Why were none of the bodies discovered on a lot in
Noble, Georgia those of people who were reported
missing? They were bodies of people who had died
under circumstances that were not suspicious and had
been sent to a crematorium to be cremated.
Urns containing the cremains of the deceased are
usually given to the families. The owner of the
crematorium had in fact given out urns. What was the
problem with these urns in light of the discovery of
the bodies in the lot near the crematorium? It was
unclear whether the material in the urns really were
the cremains of the deceased or if they were some other
filler.
There is no DNA left intact after cremation so how
was Dr. Al Hazari going to determine whether the urns
contained cremains or something else? He needed to
identify the elements present using knowledge of atomic
structure to find out whether it was consistent with
real cremains or not.
Section 3.1 Origins of the Atomic Theory: Ancient
Greek Philosophers
In 440 B.C. in ancient Greece Leucippus and
Democritus claimed that matter was made of atoms. What
were the attributes of these atoms that they
hypothesized about? Atoms according to these
ancient Greeks were small, hard, indivisible particles.
In combinations they make up all material things and
come in various sizes, shapes, and weights. The
properties of matter depended on the types of atoms
they were made up of.
This hypothesis about atoms was in conflict with
another philosopher’s ideas about matter. What
did Aristotle think was true about matter?
Aristotle believed that matter was
continuous and that matter could be
divided without limit.
What is a good way to compare these two hypotheses?
A beach looks like a solid and continuous piece of
matter from a long distance away but up close it is
clear it is made up of innumerable grains of sand.
Democritus believed that all matter was like this if
looked at closely enough. Aristotle on the other hand
thought that matter could be cut up into smaller and
smaller pieces forever without ever finding a piece
that could not be cut any more.
Did either philosophical hypothesis have any
evidence for it at that time? What was it? Atoms
are too small to see and Democritus could assemble no
evidence. If anyone had considered evidence important
at the time (which they did not, particulary) then the
evidence was on Aristotle’s side. Our experience
shows matter to be continuous. Liquid water seems to
have no “smallest possible drop” and it
seems possible to cut a piece of metal in half as many
times as you like.
Section 3.2 Foundations of a Modern Atomic
Theory
In 1785 someone finally collected the first
necessary piece of evidence for the atomic theory of
matter. Antoine Lavoisier performed experiments in
which he carefully weighed all materials (including
gases) before and after a chemical or physical change.
What is the Law of Conservation of
Matter which we now attribute to his work?
The law says that matter is neither created nor
destroyed in chemical and physical changes. For
example, when wood is burned the mass of the ashes is
much less than the original mass of the wood. This is
mysterious until the wood is burned in a closed system.
In that case the original mass of wood plus oxygen
equals the mass of the ashes plus the gases produced by
burning.
An essential advance required for the advancement
of chemistry was the careful application of a very
accurate balance to investigate chemical and physical
changes in closed systems.
Work through Worked Example 1 on pg. 59 about
producing water from hydrogen and oxygen gases.
In 1797 a man named Joseph Louis Proust performed
careful experiments investigating the elemental
composition of compounds. He found that the ratio of
the masses of the elements in a compound was always the
same for that compound. Prior investigations of this
kind had not been made to as high a level of precision
and led scientists to believe that elements could be in
any ratio and still make the same compound. This result
is now known as the Law of Definite
Proportions. How is this law critical to the
argument that matter is made of atoms? If matter is
made of atoms then compounds would be made of
combinations of atoms of different elements. Mass is
related to the number of atoms so when atoms combine in
simple whole-number ratios then the ratio of their
masses will always be the same.
Section 3.3 Dalton’s Atomic Theory
What is a scientific theory? A theory is a
hypothesis of a group of hypotheses which has been
validated by many experiments and observations. It can
be used to make predictions about future
observations.
What makes a scientific theory different from an
opinion (sometimes called a ‘theory’)?
Opinions can change and are dependent not just on
facts or on the consensus of a community but also on
personal beliefs. Theories, by contrast, are valid as
long as they are not contradicted by data or
observations. They do not depend on personal beliefs.
Scientists consider evolution of species by natural
selection to be a valid, that is, true theory
regardless of how many people don’t believe in
it.
What is John Dalton’s Atomic
Theory (published in 1803)?
