BASIC ATOMIC THEORY -- THE ATOM, ITS STRUCTURE, AND ITS PROPERTIES
The Universe consists of energy. Some of that energy has condensed to form matter, the material that we can feel or touch or see, the material that gives the universe its form. But, what is matter? How is it different from say light or sound, other forms of energy? What allows matter to combine in so many amazing creations from the beautiful form of a crystal to the incredibly complex, but reproducible forms of life? These are questions that have been asked for centuries.
Historically, as is common in scientific thought, the quest for understanding matter begins with the Greek scholar/philosophers. Many of the early ideas of science grew from thought experiments in which a question would be asked and possible answers considered. For example, what happens if you take a crystal of salt and divide it in half and then divide it again and again and again? Is there a limit to the number of divisions you could make before it was no longer salt? Or, to phrase it slightly differently, is there a smallest piece of matter that retains the quality of the matter that we can see, touch, or taste? Thinking about this in the fifth century BC, Democritus decided that there must be some smallest particle of matter that was still matter -- a limit to the process of division. He called this smallest piece an atom. Of course he had no proof that atoms existed. He had no way to keep breaking the salt crystal until there was a single atom, because the single atom of salt was much too small. Still, the idea did seem reasonable.
A more scientific proof of atoms was first provided by the early 19th century meteorologist, John Dalton. Dalton was a very predictable person in that everyday for years he followed the same pattern of going for morning walks and recording weather conditions. He expected to find patterns that would let him better understand and predict the weather.
His interest in the atmosphere led him to study the behavior of gases. By 1800, several kinds of gases had been recognized, and that the atmosphere was composed of mostly nitrogen and oxygen had been determined. Hydrogen had been separated from oxygen in water, and numerous oxides had been decomposed to produce oxygen and iron, or mercury, or lead. Methane or swamp gas was regularly exploded for excitement at popular science lectures and carbon dioxide had been discovered and used to make the bubbly and delightfully refreshing concoction -- soda water.
In addition, careful experimentation had discovered that in reaction, volumes of gases always reacted in whole number ratios. For example if you decomposed water you always got twice the volume of hydrogen compared to the volume of oxygen. Or, if you burned swamp gas, 1 liter of swamp gas combined with 2 liters of oxygen to produce 2 liters of water vapor and 1 liter of carbon dioxide.
(1 l. methane + 2 l. oxygen --> 2 l. water + 1 l. carbon dioxide.)
This may not sound exciting, but it is a rather amazing pattern. Why 1:2 or 1:1 and not 1: 2.234 or any other random ratio. This is the problem that Dalton wrestled. (It is interesting to note that the pattern might not have been so apparent if the 18th century scientist had better technology. Today we know that real gases do not behave as was thought. Volume ratios really aren't exactly 1 to 1 or 1 to 2 but more like 1 to 1.001. Had Dalton and his fellow scientist known this, it would have made the pattern and its underlying cause much harder to recognize. Some times more really isn't better.)
Dalton solved the problem by realizing that the pattern could result from gases being made of small particles that all take up the same amount space. He reasoned that these small particles were the atoms that Democritus had first proposed 2300 years earlier. Dalton's idea then was that when water is decomposed, it produces 2 atoms of hydrogen for every one atom of oxygen or in terms of volume 2 liters of hydrogen to 1 of oxygen. When methane burns, Dalton thought 1 atom of methane reacted with 2 atoms of oxygen to produce 2 atoms of water and 1 of carbon dioxide.
(1 methane + 2 oxygen --> 2 water + 1 carbon dioxide.)
This idea of the atomic particle made so much sense it was quickly accepted. Not because atoms had been proved to exist, but because thisvery sensible idea made sense proved to be useful. It was able to accurately describe and predict behavior of gases and soon other kinds of matter as well. Dalton's model of gas atoms worked which is always the test of a scientific theory. As proposed, Dalton's atomic theory needed to be refined, but it was useful and so was accepted.
One of the first refinements was to recognize that many of the atoms defined by Dalton were in fact combinations of smaller particles. These molecules, the smallest piece of the matter in question, are made of atoms. Most molecules, like a molecule of water are a combination of two or more different kinds of matter. If you subject pure water to the proper conditions, you can cause it to divide or decompose to form hydrogen molecules and oxygen molecules (H2 and O2). However, no matter what you did to the hydrogen or the oxygen, there was no way to break either of those gases into other kinds of matter. As a result, it was possible to classify materials as either compounds, matter like water that could be decomposed into simpler constituents, and elements, matter that seemed to contain only one kind of atom in its molecules. The idea that gases consist of molecules not single atoms is based on the early 19th century work of an Italian school teacher named Avogadro and represents the first significant contribution to scientific thought by an Italian after the trial of Galileo. Unfortunately, bias is as common in science as in any other field and, at least in part because he was Italian, Avogadro's ideas were largely ignored for nearly a century.
