom, it’s velocity is uncertain by ~7,300 km/s (~ 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 protons and neutrons are made of smaller

particles they named quarks (aces)

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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

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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.