All matter is made up of tiny, indivisible
particles called atoms.
Atoms cannot be created, destroyed, or
transformed into other atoms in a chemical
reaction.
All atoms of a given element are
identical.
Atoms combine in simple, whole-number ratios
to form compounds.
Quoted from Johll, pg. 60
How does Dalton’s atomic theory explain the
law of conservation of mass? Since matter is made
of atoms and atoms are not created or destroyed then no
atoms are lost or gained in chemical changes. Instead,
these changes are understood to be changes in how atoms
are bound together in compounds.
How does Dalton’s atomic theory explain the
law of definite proportions? Atoms combine in
simple whole-number ratios because it is not possible
to have fractions of atoms. For example, water has two
hydrogen atoms for every one oxygen atom
(H2O) and
carbon dioxide has one carbon atom for every two
oxygen atoms (CO2). The constant ratio
of atoms implies a constant ratio by mass since all
atoms of the same kind have the same mass.
Dalton made a prediction about the ratios in which
elements can combine. We call this prediction the
Law of Multiple Proportions. What does
it mean? It means that a pair of elements may
combine in more than one ratio of atoms and when they
do, these are different compounds. For example,
nitrogen and oxygen can combine in a variety of ways:
NO,
NO2, N2O, N2O3,
N2O4, and
N2O5. The
formula N2O
is laughing gas (nitrous oxide) and NO2 is nitrogen dioxide,
a component of smog.
Work through Worked Example 2 on pg. 61.
Section 3.4 Atomic Structure: Subatomic
Particles
Dalton’s atomic theory is not entirely
correct. Are atoms indivisible? If not, what parts make
them up? Atoms are divisible and they are made of
positively charged protons, neutral neutrons (both of
which are found in the nucleus), and negatively charged
electrons (found in orbit around the nucleus).
(Show a video clip of how a cathode-ray tube works.
It should clearly show the deflection of the beam due
to the influence of a magnet.)
What is a cathode-ray tube and what does it do?
It is a tube from which nearly all gases have been
removed. It contains two pieces of metal at opposite
ends of the tube: these are electrodes called the
cathode and the anode. When a large voltage is applied
to the electrodes a visible ray can be seen moving from
the cathode to the anode. The ray can be deflected in
one direction or another by magnetic and electric
fields.
When J.J. Thomson measured the mass-to-charge ratio
of the cathode ray particles in 1897 what did he
discover? He discovered that the particles that
make up the ray have a mass less than 1/1000 of the
mass of the lightest atom. This was the first
experimental evidence for particles smaller than atoms.
He deduced that the particles had a negative charge by
the direction they moved under the influence of
magnetic fields. It was ultimately identified it with
the already-named particle of electricity, the
electron.
Now that scientists knew that atoms could have
negative electrical charges removed from them they also
knew that atoms must also contain positive charges.
What were the two competing models for how this charge
was distributed in the atom? The so-called plum
pudding model held that atoms were balls of positive
charge with electrons buried inside like raisins in a
plum pudding. A more modern and American food that
communicates the same idea might be a chocolate-chip
cookie.
The other competing model was produced by Ernest
Rutherford and his co-workers in 1909. He held that
atoms had a very dense center of positive charge that
was much smaller than the atom as a whole. His model
is an atom that is mostly empty space with a small,
dense, positively charged nucleus.
How did Rutherford make this model? He used
alpha particles produced by the radioactive decay of
radium to make a beam of positively charged particles.
He directed his beam at a very thin gold foil. If the
plum pudding model were true, all of the alpha
particles should have passed through the gold with very
little resistance. Instead, he and his co-workers
observed that some alpha particles were deflected to
the side. Not very many, though, so they concluded that
the center of positive charge in the gold atoms must be
small since not many alpha particles were deflected. In
addition, some alpha particles were deflected almost
directly back toward the source. This could only happen
if the alpha particle made a direct hit on a very
dense, positively charged object.
See the Rutherford Scattering applet at
PHeT. Use the applet on an overhead projector to
talk about how the experiment works and how the
scientists came to their conclusions about the atom.
Note: See and discuss the observations and
conclusions on pp. 63-64 to discuss the gold foil
experiment in more detail.
