Classroom notes for: Radiation and Life

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Classroom notes forRadiation and Life

• 98.101.201
• Professor Thomas M. Regan
• Pinanski 207 ext 3283

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Basic Atomic Theory

• An atom can be defined as the most basic unit of
  a chemical element
• Remember an element can be informally defined as
  something with unique physical and chemical
  properties and something that cannot be broken
  down into any other substances.
• For example one hydrogen atom is the smallest
  subdivision that exists of the element hydrogen,
  which has properties different from all other
  elements.
• The word atom is a derivation of the Greek
  words tomos (to cut) and a (not) a
  result of the original thought that atoms are
  indivisible. http//www.nzedge.com/heroes/rutherfo
  rd.html
• Remember Democritus and Dalton?

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• Atoms are tiny.
• An average atom has a diameter of about 10-10 m.
  (Radiation and Health, Luetzelschwab, p. A4)
• A non-SI unit of length traditionally used by
  chemists is the angstrom, which equals 10-10
  meters. (General Chemistry, Ebbing and Wrighton,
  p. 13)
• Roughly, the size of an atom is to an apple as
  the size of an apple is to the earth.
• No one has ever viewed an atom using visible
  light.

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• Electron Microscope
• To resolve detail that is the size of angstroms,
  we need a wavelength on the order of angstroms.
  X-Rays have wavelengths in this range, but so far
  no practical means have been found for focusing
  them. Electrons, on the other hand, are readily
  focused with electric and magnetic fields. The
  German physicist Ernst August Friedrich Ruska
  (1906-1988) used this wave property to construct
  the first electron microscope in 1933 (for which
  he shared the 1986 Nobel Prize for physics).
  (General Chemistry, Ebbing and Wrighton, p. 254)
  and (Asimovs Chronology of Science and
  Discovery, Asimov, pp. 585-586)

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http//www.mos.org/sln/sem/tour20.html
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Transmission Electron Microscope (TEM)

• operates on the same basic principles as the
  light microscope but uses electrons instead of
  light. What you can see with a light microscope
  is limited by the wavelength of light. TEMs use
  electrons as "light source" and their much lower
  wavelength makes it possible to get a resolution
  a thousand times better than with a light
  microscope.
• You can see objects to the order of a few
  angstrom (10-10 m). For example, you can study
  small details in the cell or different materials
  down to near atomic levels. The possibility for
  high magnifications has made the TEM a valuable
  tool in both medical, biological and materials
  research.

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Magnetic Lenses Guide the Electrons

• A "light source" at the top of the microscope
  emits the electrons that travel through vacuum in
  the column of the microscope. Instead of glass
  lenses focusing the light in the light
  microscope, the TEM uses electromagnetic lenses
  to focus the electrons into a very thin beam. The
  electron beam then travels through the specimen
  you want to study. Depending on the density of
  the material present, some of the electrons are
  scattered and disappear from the beam. At the
  bottom of the microscope the unscattered
  electrons hit a fluorescent screen, which gives
  rise to a "shadow image" of the specimen with its
  different parts displayed in varied darkness
  according to their density. The image can be
  studied directly by the operator or photographed
  with a camera.

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TEM
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Scanning Tunneling Microscope

• The scanning tunneling microscope consists of a
  tungsten metal needle with an extremely fine
  point (the probe) placed close to the sample to
  be viewed. If the probe is close enough to the
  sample, electrons can tunnel from the probe to
  the sample. The probability for this can be
  increased by having a small voltage applied
  between the probe and sample. Electrons
  tunneling from the probe to the sample give rise
  to a measurable electric current. The magnitude
  of this current depends on the distance between
  the probe and the sample (as well as on the wave
  function of the atom in the sample). By
  adjusting this distance, the current can be
  maintained at a fixed value. As the probe scans
  the sample, it moves toward or away from the
  sample, in effect following the contours of the
  sample. (General Chemistry, Ebbing and Wrighton,
  pp. 257-258)
•  

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• The probe must move incredibly small distances,
  and this is done by an ingenious mechanism.
  Certain solids (piezoelectric crystals) generate
  a voltage when their length is changed (as when a
  phonograph needle rides over the grooves of a
  record). The reverse also occurs. Small voltage
  variations applied to a piezoelectric rod can
  generate corresponding small changes in the
  length. In the tunneling microscope, a variable
  voltage is applied to a piezoelectric rod to
  shorten or lengthen it. Because the probe is
  attached to this rod, the probe is moved toward
  or away from the sample. In this way the
  distance of the probe to the sample is adjusted
  by a voltage in order to maintain a constant
  tunneling current as the probe scans the sample.
  (General Chemistry, Ebbing and Wrighton, pp.
  257-258)

