A
Intersection 2
9/12/06
Reading: 1.8-1.11 p17-28
2.1-2.2 p 40-44
A
Outline
Demos, News
Law, Theories, Models
Scientific Inquiry
History of the atom
Dalton
Thompson (Plum Pudding)
Millikan
Rutherford
Representation and Scale in Chemistry
M
Demos!
A
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)
M
According to the National
Academy of Science, a Law is:
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?
M
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.
M
“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.
M
Law of Definite Proportions
compounds composed of two or more
elements in definite proportions
Joseph Proust
1754-1826
Supported Law
I also discovered
Bleach (NaOCl)
Claude Berthollet
1748-1822
Opposed 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!
A
According to the National
Academy of Science, a Theory is
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.
A
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
A
Theory vs. Model
• Theory based on facts, evidence
• Model is the picture, an analogy, a way of
describing a theory
A
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”
M
History of the Atom
A Swiss alchemy lab from:
http://www.rosicrucians.org/
salon/swiss/swiss.html
M
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.
Rumi’s poem about atoms
M
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/atomosoke.htm
M
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.
A
th
17
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
A
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.
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
A
Dalton on Elements
• Atoms are the
smallest units
of matter
• Chemical
elements are
made of atoms.
A
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
A
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?
M
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.)
M
Radioactivity
M
1897: JJ Thomson & the Cathode Ray
Tube
Picture from : www.chem.uiuc.edu/ clcwebsite/cathode.html
M
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.
M
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
M
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
A
1909: Millikan Finds the Charge of eIn 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.
A
Millikan’s Experiment
A
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:
A
M
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
M
Rutherford’s Hypothesis
Alpha (a) particles are positively charged
particles emitted by certain radioactive atoms. If
particles are shot at a thin gold foil……
M
Rutherford’s Data
Flash demo
In Rutherford's own words, "It was almost as if you fired a 15inch shell into a piece of tissue paper and it came back and hit
you."
M
Rutherford’s Conclusions
• Most of his a 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.
A
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******
A
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?
M
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
M
Macroscale, Microscale, and Nanoscale
M
Models and representations
• How are atoms represented?
12C
6 protons
6 neutrons
exactly 12 amu
• How are elements represented?
http://www.webelements.com/
M
Models of Molecules
H2O
O
H
H
M
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…”
A
Nanoscale representations of the
three states of matter
A
Concept Question 1
• 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?
A
Summary
• Atomic models have evolved as scientists
discovered more facts
• Models help to chemists with macroscopic
eyesight to conceptualize a nano- and
microscopic world
A
Reminders…
• No sandals or contacts in studio
• HW 2 Due Monday 9/18
• Pennies lab report is due next Friday 9/22