The second refinement has to do with recognizing that even the atom is not the smallest part of matter. There are sub-atomic particles, only a few, and that these are arranged in different ways to make the atoms of the elements which are then arranged in different ways to make the molecules of compounds. By the late 19th century, atoms were thought to be composed of two kinds of smaller particles called protons and electrons. Protons hold a positive electric charge. Electrons have a negative charge. And, because like charges repel, it was felt that an atom must be like a bread with nuts (protons) and raisins (electrons) more or less evenly spaced through it. The discovery and investigation of radioactivity forced chemists and physicists to change these ideas. First, radioactive elements weren't stable. The supposedly evenly spaced electrons and protons of the atom were falling apart with either small negatively charged particles or larger positively charged particles seeming to fly out of the atom at high speed.
Uranium was the first of these radioactive atoms to be discovered
and
the first to begin to yield the secrets of the atom to explorers like
the
Curies in France and Rutherford in England. It is Rutherford's
experiments
that are most germane to this course. He made two spectacular
discoveries.
First, he measured uranium's rate of decay. The uranium atom decays
spontaneously
to form a different element, thorium by emitting a high speed,
positively
charged particle called the Alpha particle. Rutherford decided to try
to
discover how rapidly the decay occurred and whether it was a regular
event.
To do this he placed a small amount of uranium in darkened room and
placed
a glass coated with a substance that glowed when struck by an alpha
particle.
He
also placed himself in the room and counted the number of times that
the
coated glass (or alpha particle detector) glowed in a certain amount
time.
His experiment showed that twice as much uranium gave off twice the
alpha
particles. Perhaps not too surprising, but important because it
demonstrates
a certain regularity to the process.
However, the Rutherford was not satisfied with his result, and he asked a second question. If, for example, 0.1 gram (g) of uranium emitted 10 million alpha particles in an hour, how long would it take until one half of the uranium had decayed? Because he knew how many uranium atoms are in 0.1 g (approximately 100,000,000,000,000,000,000), it was possible to work this out by a relatively simple calculation. The answer came to 4.5 billion years. The same result applies to the time required for half of 0.05 g of uranium because it emits half as many alpha particles as 0.1 g.
These numbers are so large, it becomes hard to comprehend. Lets,
begin
again with an imaginary radioactive element --Abingtonium, atomic
symbol
Ab. An experiment similar to Rutherford's shows that if you begin with
1000 atoms of Ab, 
500
alpha particles are emitted in the first hour. If you start with
500 atoms of Ab, 250 alpha particles are emitted in the first hour of
observation,
and if you start with 250 atoms, 125 alpha particles are emitted in the
first hour. Or, if you start with 1000 atoms, after one hour you will
have
500 atoms of Ab, after 2 hours you will have 250 atoms and after 3
hours
you will have 125. How many atoms will there be after 4 hours? Compare
your answer with the graph showing results of the Ab experiment..
The key idea is that no matter how many atoms of a radioactive element you begin with, the time necessary for half of the atoms to decay is a constant. The constant varies for different elements, but for any one kind of radioactive atom, it always remains the same. This constant, known as the half-life of the element, is 4.5 billion years for uranium. It would be only 1 hour for Abingtonium if it existed.
By asking and answering this question, Rutherford realized that he had discovered a kind of clock. If you could figure out how much uranium was in a rock when it formed and could measure how much uranium is there now, you could use the half-life of uranium to determine the rock's age. Rutherford tried to use this method to determine the age of rocks and by implication the age of the earth. Uranium when it decays eventually becomes lead. So Rutherford measured the lead in a rock and assumed that it all used to be uranium and measured the uranium that was still present. For example if the the number of atoms of lead and uranium are equal, 1 half-life had passed since the rock formed. The rock, then, is 4.5 billion years old. Unfortunately most rocks have lead that did not form by uranium decay and so most of Rutherford's ages were too old, but his idea was good. Several common minerals found in rocks allow small amounts of uranium into their crystal structure when they form, but no lead. By separating these minerals from the rock and then measuring the amount of uranium and lead in them today, it is possible to determine an age for the mineral and under some circumstances the age of the rock. The oldest earth rock to be dated by this method is 3.98 billion years old. Moon rocks and meteorites are usually about 0.5 billion years older (4.5 billion years).
