Announcements & Agenda (01/19/07)
You should currently be reading Ch 3 & starting
Ch 4, which will be relevant to lab next week
Today:
 More on Subatomic Particles (3.5)
 Isotopes & Atomic Mass (3.6)
 Radiation (Ch 9 – only responsible for notes)
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I thought the quiz was…
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A piece of cake.
Manageable.
Not too easy or difficult (what I expected).
Challenging!
Oh my goodness!!!
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Last Time: Classification of Matter
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Last Time: Know These Elements
O 65.0 %
C 18.0
H 10.0
N 3.0
Ca 1.4
P 1.0
Mg 0.50
K
S
Na
Cl
Fe
Zn
0.34
0.26
0.14
0.14
0.004
0.003
Trace Elements
As, Cr, Co, Cu, F, I, Mn, Mo, Ni, Se, Si, V
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Last Time: Atoms Rearrange in Chemical
Reactions
Methane (CH4) reacts with oxygen (O2) to produce carbon
dioxide (CO2) and water (H2O). COUNT THE ATOMS!
SEE BOB DEMO!
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Subatomic Particles
Atoms contains subatomic particles,
• protons have a positive (+) charge.
• electrons have a negative (-) charge.
• like charges repel and unlike charges
attract.
• neutrons are neutral.
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Structure of the Atom
• dense nucleus that
contains protons and
neutrons.
• of electrons in a
large empty space
around the nucleus.
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Atomic Mass Scale
On the atomic mass scale for subatomic particles,
• 1 atomic mass unit (amu) has a mass equal to 1/12 of
the mass of the carbon-12 atom.
• a proton has a mass of about 1 (1.007) amu.
• a neutron has a mass of about 1 (1.008) amu.
• an electron has a very small mass, 0.000549 amu.
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Summary
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Atomic Number = The Identity of
the Element
Counts the number
of
protons
in an atom
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All Atoms of the Same Element
Have the Same # of Protons!!!
11 protons
Symbol
11
Na
11
www.webelements.com
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Electrons in An Atom
An atom of
• an element is electrically neutral; the net charge of
an atom is zero.
• has an equal number of protons and electrons.
number of protons = number of electrons
Aluminum has 13 protons and 13 electrons. The net
charge is zero.
13 protons (13+) + 13 electrons (13 -) = 0
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Mass Number
Since protons and neutrons account for most
of the mass of the atom,
count the number
of
protons and neutrons
to determine the mass number!!!
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Nuclear Symbol
• represents a particular atom of an element.
• gives the mass number in the upper left corner
and the atomic number in the lower left corner.
Example: An atom of sodium with atomic
number 11 and a mass number 23 has the
following atomic symbol:
mass number
23
Na
atomic number
11
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Subatomic Particles Example
16
O
31
P
65
8
15
30
? p+
?n
? e-
? p+
?n
? e-
? p+
?n
? e-
Zn
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Subatomic Particles Example
16
O
31
P
65
8
15
30
? p+
?n
? e-
? p+
?n
? e-
8 p+
8n
8 e-
Zn
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Subatomic Particles Example
16
O
31
P
65
8
15
30
15 p+
16 n
15 e-
? p+
?n
? e-
8 p+
8n
8 e-
Zn
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Subatomic Particles Example
16
O
31
P
65
8
15
30
15 p+
16 n
15 e-
30 p+
35 n
30 e-
8 p+
8n
8 e-
Zn
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Isotopes
 Atoms with the same number of protons, but
different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
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Learning Check
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2) 65
3) 67
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Atomic Mass
Na
22.99
 Listed on the periodic table
 Gives the mass of “average” atom of each element
compared to 12C
 Average atom based on all the isotopes and their
abundance %
 Atomic mass (!unlike mass #!) is not a whole #
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Calculating Atomic Mass
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
(mass isotope1 x %) + (mass isotope2 x %) + …
100
100
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Example: Atomic Mass of Mg
Isotopes
24Mg
=
Mass of Isotope
24.0 amu
Abundance
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
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Why Do We Care About Isotopes?
Answer: Nuclear Stability!
“belt of stability”
Unstable nuclei


Radioactive
Can decay to
release alpha
particles, beta
particles and
positrons.
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Radiation Primer: Subatomic
Particles Revisited
Radiation comes from nucleus of an atom
Unstable nucleus emits a particle or energy

protons and/or neutrons typically lost from nucleus
changing identity of element
 alpha (particle)
 beta (particle)
 gamma (pure energy)
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Half-Life of a Radioisotope
The time for the radiation level to fall (decay)
to one-half its initial value
decay curve
initial
1
half-life
8 mg
4 mg
2
2 mg
3
1 mg
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Examples of Half-Life
Isotope
C-15
Ra-224
Ra-223
I-125
C-14
U-235
Half life
2.4 sec
3.6 days
12 days
60 days
5700 years
710 000 000 years
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Medical Uses of Radiation
Nuclear medicine has two main arenas
Diagnostic methods
Small amounts of radioisotopes
administered to help image an organ or
follow a physiological process
Therapeutic methods
Larger radiation doses to deliver fatal
punch to diseased tissue
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Diagnostic Methods
Radioisotopes behave chemically the
same as stable isotopes of the same
atom
Thus, can use to target an organ or a
physiological process
Usually -emitters because radiation
has to emerge from the body if the
imaging equipment is to see it
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Some Radio-Imaging Isotopes
P-32
Cr-51
Fe-59
Se-75
I-131
Hg-197
Eye tumors
Spleen shape and GI disorders
Bone marrow function
Pancreas scan
Thyroid malfunction
Kidney scan
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PET Scans
Positron Emission Tomography
Good for following physiological processes
Patient given short-lived +-emitter
11
6
- +
C
11
5
B +
0
1
e+
t1/2
20 min
The + hits an electron in tissues
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PET Scans
With annihilation event, a pair of gamma
rays are emitted in opposite directions
0
1
e+
+
0
-1
e-
2
Very clear images because signal
distinguished from background radiation
(only seen in one direction)
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A PET scan instrument places patient inside a ring
of detectors to see the paired, 180o gamma rays
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PET Scans
Can incorporate C-11 into a number of
organic compounds to follow
Blood flow
Glucose metabolism (Fig 9E.5, Page 259)
Oxygen uptake
Find brain areas associated with epilepsy
Find hard to spot tumors
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http://www.breastcancer.org/testing_pet.html
Abnormal
lymph
nodes
imaged by
PET scan
Also:
http://www.ldcmri.com/html/pet_scans.html
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Therapeutic Radiation
Selective destruction of pathological
cells and tissues
Rapidly dividing cells most vulnerable thus it targets cancer
Used when cancer is not well localized
Co-60 is common source of x-rays and
-rays
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Therapeutic Radiation
Can also deliver a radiation dose with
an internal administration of selected
isotopes
I-131 targets thyroid cancer - thyroxine
contains iodine
I-125 crystals implanted in prostate
gland to deliver continuous radiation (t1/2
= 60 days)
Y-90 implanted in pituitary to slow tumor
growth everywhere
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Radio-Tracer Compounds
Huge use of radioisotopes to study
chemical reactions
Can see 10-19g/L - almost individual
atoms
Photosynthesis - carbon in glucose
comes from CO2
Calcium - uptake is 90% efficient in
children; 40% efficient in adults
Zinc - uptake by trees in winter 2 ft/day
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