The mass of the atom could not be accounted for by
adding up the masses of protons and electrons. What
accounts for the missing mass? Neutrons, discovered
in 1932 by James Chadwick by his correct interpretation
of nuclear experiments that were producing heavy
neutral particles.
What are the symbols, charges, and masses of the
subatomic particles?
Particle
Charge
Mass (amu)
Symbol
electron
–1
5.486 × 10-4
e–
proton
+1
1.0073
p+
neutron
0
1.0087
n0
Note: the text has p– for
the proton. Clearly this is a typo.
What are isotopes? Isotopes
are atoms of an element with different numbers of
neutrons. Not all atoms of an element are necessarily
identical. All atoms of an element have the same number
of protons but they can have varying numbers of
neutrons. Isotopes behave chemically in an identical
way but they have slightly different masses.
What unit is used for the masses of subatomic and
atomic particles? Atomic mass units, defined as
1/12 the mass of a carbon atom with 6 protons and 6
neutrons. Usually, the masses of protons and neutrons
are rounded to 1 amu and electrons are considered
massless. They do have mass but their masses are so
small compared to the particles in the nucleus (protons
and neutrons) that we ignore their mass. Protons and
neutrons are about 1838 times more massive.
Section 3.5 Nature’s Detectives:
Isotopes
What is the mass number of an isotope? It is
the sum of the number of protons and the number of
neutrons. Since these particles have a mass of about 1
amu each the mass number is the approximate mass of the
isotope in amu. Usually the actual, exact mass is a
little bit less than the sum of protons and neutrons.
Since it would take 1838 electrons to equal 1 amu they
are disregarded in finding the mass number. The atomic
mass number is symbolized by A.
What is the atomic number of an isotope? It is
the number of protons in the nucleus. It is given the
symbol Z.
How are symbols for isotopes written so as to show
both Z and A? How can they be abbreviated? The
symbol for an isotope is written with a
left-superscript of A and a left-subscript of Z like
this:AZX
How are the names for isotopes written? They
are written with the name of element followed by a dash
and the mass number of the isotope (A). An isotope of
carbon with a mass number of 12 is called carbon-12 and
its symbol is126C
. Carbon-13 is136C
and uranium-238 is23892U
.
Work through the worked examples 3 and 4 on pp.
67-68
The elements beryllium (Be) and fluorine (F) each have only one stable isotope
but most other elements have two or more stable
isotopes. All isotopes of an element are not equally
abundant but the proportion each contributes is
generally constant in natural samples. How are the
abundances of an element’s isotopes described?
The abundances are described using the idea of
percent abundance. Natural samples of Silicon atoms
consist of 92.23% silicon-28, 4.67% silicon-29, and
3.10% silicon-30. So if you have 10,000 silicon atoms
then 9,223 of them are silicon-28, 467 of them are
silicon-29, and 310 of them are silicon-30.
How can the abundances of isotopes be used as a
tool for investigations? Since the abundances of
isotopes is generally constant for natural samples they
can serve as a baseline for comparison. If a man-made
process creates a change in the abundance of isotopes
then this can be detected and used to re-create the
history of a sample.
Section 3.6 Atomic Mass: Isotopic Abundance and
the Periodic Table
How are the masses of elements determined? They
are determined using the process of the weighted
average. The masses of the isotopes of an element are
multiplied by their abundances and then added together.
The result is a mass which no single atom will ever be
found to have but which is the average mass for an atom
of that element. Since single atoms are rarely the
objects of investigators’ scrutiny and instead
scientists work with large collections of atoms, an
average mass is just fine for nearly all purposes.
Section 3.7 Mathematics of Isotopic Abundance and
Atomic Mass
Section 3.12 Case Study Finale: To Burn or Not to
Burn
How were the cremains identified as either actual human ashes or a filler? Dr. Al Hazari reasoned that a human body would be less than 1% silicon but a filler material would have a much higher silicon content. He used an instrument that generates very high temperatures to make emission spectra. These spectra can be analyzed to determine elemental composition. It turns out the cremains were not actually human ashes.
What motivated this strange crime? No one knows but perhaps mercury poisoning from exposure to the fumes from the crematorium caused neurological damage, impairing the decision-making ability of Mr. Ray Brent Marsh.