Classroom notes for:
Radiation and Life
98.101.201
Professor: Thomas M. Regan
Pinanski 207 ext 3283
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/rutherford.html
Remember Democritus and Dalton?
Atoms are tiny.
Object
Dia. (m)
atom
1.00E-10
apple
8.00E-02
earth
1.27E+07
atom/appl
1.25E-09
appl/earth
6.28E-09
 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.
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 (Asimov’s Chronology of
Science and Discovery, Asimov, pp. 585-586)
http://www.mos.org/sln/sem/tour20.html
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.
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.
TEM
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)
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)
STEM
Four differently shaped corrals made by
iron atoms on a copper surface.
http://nobelprize.org/physics/educational/microscopes/scanning/gallery/4.html
STEM
http://nobelprize.org/physics/educational/microscopes/scanning/index.html
The Atom’s Constituents
The atom isn’t indivesible; it consists of three
basic building blocks. These building blocks
are the commonly known neutron, proton and
electron.
 Protons (p+) are one.
• Protons can be thought of as roughly spherical objects with a
mass of approximately 1.6726 x 10–27 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)
The Nucleus
 The protons and neutrons coexist in the nucleus, while the electrons “orbit”
around it.
• This is Rutherford’s planetary model of the atom; it is not entirely
accurate, but serves as a useful tool for describing the atom’s 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 (1010 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!
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) (we’ll 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 isn’t such a strict prohibition
on the numbers of neutrons, as we’ll see.
 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)
• Lead’s symbol, Pb, is based on the element’s original Latin
name plumbum, also the source of the word “plumber.” (Chemistry in
the Community 4th Ed., American Chemical Society, p. 54)
 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;
• uranium is
92U.
and
– 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.
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.
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).
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)
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: isotoPesame number of Protons
• For example, the following are all isotopes of hydrogen, because
each has one proton (Z=1).
– 1H
– 1H
– 1H
(H - hydrogen)
(D - deuterium)
(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 can’t be separated by chemical means.
• Isotopes of an element, if radioactive, will have different
radioactive properties (remember Soddy?)