Rutherford's second experiment redefined models of the atom. He "shot" alpha particles at a gold foil. Because the alpha particles are heavy and because they were moving rapidly, his expectation was that they would sail through the foil and its gold atoms much like "bullets passing though tissue paper". According to the raisin bread model, there should not be anything in the foil "solid" enough to slow down his alpha particle bullet. Expectations and reality clashed when Rutherford discovered a very few of the alpha particles bounced back. (Imagine shooting at tissue paper and having to duck out of the way because bullets bounce back at you.) It took Rutherford two years to accept the result because the positively charged alpha particle would bounce back only if it were hitting a massive, positively charged object in the gold atoms. Even though he had no way of explaining how it was possible, Rutherford finally accepted that atoms must have a massive, positively charged nucleus that holds all the protons and that the volume or size of the atom is created by a cloud of electrons. Because so few of the alpha particles bounced back, Rutherford determined that the nuclei of gold atoms are approximately 1,000,000,000,000 (one trillion) times smaller than the atom itself.
Following Rutherford's work other scientists have been able to
improve
our understanding of the atom. Their work led to the discovery of a
third
sub-atomic particle, the neutron, a neutral particle that occurs in the
nucleus with the protons. 
We
now think that a typical atom consists of a nucleus with protons and
neutrons
surrounded by electrons. The atom, itself, is about 1/100,000,000 of a
cm. in diameter. 100 million can fit end to end across your little
finger.
The nucleus is 10,000 times smaller in diameter. This means that the
size
of the atom must be derived from an electron cloud which creates a
negative
charge and repels all other atoms because they also have a negative
cloud.
However, although size is created by electrons, mass of the atom comes
from the nucleus. Protons and neutrons are more than a thousand times
heavier
than an electron. Imagine what would happen to matter if there were no
electrons -- only nuclei. A golf ball made only of nuclei would weigh
close
to 100 billion pounds.
The chemistry of the atom is determined by the number of protons in the nucleus. All hydrogen atoms have 1 proton; all carbon atoms have 6, and all uranium atoms have 92. Atoms of an element may have different numbers of neutrons in their nucleus. For example hydrogen may have nuclei with 0, 1 or 2 neutrons. Carbon may have 6, 7 or 8 neutrons, call C-12, C-13 and C-14 respectively. The number is the sum of particles in the nucleus, 6 + 6 in C-12 and 6 + 8 in C-14. Atoms of an element with different numbers of neutrons are called isotopes. Typically the neutrons act to make an atom either more stable or less stable in terms of radioactivity. Hydrogen with 1 or 2 neutrons and carbon with 8 neutrons are radioactive. All isotopes of uranium are radioactive, but they have different half-lives. The most common uranium with 146 neutrons is the most stable form.
This view of the atom is important to this course because we will use isotopes of carbon and uranium to explore the age of events and the isotopes of carbon and oxygen to better understand climatic changes in Earth history. With this in mind, let us look at the carbon-14 system in more detail. Carbon-14 or C-14 is a radioactive isotope of carbon. It is formed when nitrogen in the atmosphere is struck by a cosmic ray causing one of the seven protons in the nitrogen nucleus to change into a neutron.
(Proton ---> Positron (A "positive electron") + Neutron)
Common nitrogen has 7 neutrons and 7 protons in its nucleus, by changing one of the protons to a neutron, the atom takes on the chemistry of carbon because it now has 6 protons. It is called C-14 because it has a total of 14 particles in its nucleus, 6 protons and 8 neutrons.
C-14 is unstable. In 5730 years, half of all the C-14 on Earth decays. (A neutron emits an electron and becomes a proton.) Therefore, the concentration of C-14 in the atmosphere will be controlled by the rate of production balanced by the rate of radioactive decay. So long as the amount produced remains constant, the amount in the atmosphere should also remain constant.
Because C-14 exists in the atmosphere, it is gradually incorporated into living organisms through photosynthesis. Plants take in atmospheric carbon dioxide and convert it to sugar and other molecules. Animals eat the plants, and the C-14 of the plants is incorporated in the tissues of the animal. So long as the organism is alive, it is mixing the carbon of its body with the carbon of the atmosphere and so should have the same concentration of C-14 relative to normal carbon (C-12) as the atmosphere. But when the organism dies, the mixing ends. A dead plant does not take in carbon dioxide. Isolated from the atmospheric reservoir of carbon dioxide, the amount of C-14 in the organism begins to decrease according to its half-life. After 5730 years, it should have just half the atmospheric concentration of C-14 and after 11460 years a quarter, etc. It is possible, therefore, to use the C-14 content of a tree limb or a human bone to determine when the tree or the person died.
Reading Question: How old is a tree limb that has 0.25 the normal
(current) concentration
of C-14? Refer to the graph labeled "Half-lives".