Intersection 2
9/05
Reading: 1.8-1.11 p17-28
2.1-2.2 p 40-44
Questions
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Turning in course pack
Lab reports
Calendar
Registration for CHEM 126
Outline
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Review
Concept questions
Representation and Scale in Chemistry
History of the atom
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Dalton
Thompson (Plum Pudding)
Millikan
Rutherford
Bohr
Review of Week 1
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”
Pictures from:
http://images.google.com/imgres?imgurl=http://antiquescientifica.com/anatomical_model__head_Pichlers_Witwe_and_Son.jpg&imgrefurl=http://antiqu
escientifica.com/archive34.htm&h=652&w=501&sz=100&tbnid=aT8PH6LpljMJ:&tbnh=135&tbnw=103&hl=en&start=4&prev=/images%3Fq%3Danat
omical%2Bmodel%26svnum%3D10%26hl%3Den%26lr%3D
And http://images.google.com/imgres?imgurl=http://www.picture-newsletter.com/modelplanes/model-airplanefg6e.jpg&imgrefurl=http://www.picturenewsletter.com/modelplanes/&h=779&w=1200&sz=120&tbnid=_vVnAtgUr6wJ:&tbnh=97&tbnw=150&hl=en&start=1&prev=/images%3Fq%3Dmode
l%2Bairplane%26svnum%3D10%26hl%3Den%26lr%3D
Why do chemists use models?
• 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
Macroscale, Microscale, and Nanoscale
Models of microscale and nanoscale because we can’t see with our
naked eye. (Visual learners)
Models and representations
• How are atoms represented?
12C
6 protons
6 neutrons
exactly 12 amu
• How are elements represented?
• (http://www.webelements.com/)
Concept Question 1
Antoine Lavoisier, the "father of chemistry", listed lime
as a chemical element in his table of 33 known
elements. Which of the following observations best
shows that lime cannot be an element?
(a) Lime reacts with water, generating a large amount of
heat.
(b) Lime and carbon dioxide are produced when limestone is
roasted.
(c) When a certain soft metal is burned in oxygen, lime is
produced (with no other products).
(d) Lime melts at a temperature of 2572°C.
Elements
• Element: A substance that cannot be decomposed into
two or more new substances by chemical or physical
means
• The diatomic elements:
– H, O, N, and the halogens (H2, O2, N2, F2, Cl2, Br2, I2)
• Allotropes are different forms of the same element
in the same physical state at the same temperature
and pressure.
– O2 vs. O3
– diamond, graphite, C60 buckyballs
•
•
http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/diamond.htm
http://www.chem.yorku.ca/hall_of_fame/essays98/buckyball/bucky1/bucky.htm
IrReSPONSiBILiTiEs is the longest word that
can be spelled entirely using chemical
symbols without reusing any element...
Ir - iridium
Re - rhenium
S - sulphur
P - phosphorus
O - oxygen
N - nitrogen
Si - silicon
Bi - bismuth
Li - lithium
Ti - titanium
Es – einsteinium
Concept Question 2
Which of the following samples could be methane,
CH4? A sample that contains:
(a) 25% hydrogen by weight; 75% carbon by weight
(b) 4.0 g of H atoms and 1.0 g of C atoms
(c) 0.40 mole of H atoms and 1.0 x 1023 C atoms
(d) 0.40 mole of H2 molecules and 0.20 moles of C atoms
Models of Molecules
H2O
O
H
H
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…”
Nanoscale representations of the
three states of matter
Concept Question 3
• 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?
History of the Atom
A Swiss alchemy lab
from:
http://www.rosicrucians.or
g/salon/swiss/swiss.ht
ml
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.
• 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
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 17th century
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.
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.
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
Flaws in Dalton’s Model
• 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?
Electricity 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.)
Radioactivity
JJ Thomson & the Cathode Ray Tube
Picture from : www.chem.uiuc.edu/ clcwebsite/cathode.html
Thompson’s Experiments
1) Thompson showed that the charge and the rays were
inseparable.
2) The rays (carrying negative charge) bent towards the
positive electrode.
3) By adjusting the strength of the electric and magnetic
fields, he was able to make their effects cancel out and
the cathode rays follow their original path. By measuring
how much the rays were bent by a magnetic field and
how much energy they carried, 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.
Thompson Adds to Atomic
Theory
1) Cathode rays are charged particles called
corpuscles (today's called electrons)
2) Corpuscles are constituents of the atom
3) Corpuscles are the only constituents of the
atom
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.
Millikan’s Experiment
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.
What is your method?
• Need 15-20 volunteers to grab a handful of
pennies and weigh them:
• Sort the data from smallest to largest and
find the difference between neighboring
measurements.
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
Rutherford’s Hypothesis
Alpha (a) particles are
positively charged
particles emitted by
certain radioactive
atoms. If particles are
shot at a thin gold
foil……
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."
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 by
a like charged nucleus.
• Nucleus was only 1/10,000th the size of the
entire atom, but contained nearly all the
mass.
Bohr Model
• This model was a planetary model, in which the
electrons orbit around protons and neutrons that
occupy a central space. In Bohr’s model, there are
different electron levels, each holding a different
number of electrons. The first level holds 2
electrons, the second holds 8, and the third holds
18. The farther the electron level is from the
“nucleus” of the atom, the more energy it has. The
electrons usually fill the electron levels from
closest to furthest out.
Bohr Model Flaws
Ineffective explanation for bonding between
atoms.
Little ability to predict molecular shapes.
Could not predict how many bonds a
particular atom is likely to form
Atomic Orbital: Today’s
Working Model of an Atom
Atomic orbital theory places the electrons in
specific regions of space called orbitals.
These orbitals can be mathematically
derived through quantum mechanics.
Subatomic Particles
Mass (g)
electron 9.10938188 x10-28
proton
1.6762158 x10-24
neutron 1.67492716 x10-24
Charge
-1
+1
0
Summary
• Models are used to give chemists a
macroscopic view of a nono- and
microscopic world
• Atomic models have evolved as scientists
discovered more facts
As You Go…
• No sandals (cooler weather is on the way to
help…)
TUESDAY
WEDNES
DAY
THURSDA
Y
FRIDAY
SATURDA
Y
HIGH
HIGH
HIGH
HIGH
HIGH
90
80
76
73
76
LOW
LOW
LOW
LOW
LOW
61
55
57
59
• Scientific report due today in discussion
• HW 2 Due next Tuesday
• Chemical Scholarship assignment due next
Wednesday in discussion