Study Guide #2
Note: covers lecture material only; be sure you have done the assigned reading as well!
STRUCTURE OF ATOMS
Electrical charge
 Charges exert forces on one another (likes repel, opposites attract)
 There is a basic unit of charge; all charges are a multiple of this unit
Cathode Rays
 J. J. Thomson studies “cathode rays,” shows that they are particles and determined the
ratio of their charge to mass
 They come from atoms
 Called “electrons”
 Millikan determined charge and hence mass, about 1/2000 times less massive than a
hydrogen atom
Thomson’s model of the atom – plum pudding or blueberry muffin, static
Explored by Rutherford using scattering experiments
 Shot alpha particles through gold foil, get info on inner structure of atoms from the
pattern of scattered alpha particles (know the basic ideas here)
 Surprising thing was that some alphas were scattered a lot
 Concluded that there was a heavy, positively charged core of the atom – the nucleus
 Measured the size of the nucleus to be about 10–15 m (100,000 times smaller than the
atom as a whole)
 All of the positive charge and almost all of the mass are concentrated in the nucleus
Leads to “planetary” model of the atom – like a miniature solar system
 Law of electrical attraction is very similar to the law of gravitational attraction (in both
the force falls off like the square of the distance)
 Problem: instability due to radiation by the electron(s)
 No resolution of this given the laws of physics then known
QUANTUM MECHANICS
 A more fundamental physical theory than Newton’s; Newton’s laws are an approximate
form of QM, valid for systems that are large compared to the atomic scale
 Understand the basic results of the two-slit experiment
 Lessons
o Particles to not travel along definite paths (they don’t go either through one slit or
the other, they somehow “go through” both
o The act of observation always affects the observed system in a basic and
irreducible way (Heisenberg Uncertainty Principle)
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o Atomic-scale systems exhibit “wave-particle duality”, i.e. they behave in some
ways like particles (they are lumps, indivisible) and in other ways like waves
In QM we describe electrons using a type of wave, which can spread out and exhibit
typical wave phenomena like interference. This wave tells us how likely we are to find
the whole electron at any given location, if we look for it. We can only calculate the
probability that the electron will be at any given location. Before we measure its
position, it is not really anywhere.
Principal later architects of QM: Heisenberg, Born, Schroedinger. (Also important,
though not for the test, were: Einstein, Bohr, Dirac, Pauli, Jordan.)
HESENBERG UNCERTAINTY PRINCIPLE
 In QM a particle is represented by a wave “packet”, a mixture of waves with different
wavelengths (representing different speeds)
 Has an extent in space (or position “uncertainty”) x—the particle can be found
anywhere in this range
 Also an uncertainty in speed s
 Uncertainty principle says the product xs=h/m, where h is Planck’s constant and m is
the particle’s mass
 This product gives the “realm of possibilities” (RoP) for the particle, i.e. a range in x and
s that the particle can have
 Smaller x means bigger s and vice versa
 Also, bigger m means a smaller RoP; hence macroscopic objects like baseballs
effectively have a well-defined position and speed (the rectangle is tiny!), just as we
experience
 Also explains why atoms are stable in QM
QUANTUM MECHANICS OF ATOMS
 Bohr model
o Only particular orbits are allowed
o Labeled by an integer n
o There are n2 orbits total for a given n; all the orbits with the same n have the same
energy
o Electrons in these orbits do not radiate light (by assumption!)
o They can “jump” from one orbit to another though.
