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Chapter 8
The Nucleus
More than 2000 years ago, ancient Greek philosophers pondered the nature
of matter. They noted that some materials seemed to be mixtures of more
elementary matter and some materials seemed to be pure substances.
Salt seems to disappear when mixed with water, but when saltwater is heated
the water seems to disappear and crystals of salt remain, so saltwater is not
an elementary material, but is a mixture.
Gold, on the other hand, comes from a mineral mixture called an ore. When
the ore is heated, liquid gold comes out while the rocky material remains
more or less the same.
Gold can be heated and solidified over and over without changing in any
way.
Gold seems to be an elementary material.
Some Greek philosophers wondered how many times a piece of gold could
be cut and still be gold.
In other words, if a piece of gold is cut in two, the two pieces obviously are
gold, but when those pieces are cut, would there eventually be a piece that
would be the smallest piece possible, and would therefore be uncuttable?
Some philosophers suspected that there was in fact a smallest particle for
each kind of pure matter and called it an atom, (not cuttable).
The saltwater solution could be explained by the particle theory of matter, as
could the evaporation of perfume.
The alternative theory was the continuous theory of matter.
The atomic theory of matter was the result of philosophical speculation,
and it was not until the 1840’s that chemists began to use the atomic theory
to explain why materials combined in definite proportions instead of random
proportions.
When hydrogen and oxygen are mixed, for example, only a certain number
of grams of hydrogen will combine with any particular amount of oxygen to
produce water.
It was not until the 1980’s that atoms could be seen with microscopes.
Scientist do not need to see something to believe it, they need to see an idea
work, to explain something, like how materials react with each other.
By 1911 the atomic theory was widely accepted and several things were
known about atoms.
1. They are very small.
2. They are electrically neutral.
3. They contain electrons.
The electron had been “discovered” by an English scientist named J. J.
Thomson in 1897.
Since Thomson knew the atom is neutral, he speculated that the atom must
contain positive matter of some sort as well as negative electrons and
suggested that the electrons are imbedded in the positive matter in a mixture
resembling a “plum pudding” or fruit cake, as we would call it.
Figure 7-1
Rutherford model of the atom
The most direct way to probe the structure of a fruitcake would be to stick a
finger in it as a probe.
In 1911 a New Zealand scientist named Ernest Rutherford proposed an
experiment to probe the structure of an atom.
A few years earlier it was discovered that some minerals emit a stream of
very small particles called alpha particles. These particles are about 8000
times more massive than electrons, but still less massive than most atoms.
Rutherford believed that he could send these particles into atoms to test their
density and internal structure.
The Rutherford Experiment-Explain
Figure 7-2
Figure 7-3
Analogy with star entering the solar system (atom is relatively more
empty)
Nuclear structure
Eventually it was discovered that the positive matter in the nucleus consists
of particles, called protons.
Protons have exactly the same electric charge as electrons, but a mass that is
1836 times more than that of an electron.
In time it was discovered that the nucleus also contains neutral particles
called neutrons.
These have slightly more mass than protons: 1839 times that of the electron.
Each elementary material, called an element, is composed of atoms whose
nuclei all contain exactly the same number of protons.
The number of protons is called the atomic number of the element.
Figure 7-4
The number of protons determines the number of electrons.
The number of electrons determines most of the properties of an element.
There are over 100 different atomic numbers, thus over 100 elements.
At room temperature and normal air pressure, 11 elements are gases, 2 are
liquids and the rest are solids.
Isotopes
All atoms of an element have the same number of protons, but not
necessarily the same number of neutrons.
Atoms of the same element with different numbers of neutrons are called
isotopes of the element.
Example: hydrogen, deuterium, tritium
Figure 7-5
A nucleus with a particular composition is called a nuclide.
Each nuclide is represented by a symbol:
A
X = atomic symbol Z = atomic number A = mass number
ZX
Mass number is the number of nucleons in the nucleus.
A common nuclide of chlorine is 3517Cl.
This same nuclide could also be called Cl-35.
Radioactivity
In 1896 Henri Becquerel was experimenting with the element uranium.
He put some uranium ore in a desk drawer along with some photographic
film.
The film was wrapped in paper to keep it from being exposed, but when
Becquerel opened the paper, he found that the film was exposed anyway.
He wondered if the uranium had anything to do with it, and eventually
discovered that uranium gives off some kind of invisible radiation that can
penetrate paper, expose film, ionize gases and cause zinc sulfide to glow in
the dark.
The property of uranium which allowed it to do these things was called
radioactivity. This term was invented by a scientist named Marie Curie.
Marie Curie and her husband Pierre Curie worked in the same laboratory as
Becquerel.
They worked on an ore called pitchblende and eventually discovered 2 new
elements in the ore.
