Download Atomic Structure

Atomic Structure 1
The Atom and Isotopes
http://commons.wikimedia.org/wiki/File:Stylised_Lithium_Atom.png
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Can We “See” Atoms?
http://www.aip.org/history/einstein/atoms.htm
Scanning Tunnelling Microscope Image of Silicon Atoms
(X 34,000,000)
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Can We “See” Atoms?
New Scientist
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Changing Models of the Atom
http://www.docstoc.com/docs/2197377/History-of-the-Atom-Timeline
TOK: If we can’t see these particles,
how do we know that atoms are really like this?
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A Grade 9 Version of the Atom
energy levels
proton
nucleus
electron
neutron
What is the name of this atom?
carbon-12
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C
What is incorrect about this representation?
many things - sizes, colours, spacing + ?
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Subatomic Particles
approximate
particle
location
charge
proton
nucleus
+1
1
neutron
nucleus
0
1
electron
energy levels
-1
1/2000
mass*
* in atomic mass units (amu)
1 amu = 1.67 x 10-24 g
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Terminology Related to Atomic Structure
Define: atomic number (Z)
= the number of protons in an atom
• unique for each element
mass number (A)
= the number of protons PLUS neutrons in an atom
• always a whole number
• NOT the atomic mass on the Periodic Table
Symbols:
A
Z
X
charge (ions only)
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Terminology Related to Atomic Structure
Types of “particles”
ATOMS
IONS
= particles with the same number
of protons (+) and electrons (-)
= charged particles
= formed when an atom loses or gains electrons
+ ions
(cations)
ISOTOPES
– ions
(anions)
... wait for a few slides ...
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Terminology Related to Atomic Structure
Deduce:
number of protons = atomic number (Z)
number of neutrons = mass number (A) - atomic number (Z)
number of electrons
for neutral atoms: # e = number of protons
for cations (+ ions): # e = number of protons - charge
for anions (- ions): # e = number of protons + charge
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Terminology Related to Atomic Structure
Give the number of protons, neutrons and electrons in:
32
P
p = 15
n = 32 - 15 = 17
e = 15
I
p = 53
n = 131- 53 = 78
e = 53
131
18 O 2-
p=8
27 Al 3+
p = 13
carbon-14
p=6
n = 18 - 8 = 10
e = 8 + 2 = 10
n = 27 - 13 = 14
n = 14 - 6 = 8
e = 13 - 3 = 10
e=6
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Isotopes
Define:
Isotopes are atoms of the same element with
different numbers of neutrons.
Isotopes are atoms with the same atomic number but
different mass numbers.
http://education.jlab.org/glossary/isotope.html
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1
H
2
1
H
3
1
H
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Isotopes - Properties
chemical properties
• properties related to how a chemical reacts with
other chemicals
• examples = flammability, reactivity, pH
• isotopes of an element have the SAME chemical
properties
example - use of iodine supplements
The thyroid gland regulates your metabolism using a hormone called thyroxin
which contains iodine.
Radioactive iodine isotopes are waste products from the decay of uranium. If
you ingest radioactive iodine, it accumulates in your thyroid gland when cells
there produce and store thyroxine. This is an example of how both the
radioactive and non-radioactive isotopes of iodine have the same chemical
properties - they react in the same way. Over time, the build up of thyroxine
with radioactive iodine atoms can lead to thyroid cancer.
In order to prevent the accumulation of radioactive iodine in the thyroid, people
may take iodine supplements which contain non-radioactive iodine. If there is
more non-radioactive iodine available, then more of it will be used to make
thyroxin.
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Isotopes - Properties
physical properties
• properties related to how a chemical looks, feels,
tastes
• examples = boiling and melting point, density, solubility
• isotopes of an element have DIFFERENT physical
properties
example - uranium enrichment
These are fuel rods in a nuclear reactor and they contain radioactive uranium-235.