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STEM
Four differently shaped corrals made by iron
atoms on a copper surface. http//nobelprize.org/p
hysics/educational/microscopes/scanning/gallery/4.
html
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STEM
http//nobelprize.org/physics/educational/microsco
pes/scanning/index.html
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The Atoms Constituents

• The atom isnt indivesible it consists of three
  basic building blocks. These building blocks are
  the commonly known neutron, proton and electron.

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• Protons (p) are one.
• Protons can be thought of as roughly spherical
  objects with a mass of approximately 1.6726 x
  1027 kg, or slightly more than one atomic mass
  unit (amu), and a 1 charge. (Radiation and
  Health, Luetzelschwab, p. A1)
• Note that one amu 1.66043 x 10-27 kg.
  Measuring mass in amu is simply a more convenient
  method when dealing with such small objects. One
  amu is a tiny fraction of one kilogram, in the
  same way that an ounce is a fraction of a pound.
• Neutrons (n0 or n) are a second.
• Neutrons can be thought of as roughly spherical
  objects with a mass of approximately 1.6750 x
  10-27 kg, or slightly more than one amu, and no
  charge (they are electrically neutral. (Radiation
  and Health, Luetzelschwab, p. A1)
• Both protons and neutrons are known to be made of
  more fundamental particles known as quarks
  (Radiation and Health, Luetzelschwab, p. A1)
  however consideration of these particles is
  beyond the scope of this course.
• Electrons (e-) are a third.
•  Electrons have a -1 charge, and a mass of
  approximately 1/1837th of either a proton or a
  neutron their size and shape in comparison to
  the proton and the neutron is not well
  understood. (Radiation and Health, Luetzelschwab,
  p. A1)

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The Nucleus

• The protons and neutrons coexist in the nucleus,
  while the electrons orbit around it.
• This is Rutherfords planetary model of the atom
  it is not entirely accurate, but serves as a
  useful tool for describing the atoms structure.
  In fact, the best we can do is to specify a
  region of space in which there is a given
  probability of finding an electron.
• The nucleus holds all of the positive charge and
  essentially all of the mass of the atom but it
  is tiny compared with the overall size of the
  atom.
• The diameter of a medium-sized nucleus is about
  10 femtometers (10-14 m) on average, the
  diameter of this atom is about one-tenth
  nanometer (10-10 m). This means that the atom
  as a whole is about 10,000 times larger than the
  nucleus, but mostly empty space. (Radiation and
  Health, Luetzelschwab, p. A4)
• If the nucleus were a grain of sand at center
  court in a school gym, the electrons would be in
  orbit in the stands.
• (1/16 inch assumed diameter)(a factor of 10,000
  times larger) / (12 inches/foot)
• If the nucleus were a large marble, the electrons
  would be orbiting over 400 feet away.(1 inch
  assumed diameter)(a factor of 10,000 times
  larger) / (12 inches/foot)
• What prevents my hand from passing through the
  desk/table when I hit it? The negatively charged
  electrons in the desk repel the negatively
  charged electrons in my hand!

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Building an Atom

• A chemical element is defined by the number of
  protons in the nuclei of its atoms.
• The simplest element is hydrogen (H) its atoms
  contain one proton in the nucleus.
• The next simplest element is helium (He) its
  atoms contain two protons in the nucleus.
• The most massive element that exists naturally in
  any significant quantities is uranium (U) (well
  hear a lot about uranium throughout the course)
  its atoms contain 92 protons in the nucleus.
• Remember, the definition of an atom is such that
  the number of electrons always balances the
  number of protons however, there isnt such a
  strict prohibition on the numbers of neutrons, as
  well see.