They named one of the elements polonium, after Marie’s native country,
Poland.
The second element was called radium, because it was extremely
radioactive.
Scientists discovered that the rate of radiation was unaffected by chemical
reactions, heating or cooling.
It was apparent that the radioactivity must be coming from something deep
inside the atom that is not affected by external conditions.
Radioactivity is a property of the nucleus.
Some nuclides are inherently unstable and have a tendency to rearrange
themselves in some way. This process leads to the release of energy or
particles that is the basis of radioactivity.
There are about 7000 theoretical combinations of nucleons or 7000 possible
nuclides.
About 2000 nuclides have actually been found or created in the lab.
Only about 256 nuclides are stable and not radioactive.
Even so, most elements in nature do not have radioactive isotopes.
Some elements have some radioactive and some non-radioactive.
And some elements with large atomic numbers such as uranium have all
radioactive isotopes.
Scientists can artificially produce radioactive isotopes of certain elements
which are not usually radioactive. These isotopes can be placed in a
person’s body where they will behave like non-radioactive atoms, but their
radioactivity will allow doctors to follow their paths in the body.
These isotopes are called tracers.
Figure 7-6
Radioactive decay
Early experimenters found that a magnetic field can separate the radiation
from highly radioactive elements such as radium into 3 components.
Figure 7-7
The radiation that behaves as a positive particle would behave is called
alpha radiation, which is composed of a stream of alpha particles.
We now know that these particles are composed of 2 protons and 2 neutrons,
just like a nuclide of Helium. (Sometimes alpha particles are called helium
nuclei, but in fact alpha particles usually do not come from a sample of
helium at all.)
The radiation that behaves as a negative particle would behave is called beta
radiation, which is composed of a stream of beta particles.
Beta particles seem to be identical to electrons, but these particles come
from the nucleus, not the electron cloud.
The radiation that is unaffected by a magnetic field is obviously uncharged.
However, this radiation does not consist of particles, not even neutrons.
This radiation consists of high-frequency EM radiation, called gamma
radiation.
Why decays occur
A nuclide that is radioactive is said to undergo a decay, because it changes
in some way, and releases radiation in the process of change.
Figure 7-8
Gamma decay: nucleus has excess potential energy
Alpha decay: nucleus is too large
Beta decay: nucleus has too many neutrons relative to protons
Electron capture: nucleus has too many protons relative to neutrons
Positron emission: nucleus has too many protons relative to neutrons
Neutrons outside the nucleus are unstable. They decay into a proton and an
electron.
However, a neutron is not a combination of a proton and an electron. It is a
unique particle, and it cannot be formed by trying to combine the proton and
electron. A hydrogen atom would be the result of a forced combination.
Apparently, a proton is stable, even outside the nucleus.
When a decay occurs, we can usually predict the changes that occur in the
original nuclide.
Example 7.1
Example 7.2
Half-life
Unstable nuclides are called radionuclides.
Each radionuclide decays at a characteristic rate.
The rate is indicated by a unit called half-life, the time needed for one half
of the atoms in a sample to decay.
Figure 7-10
The half-life is based on the structure and composition of the nucleus and is
not affected by external events.
Some half-lives are as short as a millionth of a second and others are billions
of years long.
One of the biggest problems related to nuclear power is the problem of
waste disposal, since the waste is still radioactive, even though it can no
longer boil water.
Radiometric dating
The slow steady decay of radionuclides forms the basis of a technique for
measuring the age of materials.
Radiometric dating uses information about the rate of decay of
radionuclides.
By measuring the concentration of the products of radioactive decay and
comparing these products to the concentration of the original radioactive
material, we can calculate how long a radioactive substance has been
decaying.
(Much more will be discussed in Earth Science.)
Radiation hazards
All radiation from radioactive materials is powerful enough to ionize the
matter it passes through.
Radiation dosage is measured in sieverts, or millisieverts, where 1 Sv is the
amount of any radiation that has the same biological effects as those
produced when 1 kg of body tissue absorbs 1 joule of x-rays or gamma rays.
A similar unit is called a rem, or millirem. This unit is somewhat
outmoded, but is still used.
All persons are exposed to radiation.
Figure 7-11
The average dose is about 3.6 mSv/y.
The dosage varies with location. In places where there is a lot of granite the
dosage is higher because granite often contains uranium.
At high altitudes, people are more likely to be irradiated by cosmic
radiation. In fact, astronauts may receive as much as 1 mSv per day!
X-rays are an important part of the radiation dosage of human beings.
The necessity of x-rays must always be balance against their risk of harm.
This is especially true of the concentrated form of x-rays called CT scans,
(computed tomography), which are equivalent in dosage to hundreds of
ordinary chest x-rays.