However, natural uranium contains more than 99% non-radioactive 238U and only
0.71% radioactive 235U. Uranium-235 is needed for the fission reactions that generate
energy in nuclear power plants.
In order to increase the amount of 235U, a process of isotope separation is used.
Chemicals with different physical properties can be separated from each other. One
physical property that can be used to separate the isotopes of an element is density.
A process called centrifugation can be used to separate chemicals with different
densities. Uranium enrichment processes take advantage of the different densities of
235U and 238U to increase the amount of radioactive 235U in a sample - up to 3-4% in
reactor grade uranium and up to 90% in weapons-grade uranium.
http://www.world-nuclear.org/info/inf28.html
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Radioisotopes
Radioisotopes are isotopes that have “unstable” nuclei
Unstable nuclei emit “radiation” as they try to become stable.
14C
(unstable):
6p
8n
14N
(stable):
7p
7n
beta radiation
http://www.windows2universe.org/physical_science/physics/atom_particle/radioactive_decay.html
In this example of a nuclear reaction, a neutron is converted to a proton, with
the release of beta radiation. There are many other types of nuclear reactions.
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Types of Radiation
“Radiation” refers to certain particles or energy released
when an unstable nucleus decays.
1. alpha radiation
• helium nuclei
• symbol = 4 2 He
• low energy, but particles have mass
1I. beta radiation
• high speed electrons
• symbol = 0-1 e
• high energy, but particles have little
mass
1II. gamma radiation • a form of electromagnetic radiation
• symbol =
• very high energy, but no particles
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Types of Radiation
penetrating power of different forms of radiation
Different types of radiation have different “penetrating powers”, depending on their mass and energy.
Alpha particles (helium nuclei) are large and slow, and can blocked by paper or skin.
Beta particles (high speed electrons) are small but fast, and are blocked by aluminum sheets.
Gamma rays are not particles and have high energy, and are only blocked by lead sheets.
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Uses of Radioisotopes
general uses:
energy production
sterilization of surgical equipment
food preservation
detect cracks in structures
smoke detectors
medical diagnosis and treatment
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Uses of Radioisotopes
carbon-14: radioactive dating
The amount of carbon-14 in a living organism is
constant. But when it dies, the levels of carbon-14
decrease as the radioisotope decays. This decay
occurs at a regular rate, known as the half-life of the
radioisotope. The half life of carbon-14 is 5730 years.
The amount of carbon-12, however, is the same in both
living and dead matter. By measuring the ratio of
carbon-12 to carbon-14 in a specimen, scientists can
calculate the age of the specimen.
http://commons.wikimedia.org/wiki/File:Guanajuato_mummy_02.jpg
cobalt-60 : cancer radiation therapy
Cobalt-60 releases gamma radiation, which is very
effective at destroying the fast dividing cells in cancerous
tumours. Normal cells may also be affected, but they
are able to recover if treatment is carefully controlled. It
has been used for over 50 years to treat a variety of
types of cancers, including skin, larynx, brain, breast and
uterine cancers, as well as leukemia (a cancer of the
blood).
http://www.medscape.com/viewarticle/580420_3
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Uses of Radioisotopes
iodine-131: medical diagnosis/tracer
Radioisotopes have the same chemical properties as other atoms of
the same element. This means they will be used by cells to make
compounds in the same way as non-radioactive isotopes. However,
the radioactive isotopes are easily detected and this makes them
useful as a “medical tracer”. They can be used to track the movement
or accumulation of a particular chemical in the body, as shown in the
photo to the right. This is the result of a test for thyroid cancer, where
radioactive sodium iodide is injected into the patient. The dark areas
show the presence of thyroid tumours. This isotope has a short halflife of 8 days, so the small amount used in the test be cleared from the
body quickly. Iodine-131 is also used to
treat thyroid cancers.
http://www.medscape.com/viewarticle/580420_3
iodine-125: cancer radiation therapy
Iodine-125 has a longer half-life (80 days) than iodine-131. It is used in the treatment of prostate
and brain cancer.
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