When discussing nuclear radiation, we are only concerned with the
very small nucleus that consists of positive protons and neutral
neutrons. There are no electrons in the nucleus and electrons do not
make an atom radioactive.
http://commons.wikimedia.org/wiki/File:Atom-struc.svg
These are all isotopes of hydrogen. Hydrogen-1 has one proton and
no neutrons. H-2 still has one proton, but also has one neutron. It is
heavier (more massive) than H-1. H-3 has one proton and two
neutrons and has 3 times the mass of H-1. H-1 is very common, H-2 is
uncommon but not unusual, H-3 is quite rare. A common student
misconception is that H-1 is not considered an isotope. They think
only H-2 and H-3 are isotopes of hydrogen and that H-1 is the
“standard”. In fact, all three are isotopes of each other. Chemical
tests can’t differentiate between isotopes, but mass spectrometers
are sensitive instruments that can separate isotopes by mass. Why
important? For example, nuclear reactors need Uranium-235 but the
most common form of Uranium is U-238. A mass spectrometer could
be used to separate these isotopes and produce enriched Uranium.
Carbon has six protons so C-14 has (14 – 6 = 8) eight neutrons.
The ratio of neutrons to protons determines if an atom is radioactive.
Larger atoms are stable when they have more neutrons than protons
because the strong and weak nuclear forces (the attractive forces
that act between all nucleons) overcomes the electrostatic repulsion
of the positive protons. Too many neutrons, however, and the
nucleus will decay. Bigger atoms usually decay by getting rid of 2
protons and 2 neutrons (a Helium nucleus). Smaller atoms (having a
smaller volume nucleus) are stable when the number of protons is
about the same as the number of neutrons. Too many neutrons and a
smaller atom decays by changing a neutron into a proton and
emitting an electron from the nucleus.
http://commons.wikimedia.org/wiki/File:Radiation_warning_symbol.
svg
Have students share answers to the Summarizing Questions part of
Radioactivity -1. These are not in their guided notes because they are
on the simulation handout.
Answers 1: The number of neutrons determines the mass number,
which is part of the name of an isotope. The number of neutrons,
compared to the number of protons, determines whether a nucleus is
stable or radioactive.
Answer 2: An isotope is an atom of an element that has the same
number of protons as all other atoms of that element, but has
different numbers of neutrons as some atoms of that element.
Answer 3: No
A quantum is a bundle (often a bundle of energy). When something is
emitted as quanta, that means it comes out as a chunk. Electrons,
protons, and neutrons are also quanta. All students need to know is
that each time a particle comes out of the nucleus of the atom it
makes a single click on the Geiger counter. The terms quantum and
quanta are here only as an introduction to the terms.
http://commons.wikimedia.org/wiki/File:Geiger_counter_2.jpg
One of the high energy particles that come out of the nucleus is the
alpha particle. It is the most massive of the ejected particles and is
actually a Helium nucleus (2 protons and 2 neutrons). Because it is so
massive, it can actually be stopped by a piece of paper! Alpha
radiation is most dangerous when a radioactive material (like Radon
gas) is breathed into the lungs. There the alpha particles that are
emitted as Radon turns into Polonium can harm the tissue in the
lungs.
When an alpha particle is ejected from the nucleus, the mass number
of the original nucleus decreases by 4. The number of protons and
the number of neutrons each goes down by 2. A new element is
formed. Nuclear reactions release HUGE amounts of energy (think
atomic bombs and fusion in the sun) because the nuclear forces
(strong and weak) are very strong. Energy is released when these
bonds are broken. The energy comes out as kinetic energy of the
alpha particle.
The amount of chemical energy typically released (or converted) in a
chemical explosion is: 5 kJ for each gram of TNT
The amount of nuclear energy typically released by an atomic bomb
is: 100,000,000 kJ for each gram of uranium or plutonium
http://commons.wikimedia.org/wiki/File:Disintegration_(PSF).png
http://commons.wikimedia.org/wiki/File:Disintegration_(PSF).png
In the diagram, the electron is the beta particle. It does not come out
of the atomic orbitals, but comes out of the nucleus. Beta particles
can go through paper, but can be stopped with Aluminum foil or a
few cm of flesh. They have so much kinetic energy that they will burn
the skin or tissues that they strike.
Carbon-14 decays by emitting a beta. One neutron becomes a proton
with a resulting atom of nitrogen-14 with 7 p and 7 n. The electron is
emitted from the nucleus with a very high kinetic energy.
http://commons.wikimedia.org/wiki/File:Beta-minus_Decay.svg
Gamma rays, gamma particles, gamma decay – all the same thing.
Gamma rays are the highest energy wave on the electromagnetic
spectrum. They are thought of as particles (wave/particle duality of
quantum physics) because they can be counted as they are emitted
from the nucleus. When a gamma ray is emitted, there is a decrease
in nuclear vibration (the nucleus “settles down”, but the number of
protons and neutrons in the nucleus do not change. With an energy
higher than x-rays, gamma rays are very dangerous to cells in the
human body, and are usually emitted along with alpha and beta
particles. It takes a block of lead several feet thick to stop a gamma
ray.
The questions are animated – press enter to show the next question.
These questions are not on the guided notes but are on the handout
for Simulation 2.
Answer 1: No, they are the radiation that comes off a radioactive
material, but are not radioactive themselves. If they land on you, you
do not become radioactive, however, they can do damage to your
cells because they hit you with so much kinetic energy.
Answer 2: Need to be careful here with answers – if the alpha, beta,
gamma particles do not make things radioactive, but if you rubbed
radioactive Uranium on you, some of the U might get on your skin
and it will be radioactive. If you drink radioactive Barium, you become
radioactive because the Barium is in you.
Answer 3: Maria is correct. Atoms are radioactive. They only decay
once, then become a different element (which might also decay), but
they are radioactive before they decay. Radioactive materials keep
emitting particles because there are so many atoms in the materials
(might be a good time to mention half-life).
Use as many of these slides as needed for practice.
1a. U-235 has 92 protons
1b. U-235 has 143 neutrons (235 – 92 = 143)
1c. Thorium-231 is left. (p: 92 – 2 = 90 so Thorium, n: 143 – 2 = 141
OR 231 – 90 = 141)
Use as many of these slides as needed for practice.
2. Thorium-231 is left! Gamma radiation does not change the number
of protons or neutrons in the nucleus.
Use as many of these slides as needed for practice.
3. Radium-227 (227Ra) (p: 90 – 2 = 88 so Radium, n: 141 – 2 = 139 OR
227 – 88 = 139)
Radium decays by alpha to Radon-223. The Radon in our basements
comes from Uranium in the ground.
Use as many of these slides as needed for practice.
4a. Nitrogen has 7 protons
4b. N-16 has 9 neutrons (16 – 7 = 9)
4c. The new atom is Oxygen-16 (p: 7 + 1 = 8 so Oxygen, n: 9 – 1 = 8).
O-8 is stable. Notice that mass number did not change.