Intersection 2

9/12/06

Reading: 1.8-1.11 p17-28

2.1-2.2 p 40-44

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OutlineDemos, NewsLaw, Theories, ModelsScientific InquiryHistory of the atom

DaltonThompson (Plum Pudding)MillikanRutherford

Representation and Scale in Chemistry

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Demos!

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News bit

• From C&E News July 3, 2006– Copper topped $8000/metric ton (the highest

price since 1870)– Pre 1992 1p and 2p coins (UK) 97% copper

• 1 metric ton of coins = $5400

• Smelted into copper, 1 metric ton of coins = $8700

– Post 1992 are steel electroplated with copper (magnetic)

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A descriptive generalization about how some aspect of the natural world behaves under stated circumstances

How are a fact and a law related?

Can you think of any scientific laws?

According to the National Academy of Science, a Law is:

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Some Examples of Laws as stated by Dalton

Law of Conservation of Matter

• In an ordinary chemical reaction matter is neither created nor destroyed.

• The sum of the masses of the reactants equals the sum of the masses of the products.

Law of Constant Composition

• A chemical compound always contains the same elements in the same proportions by mass.

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“Laws” Can Be Overturned !

Which Law from previous page is now known to be incorrect ?

Law of Conservation of Matter Law of Constant Composition

The “Law of Constant Composition” also has an older name given by Joseph Proust.

The Law of Definite Proportions

Once a Law has become accepted, it is very difficult to get it convince the scientific community to discard it. Hence, this one still appears in your textbook despite its limitations. This

law is only true for simple, small molecules.

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Law of Definite Proportions

Joseph Proust1754-1826

Claude Berthollet1748-1822

Supported LawOpposed Law

But conceded to Proust

For most compounds discussed in Chem 130, Proust was correct !

However, more complex materials violate this Law (for example, LEDs used later in course)Today, Berthollet is believed to be correct! He was vindicated ~160 years after his death!

I also discoveredBleach (NaOCl)

compounds composed of two or more elements in definite proportions

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In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses. Theories must be falsifiable.

According to the National Academy of Science, a Theory is

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According to the National Academy of Science, a Model is

A description or analogy used to help visualize something (as an atom) that cannot be directly observed

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Theory vs. Model

• Theory based on facts, evidence

• Model is the picture, an analogy, a way of describing a theory

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NAS Building in Washington, DC

“The investigation of truth is in one way hard and in another way easy. An indication of this is found in the fact that no one is able to attain the truth entirely, but everyone says something true about the nature of things, and by the union of all a considerable amount is amassed.”

-Aristotle in “Metaphysics”

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History of the Atom

A Swiss alchemy lab from:

http://www.rosicrucians.org/salon/swiss/swiss.html

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Early history of the atom• The word atom dates to 420 BC.

• Democritus and his teacher Leucippus proposed the idea that space was either empty (as in a vacuum) or occupied by atoms that were eternal, invisible, and so small that they could not be further diminished.  

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Rumi’s poem about atoms

Aristotle and Plato disagreed, claiming that the four basic elements of earth, wind, fire, and water made up the material world and that all things could be derived of some combination of the four

Picture from http://www.npp.hu/tortenelem/atomosok-e.htm

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Alchemy• ~300 BC-1650• Based on the idea that everything was made by some

combination of earth, air, fire, and water• Attempts to transmute material to gold: the perfect

substance• Sought universal solvent• Looking for an Elixir to extend life• Aristotle’s and Plato’s incorrect ideas held sway for

almost 2000 years. Even though correct concepts were proposed and discussed! Another example of how Scientific Laws evolve.

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17th Century Elements

Element: a substance homogenous in its properties and not be divisible into even more fundamental elements

Only 12 elements were known in 1700:

Carbon, copper, gold, iron, lead, mercury, silver, sulfur, tin, zinc

Water, potash, air believed to be elements*

*Note: data gathered and evaluated and fact changed

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Phlogiston

• Late 17th century phlogiston was put forth by Beecher and advocated by Stahl.  They claimed when wood is burned, it releases "phlogiston" into the air.  If the wood is burned in a jar, the flame eventually goes out when the air is saturated with "phlogiston."  

• Once scientists were better able to study gases and carry out quantitative research, they discovered that oxygen was the critical component of all of the reactions involving phlogiston.    Phlogiston became the opposite of oxygen.  

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1800s: Dalton’s Theory

In the early 1800s, Dalton began to formulate his theory and model. His theory had five main principles:1. Chemical elements are made of atoms.

2. The atoms of an element are identical in their masses

3. Atoms of different elements have different masses

4. Atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

5. Atoms can be neither created nor destroyed

Dalton on Elements

• Atoms are the smallest units of matter

• Chemical elements are made of atoms.

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Dalton on Compounds

- Compounds are combinations of different elements, and that in these compounds there is a constant ratio of atoms

- Changing their physical state could not separate these compounds

- Chemical reactions occurred due to a rearrangement of combinations of atoms

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Dalton’s Model Leaves Unanswered Questions

• What makes the atoms of each element different?• Why do atoms combine to form compounds?• Why do they combine only in integer ratios?• Why are specific ratios of atoms observed in

compounds?• Why do groups of elements have such similar

properties and reactivities?

