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ATOMIC REFRESHER
Once again, an atom consists of an extremely small, positively
charged nucleus surrounded by a cloud of negatively charged
electrons. Although typically the nucleus is less than 1/10,000th
the size of the atom, the nucleus contains more that 99.9%
of the mass of the atom. Nuclei consist of positively charged
protons and electrically neutral neutrons held together by
the so-called strong or nuclear force. This force is much
stronger than the familiar electrostatic force that binds
the electron to the proton in the nucleus, but its range is
limited to the diameter of the nucleus.
The number of protons in the nucleus, Z, is called the atomic
number. This determines what chemical element the atom is.
The number of neutrons in the nucleus is denoted by N. The
atomic mass of the nucleus, A, is equal to Z + N. A given
element can have many different isotopes, which differ from
one another by the number of neutrons contained in the nuclei.
In a neutral atom, the number of electrons orbiting the nucleus
equals the number of protons in the nucleus. Since the electric
charges of the proton and the electron are +1 and -1 respectively
(in units of the proton charge), the net charge of the atom
is zero. At present, there are 112 known elements which range
from the lightest, hydrogen, to the recently discovered and
yet to-be-named element 112. All of the elements heavier than
uranium are man made. Among the elements are approximately
270 stable isotopes, and more than 2000 unstable isotopes.
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In 1896, Henri Becquerel was working with compounds containing
the element uranium. To his surprise, he found that photographic
plates covered to keep out light became fogged, or partially exposed,
when these uranium compounds were anywhere near the plates.
This fogging suggested that some kind of ray had passed through
the plate coverings. Several materials other than uranium were also
found to emit these penetrating rays. Materials that emit this kind
of radiation are said to be radioactive and to undergo radioactive
decay. In 1899, Ernest Rutherford discovered that uranium compounds
produce three different kinds of radiation. He separated the
radiations according to their penetrating abilities and named them
alpha, beta, and gamma radiation, after the first three letters
of the Greek alphabet. The alpha radiation can be stopped by a sheet
of paper. Rutherford later showed that an alpha particle is the
nucleus of a He atom. Beta particles were later identified as high
speed electrons. Six millimeters of aluminum are needed to stop
most bata particles. Several millimeters of lead are needed to stop
gamma rays, which proved to be high energy photons.
ALPHA DECAY.
Since alpha particles contain 2 protons and 2 neutrons, they
must come from the nucleus of an atom. A change in nuclear
charge means that the element has been changed into a different
element. Only through such radioactive decays or nuclear reactions
can transmutation actually occur. The mass number
of an alpha particle is 4, so the mass number of the decaying
nucleus is reduced by 4. The atomic number of an alpha
is 2, so the number of protons, is reduced by 2. |
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BETA DECAY
If there is an excess of neutrons in a nucleus, the isolated
neutron can become unstable and decay into a proton (+) and
en electron (-) (this is why the neutron is a little heavier
than a proton). Since the atom essentially gains a proton
it transmutes into the next element in the periodic sequence. |
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GAMMA DECAY
Gamma rays are a type of electromagnetic radiation that results
from a redistribution of electric charge within a nucleus.
For complex nuclei there are many different possible ways
in which the neutrons and protons can be arranged within the
nucleus. Gamma rays can be emitted when a nucleus undergoes
a transition from one such configuration to another. For example,
this can occur when the shape of the nucleus undergoes a change.
Neither the mass number nor the atomic number is changed. |
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A proton or neutron can also be ejected. If a proton is ejected
then the atom transmutes into the next element down the periodic
table. An ejected neutron will simply just change the overall mass
of the atom, reducing it by one. The atom remains the same type
only a different isotope.
½-LIFE
Starting with a pure sample of a specific radioactive isotope,
the amount of time that elapses for half of the sample to decay
into other daughter elements is defined as ½-life.
Notice that after 6 hours half has decayed (32 grams) and after
another 6 hours half of that decays (16 grams), NOT the entire
sample. It's like flipping 100 pennies and expecting about half
to be heads. If you weeded out the tails and flipped the remaining
heads you would not expect all 50 to be heads again but only about
half of what remained.
| 0 hours |
6 |
12 |
18 |
24 |
30 |
36 |
42 hours |
| 64 grams |
32 |
16 |
8 |
4 |
2 |
1 |
.5 grams |
You could also predict ½-life by charting how much decayed
in a specific time period.
| 0 days |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
| 1000 atoms |
900 |
810 |
729 |
656 |
591 |
531 |
478 |
431 |
387 |
The trend above indicates that about 10% of the remaining sample
decays each day. If you started with 1000 atoms then the half life
would occur when half (500 atoms) remained. Notice this occurs between
day 6 and day 7. You could plot this data precisely on a graph and
narrow your answer down even farther. A good guess would be about
6.6 days (as it is closer to day 7). The actual equation
for determining half life is t½ = .693 / ln
(m1/m2). m1
is the starting mass and m2 is the final mass,
t½ will be in the same units as the time from
m1 to m2 (days, seconds,
years...). .693 is approximately ln 2.
FISSION
When heavy atoms like Uranium are split by incoming neutrons
they can divide and release more stray neutrons. If the Uranium
atoms are packed together tightly, these stray neutrons can hit
other Uranium atoms and start a chain reaction releasing
massive energy. If the reaction is controlled carefully you
have a nuclear power plant. If uncontrolled it can
erupt with explosive force (atomic bomb).
FUSION
When lighter elements are tightly compressed and fuse together
a tremendous amount of energy is also released. This process is
called nuclear fusion. This process occurs naturally in the sun
where the tremendous mass supplies the immense gravitational forces
to squeeze hydrogen isotopes together to form helium and energy.
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