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• Scientists have developed a shorthand notation
  that conveys all of this information
• ZX
• X chemical symbol of the element.
• Many symbols come from the original Latin names
  for the elements, or from Greek or Latin words or
  phrases that somehow tie in with the element.
• sodium Na- natrium (L.)
• ruthenium Ru- Ruthenia, Russia (L.)
• silver Ag- argentum (L.)
• gold Au- aurum, shining dawn (L.)
• lead Pb- plumbum (L.)
• (Handbook of Chemistry and Physics, 53rd Edition)
• Leads symbol, Pb, is based on the elements
  original Latin name plumbum, also the source of
  the word plumber. (Chemistry in the Community
  4th Ed., American Chemical Society, p. 54)

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• Z atomic number number of protons in the
  nucleus
• A mass number total of number of protons
  number of neutrons in the nucleus
• By this notation
• hydrogen is 1H
• helium is 2He and
• uranium is 92U.
• The shorthand can be further simplified. For
  example, we know Z 2 for helium, so we can omit
  writing it for helium 4He.
• We can even write it as He-4.

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The Electrons

• Electron Shells
• The electrons exist without radiating energy in
  electron shells around the nucleus.
• The shells are labeled K, L, M, N, O, P, and Q.
• Generally, the electron energies in each shell
  increase from K to Q for instance, L shell
  electrons are more energetic than K shell
  electrons.
• Think of the K shell as being the innermost
  shell every atom except hydrogen has two
  electrons in the K shell.
• The shells beyond the K shell can hold maximum
  numbers of electrons dictated by quantum
  mechanics, although the valence (outermost)
  shell will never contain more than eight
  electrons.

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Chemical Bonds

• The type of interaction between atoms depends on
  the electrons and is known as a chemical bond.
• If atoms come together and bond, there should be
  a net decrease in energy, because the bonded
  state should be more stable and therefore at a
  lower energy level. (General Chemistry, Ebbing
  and Wrighton, p. 315)
• There are three main types of chemical bonds
  ionic, covalent, and metallic. (General
  Chemistry, Ebbing and Wrighton, p. 312)
• One type is an ionic bond.
• Consider the example of chlorine and sodium
  atoms.
• The chlorine has a negative charge, because it
  gained an electron, while the sodium atom now has
  a positive charge, because it lost an electron.
• Both are now ions an ion is an electrically
  charged particle obtained from an atom by adding
  or removing electrons. (General Chemistry, Ebbing
  and Wrighton, pp. 47-48) The chlorine atom is
  considered an anion (negative ion), and the
  sodium atom is considered a cation (positive
  ion). (General Chemistry, Ebbing and Wrighton, p.
  313)
• The oppositely charged ions attract each other
  and stick together in an ionic bond, forming
  sodium chloride (NaCl- table salt).

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covalent bond

• A covalent bond is formed by the sharing of a
  pair of electrons between atoms. (General
  Chemistry, Ebbing and Wrighton, p. 323)
• Draw an H2 molecule as an example- neither atom
  has gained or lost electrons there are no
  ions.
• Consider the formation of a covalent bond between
  two H atoms to give the H2 molecule. As the
  atoms approach one another, their 1s orbitals
  begin to overlap. Each electron can then occupy
  the space around both atoms. In other words, the
  two electrons can be shared by the atoms. The
  electrons are attracted simultaneously by the
  positive charges of the two hydrogen nuclei.
  This attraction that bonds the electrons to both
  nuclei is the force holding the atoms together.
  Thus, while ions do not exist in H2, the force
  that holds the atoms together can still be
  regarded as arising from the attraction of
  oppositely charged particles nuclei and
  electrons. (General Chemistry, Ebbing and
  Wrighton, pp. 323-324)
• A polar covalent bond is a covalent bond in which
  the bonding electrons spend more time near one
  atom than the other. (General Chemistry, Ebbing
  and Wrighton, p. 327)

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Isotopes

• A chemical element is defined by atomic number
  (Z).
• However, for a given atomic number, there may be
  several possible values for the mass number (A)
  i.e., the element can have nuclei with different
  numbers of neutrons.A useful mnemonic is
  isotoPe- same number of Protons
• For example, the following are all isotopes of
  hydrogen, because each has one proton (Z1).
• 1H (H - hydrogen)
• 1H (D - deuterium)
• 1H (T - tritium radioactive)
• There are no isotopes of hydrogen with three
  neutrons because nuclei with large imbalances
  between the number of protons and neutrons will
  not exist very long this point will be briefly
  touched upon again.
• Isotopes of an element have nearly the same
  chemical properties (General Chemistry, Ebbing
  and Wrighton, p. 42), because they have the same
  number of electrons arranged in the shells in
  essentially the same fashion.
• Thus, isotopes of an element cant be separated
  by chemical means.
• Isotopes of an element, if radioactive, will have
  different radioactive properties (remember
  Soddy?)