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1890s: Electrical Charge is Key to Atomic Structure• Charges of the same type repel one another; charges of the opposite

type attract one another• 1891 -G. Johnstone Stoney term "electron" coined for the unit of

electrical charge found when current was passed through chemicals • 1896 -Henry Bacquerel discovers that uranium ore emits rays that

exposed a photographic plate through protective black paper• 1898 -Marie and Pierre Curie isolate polonium and radium which emit

the same rays.  Radioactive elements emit three types of radiation (alpha (+), beta (-), and gamma (neutral) rays) which can be separated by passing them through electrically charged plates.  Alpha and beta rays have mass.  

• Conclusion:  Radioactive elements have atoms which are made of something smaller (alpha and beta particles.)

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Radioactivity

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1897: JJ Thomson & the Cathode Ray Tube

Picture from : www.chem.uiuc.edu/ clcwebsite/cathode.html

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Thompson’s Experiments

1) Cathode rays were generated across an evacuated tube by heating up the cathode and applying a high voltage across the tube.

2) Thompson showed that rays mass and negative charge using magnetic and electric fields

3) Thompson calculated the ratio of the mass of the particles to their electric charge 5.6x10-9 g/coulomb

4) Using different metals as cathodes, all cathode rays consisted of particles with the same mass to charge ratio.

5) This estimate of mass implied that electrons were was ~1000 times less massive than a hydrogen atom.  This estimate was confirmed by multiple independent experiments supporting its surprising conclusion.

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Thompson’s Addition to Atomic Theory

1) Cathode rays are charged particles called corpuscles (now called electrons)

2) Corpuscles are constituents of the atom

3) Corpuscles are the only constituents of the atom

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Plum Pudding Model of an Atom

Thompson described an atom as consisting of small, negatively charged corpuscles situated inside a positively charged field by electric static forces.

http://nobelprize.org/physics/educational/quantised_world/structure-1.html

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1909: Millikan Finds the Charge of e-

In 1909 Robert Millikan determined the charge of the electron in his "oil drop experiment“

Charge was always a multiple of -1.6 x 10-19 C.  He proclaimed that this value was the correct value for the charge of an electron.

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Millikan’s Experiment

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Question: How did Millikan determine the charge on one electron if he didn't know how many electrons an oil drop acquired?

Your challenge: to determine the number of pennies in this beaker without counting the pennies.

The rules:1. Pennies may be removed from the beaker, but they cannot be counted at any time.2. All pennies have to be returned to the beaker.

Procedure:Need 15-20 volunteers to grab a handful of pennies

and weigh them:

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Plum Pudding Model of an Atom

Thompson described an atom as consisting of small, negatively charged corpuscles situated inside a positively charged field by electric static forces.

http://nobelprize.org/physics/educational/quantised_world/structure-1.html

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Rutherford’s Hypothesis

Alpha () particles are positively charged particles emitted by certain radioactive atoms. If particles are shot at a thin gold foil……

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Rutherford’s Data

In Rutherford's own words, "It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you."

Flash demo

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Rutherford’s Conclusions

• Most of his particles passed through the foil without encountering the atomic nucleus, but a few came near enough to the nucleus to be deflected by the repulsion of a like charged nucleus.

• Nucleus was only 1/10,000th the size of the entire atom, but contained nearly all the mass.  

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Question: How did Rutherford see the nucleus of an atom?Your challenge: to determine the location and size

of an object in a closed box.The rules:

You should observe slits cut in four sides of your box with white paper covering three of the four slits. This design will allow you to use a laser pointer as a light emitting probe to collect data.

Rulers are available for data collection.*****It goes without saying that the laser pointer is

for answering the questions posed above, not for pointing at people or in anyone’s eyes******

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Object in a Box

1. Describe the manner in which you collected data.2. Sketch a figure or diagram that would be most

useful in reporting your data.3. What percent of the volume of the box is

occupied by the object?4. The object in a box is analogous to Rutherford's

experiment. What components of the experiment correspond to which parts of the object in the box?

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Why do chemists use models/analogies?

• macroscale– physical properties that can be observed by the

unaided human senses

• microscale– samples of matter that have to be viewed with a

microscope

• nanoscale– samples that are at the atomic or molecular

scale where chemical reactions occur

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Macroscale, Microscale, and NanoscaleM

• How are atoms represented?

• How are elements represented?

http://www.webelements.com/

Models and representations

      exactly 12 amu

12C

6 protons6 neutrons

        

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Models of Molecules

H

O

H

H2O

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Chemical Formula for water

Chemistry Teacher: “Johnny, what is the chemical formula of water?”

Johnny: “HIJKLMNO.”

Chemistry Teacher: “That’s wrong!”

Johnny: “But yesterday you said it was H to O…”

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Nanoscale representations of the three states of matter

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• The circle on the left shows a magnified view of a very small portion of liquid water in a closed container.

• What would the magnified view show after the water evaporates?

Concept Question 1A

Summary

• Atomic models have evolved as scientists discovered more facts

• Models help to chemists with macroscopic eyesight to conceptualize a nano- and microscopic world

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Reminders…• No sandals or contacts in studio

• HW 2 Due Monday 9/18

• Pennies lab report is due next Friday 9/22

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