o Jumping down from one orbit to a lower-energy orbit happens spontaneously and
results in light of a particular energy or wavelength being emitted; explains why
only particular wavelength are observed (spectral lines)
o Jumping up requires being “hit” by light of just the right energy (wavelength)
o Allowed orbits can be thought of as “standing waves”, analogous to the waves on
a violin string
 Pauli Exclusion Principle
o Only two electrons are allowed in a given orbit
o Comes from relativity
 With Bohr plus the Pauli principle, we can understand the basic structure of the Periodic
Table
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o Electrons settle into the lowest orbits they can, taking the Pauli principle into
account
o “Closed shells” are very stable and correspond to inert (noble) gases
o Elements with the same number of extra or missing electrons are chemically
similar to one another
o Rules for forming compounds follow from allowing elements with extra electrons
to “share” them with elements that want electrons (i.e. are missing them)
Schroedinger’s Cat thought experiment
Mendeleyev’s Periodic Hypothesis
 The elements, if arranged according to their atomic weights, exhibit an evident
periodicity of properties
 Elements which are similar as regards their chemical properties have atomic weights
which are either of nearly the same value (e.g., platinum, iridium, osmium) or which
increase regularly (e.g., potassium, rubidium, cesium)
 The magnitude of the atomic weight determines the character of the element just as the
magnitude of the molecule determines the character of a compound body
 We must expect the discovery of many yet unknown elements, for example, elements
analogous to aluminum and silicon, whose atomic weight would be between 65 and 75
 The atomic weight of an element may sometimes be amended by a knowledge of those of
the contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and
126, and cannot be 128
 Certain characteristic properties of the elements can be foretold from their atomic weights
The Life of Mendeleyev
 Covered in chapter 9 of The Last Sorcerers
The Power of Mendeleyev’s Table (or why he got most of the credit)
 Predicted the existence of no less than eight elements that were not known in 1869 –
several were found within a few years
 Predicted the properties (atomic weight, density) of several elements by looking at the
elements directly above and below the “missing” element
 Rearranged several elements into their correct order (not atomic weight order) and
assumed that this was due to the incorrect determination of atomic weights
Atomic weight vs. equivalent weight
 Equivalent weight – defined as the quantity of an element will react with a fixed amount
of another
o example: 39 grams of potassium (K) is “equivalent” to 20 grams of calcium (Ca)
o but the atomic weights of K and Ca are 39 and 40, respectively
 This is because potassium and calcium have different “valencies” – they react with
oxygen to form K2O and CaO
Chemical Periodicity
 Arranging the elements by the way they react with a particular element (to form hydrides,
for example) gives a different view of the periodic table
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The Periodic Table is not simply a convenient arrangement of the elements, it shows the
striking similarities in chemical reactivity within a group (top to bottom) and the regular
progression of properties as you go along a period (left to right)
Physical Periodicity (trends in ionization energies and atomic sizes)
 Ionization energy (IE) = the energy required to remove an electron from an atom
 Ionization energies go down as you move from top to bottom in a group
 Ionization energies go up as you move left to right in a period
 Atomic size increases as you move from top to bottom in a group
 Atomic size decreases as you move left to right in a period
Atomic Weight vs. Atomic Number
 Atoms are made up of protons, neutrons and electrons
 The atomic weight of an isotope is determined by the number of protons + the number of
neutrons
 Different isotopes have different atomic weights
 The atomic weight of an element is an average of the weights of the naturally occurring
isotopes
 Arranging the elements strictly by atomic weight puts some elements out of order
 It was only after the structure of the nucleus was understood that chemists realized that
the best way to arrange the elements by atomic number = # of protons
Origins of Periodicity
 The energies of the electrons within atoms ultimately determines the chemical properties
of all the elements
 The laws of quantum mechanics define these energies (analogous to the harmonic series
for the musically inclined)
 Creates a pattern of 2, 6, 10, 14 that appears in the table
Alternate Arrangements of the Periodic Table
 Well over 700 versions of the Periodic Table have been created
 In spite of this the modern table closely resembles the arrangement first proposed by
Mendeleyev in 1869
 Some highlight the symmetry and order (Janet) while others (Benfey and 3D
arrangements) focus on the continuous nature of the table
Scanning-Tunneling Microscopy
 Background material discussed in Scanning Tunneling Microscopy lab handout
 tunneling – quantum-mechanical effect where particles move through a barrier that they
(classically) should not have enough energy to move over
 piezoelectric effect – used to move the scanning tip with high accuracy over very small
distances
 feedback loop – STM operated with feedback loop to maintain “constant current”
 the structure of graphite – some atoms show up brighter because of the arrangement of
carbon atoms in alternate layers of graphite
Structure-Property Relationships
 Background material discussed in Structure-Property Relationships lab handout
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The structure of graphite (layers) makes it soft, slippery and electrically conductive,
while the structure of diamond (three-dimensional interconnected network) makes it one
of the hardest materials known
The structure of ice (an open structure held together by hydrogen bonds) makes it less
dense than liquid water
Arrangements of Atoms in Solids
 Amorphous – no long range order
 Crystalline – regular repeating pattern
Types of Solids
 molecular solids - discrete molecules (I2)
 metallic solids - metal atoms, metallic bonding (Fe)
 network covalent solids - strong interatomic bonding (examples include diamond and
graphite)
 ionic solids - individual ions are held together by electrostatic forces
The Unit Cell
 Definition: the unit cell is a collection of atoms (or fractions of atoms) that can be
systematically translated through space to create the entire solid
 In two dimensions, unit cells take the shape of parallelograms
 In three dimensions, they are parallelepipeds
Modern Materials
 Liquid Crystals
 Polymers (addition/condensation, x-link)
 Metals (memory metal)
 Ceramics (superconductors)
 Thin films (MOCVD)
 Semiconductors
Materials Chemistry
 Basic types of materials
metals
polymers / plastics
ceramics
 Applications
electronics
automobiles
clothing fibers
containers
coatings
biomaterials
sports equipment
superconductors, etc.
Polymeric Materials
 Polymers are long-chain molecules formed by the polymerization of small “monomers”
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Types of polymerization
Addition (polyethylene, polypropylene, Teflon, etc.)
Condensation (Nylon, polyesters, Kevlar, etc.)
Electrically Conducting Polymers
 Addition polymerization of acetylene (C2H2) yields polyacetylene
 When polyacetylene is oxidized or reduced (doped), it becomes electrically conductive!
Radioisotopes in Medicine
 The nucleus of every atom is made up of protons and neutrons. The specific combination
of protons and neutrons determines the isotope
 Some nuclei are unstable and emit one or more of the following
an alpha particle (group of 2 protons + 2 neutrons)
a beta particle (electrons)
positron (short-lived, positively-charged particle with the same mass as an electron)
electromagnetic radiation (gamma rays)
 Approximately 200 radioisotopes are used on a regular basis
 Making radioisotopes
most common: neutron capture in a nuclear reactor - cobalt-60 used for radiation
therapy
proton capture in a cyclotron
Specific Examples (Radiotracers)
 Technetium-99
half-life of six hours
one of the most widely used radioisotopes (80% of all procedures)
combination of short half-life, extensive chemistry and favorable decay processes that
produce relatively low-energy particles makes it ideal
used to image the heart, bones, liver and lungs
 Iodine-131
half-life of 8.04 days
used to determine thyroid activity
 Carbon-11 (20.4 min), nitrogen-13 (10 min), oxygen-15 (2 min), and fluorine-18 (110
min)
isotopes are incorporated into molecules such as sugars and rapidly injected
positron emitters used in PET for studying brain physiology and pathology
fluorine-18 has become very important in detection of cancers and the monitoring of
progress in their treatment (incorporated into 2-fluoro-2-deoxy-D-glucose)
Boron Neutron Capture Therapy (BNCT)
 BNCT is a form of Targeted Alpha Therapy (TAT), a means to selectively deliver high
doses of radiation to cancer cells
 BNCT uses boron-10 compounds that selectively concentrate in malignant brain tumors
 The patient is irradiated with thermal neutrons which are strongly absorbed (“captured”)
by the boron-10
 The resulting boron decays, producing high-energy alpha particles which kill the cancer
 Advantages: potentially highly targeted, localized radiation
 Disadvantages: the procedure requires the patient to be brought to a nuclear reactor,
rather than the radioisotopes being taken to the patient.
Magnetic Resonance Imaging (MRI)
 The nuclei of atoms have quantum properties just like atoms and electrons
 One quantum-mechanical property called “spin” causes the nuclei to act like little
magnets
 In the presence of a magnetic field the spin of the nuclei can either line up parallel or
anti-parallel to the magnetic field
 MRI uses radio waves to probe the difference in energy between these two arrangements
Anti-Cancer Drugs – cisplatin
 One of the most-prescribed anticancer drugs, cisplatin (cisdiamminedichloroplatinum(II)), was discovered by accident while scientists were
studying the effects of electric fields on the growth of cells
 Several platinum compounds were formed from the combination of the platinum
electrodes and the electrolyte
 The most active anti-tumor compound was found to be cis-diamminedichloroplatinum(II)
 “Testicular cancer went from a disease that normally killed about 80% of the patients, to
one which is close to 95% curable. This is probably the most exciting development in the
treatment of cancers that we have had in the past 20 years. It is now the treatment of first
choice in ovarian, bladder, and osteogenic sarcoma [bone] cancers as well.” - Dr. Barnett
Rosenberg
Anti-Cancer Drugs – Taxol
 1962 – large-scale screening of plant compounds shows that an extract of the yew tree is
active against tumors
 1967 – compound is identified
 1971 – structure of Taxol determined by x-ray crystallography
 1983 – Phase I clinical trials begin, but the extract is in short supply due to limited
number of Pacific Yew trees
 1990 – synthesis route discovered that allows production of Taxol from the European
Yew tree
 1992-1994 – Taxol is approved for treatment of ovarian and breast cancers