Transcript Nuclear Chemistry ppt 2012-2013

Nuclear Chemistry
(Topic for Regents exam, SAT II
exam and AP exam)
Video Animations
• Online resources from our TB for ch 21
(discovery: alpha, beta and gamma radiation)
• Chem tours chapter 20 of Gilbert’s book see:
http://www.wwnorton.com/college/chemistry
/gilbert2/contents/ch20/chemtours.asp
• Modes of radioactive decay, Balancing nuclear
reactions and Fusion of Hydrogen
The Nucleus
• Remember that the nucleus is comprised of
protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
the mass of the atom.
Isotopes
• Not all atoms of the same element have the
same mass due to different numbers of
neutrons in those atoms.
• There are three naturally occurring isotopes
of uranium:
– Uranium-234
– Uranium-235
– Uranium-238
Stable Nuclei
• The shaded region in the
fig. shows what nuclides
would be stable, the socalled belt of stability.
• It is the ratio of neutrons
to protons that determines
the stability of a given
nucleus.
Radioactivity
• It is not uncommon for some nuclei to be
unstable, or radioactive.
• There are no stable nuclei with an atomic
number greater than 83.
• Radioisotopes = isotopes that are unstable
and thus radioactive
• There are several ways radionuclides can
decay into a different nuclide
• (a nuclide is a nucleus with a specified number of protons and
neutrons (TB, p. 902) .
Predicting the mode of radioactive decay.
In general:
•neutron-rich nuclei tend to emit beta particles
•proton-rich nuclei tend to either emit positrons or
undergo electron capture
•heavy nuclei tend to emit alpha particles.
•The presence of magic numbers of nucleons and an even number of
protons and neutrons also help determine the stability of a nucleus.
Radioactive Series
• Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation.
• They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
• Transmutation = the
reaction by which the
atomic nucleus of one
element is changed into
the nucleus of a different
element
Types of Radioactive Decay
Alpha Decay
= Loss of an -particle (a helium nucleus)
4
2
238
92
U
Atomic # decreases by 2
Mass # decreases by 4
# of protons decreases by 2
# of neutrons decreases by 2

He
234
90
4
2
Th + He
Which element undergoes alpha
decay to form lead-208?
• Analyze
• Plan
• Solve
• Check
Types of Radioactive Decay
Beta Decay
= Loss of a -particle (a high energy electron)
0
−1
131
53
Atomic # increases by 1
# of protons increases by 1
# of neutrons decreases by 1
Mass # remains the same
I


0
−1
or
131
54
e
Xe
+
0
−1
e
Types of Radioactive Decay
Positron Emission
= Loss of a positron (a particle that has the
same mass as but opposite charge than an
electron)
0
1
11
6
Atomic # decreases by 1
# of protons decreases by 1
# of neutrons increases by 1
Mass # remains the same
C

e
11
5
B
+
0
1
e
Electron capture
• Capture by the nucleus of an electron from
the electron cloud surrounding the nucleus
(effectively converts a proton to a neutron).
• Ex: Rubidium-81 is converted to Krypton-81
by this process (Atomic numbers: Rb = 37, Kr = 36)
• Nuclear equation:
Types of Radioactive Decay
Gamma Emission
= Loss of a -ray (a photon of high-energy
light that has no mass or charge & that
almost always accompanies the loss of a
nuclear particle; often not shown when
writing nuclear equations)
0
0

Artificial Transmutation
= done by bombarding the nucleus with high-energy particles
(such as a neutron or alpha particle), causing transmutation
40
96
20Ca
+ _____ ----->
40
19K
+ 11H
2 H -----> 1 n + _____
Mo
+
42
1
0
**Natural transmutation has a single nucleus undergoing
change, while artificial transmutation will have two reactants
(fast moving particle & target nuclei.**
Nuclear Fission
• Nuclear fission is the type of reaction carried out in
nuclear reactors.
• = splitting of large nuclei into middle weight nuclei and
neutrons
Nuclear Fission
• Bombardment of the radioactive nuclide with a
neutron starts the process.
• Neutrons released in the transmutation strike
other nuclei, causing their decay and the
production of more neutrons.
• This process continues in what we call a nuclear
chain reaction.
Nuclear Fusion
• = the combining of light nuclei into a heavier
nucleus
•
2
1H
+ 21H  42He + energy
• Two small, positively-charged nuclei smash
together at high temperatures and pressures to
form one larger nucleus.
Half-Life
= the time it takes for half of the atoms in a
given sample of an element to decay
- Each isotope has its own half-life; the
more unstable, the shorter the half-life.
- Table T Equations:
fraction remaining = (1/2)(t/T)
# of half-lives remaining = t/T
Key: t = total time elapsed
T = half-life
Sample Half-Life Question 1A - Regents
Most chromium atoms are stable, but Cr-51 is an unstable
isotope with a half-life of 28 days.
(a) What fraction of a sample of Cr-51 will remain after 168
days?
Step 1: Determine how many half-lives elapse during 168 days.
Step 2: Calculate the fraction remaining.
Sample Half-Life Question 1B- Regents
(Hint:1st use Regents tables to find half-life)
(a) If a sample of Cr-51 has an original mass of 52.0g, what
mass will remain after 168 days?
Step 1: Calculate the mass remaining:
mass remaining = fraction remaining X original mass
(Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each
half-life.)
Sample Half-Life Question 2- Regents
How much was present originally in a sample of Cr-51 if 0.75g
remains after 168 days?
Step 1: Determine how many half-lives elapsed during 168 days.
Step 2: Multiply the remaining amount by a factor of 2 for each half-life.
Equations to learn for calculations
based on half-life (AP)
• Radioactive decay
is a first order
process;
• ln (Nt/No) = -kt
• Nt = No e-kt
• k = 0.693/t1/2
• or t1/2 = 0.693/k
• (kinetics topic)
• k = ? (rate
constant or decay
constant)
• N=?
For time interval t
Energy changes in Nuclear Reactions
E =mc2
• Einstein E =mc2
• mass defect
• For nuclear
reactions
∆E =c2∆m
• E = energy in Joules
(J = kg•m2/s2)
• m = mass in kg
• C = speed of light
• (2.9979 x 108 m/s)
Some practical uses of Radioisotopes
(dating, chemical tracers, industrial applications, medical
applications, nuclear power plants)
Medical Uses
Other Uses
• 60Co (cobalt-60) used in
• 14C archaeological dating
cancer treatments and used
(of once living things) and
to kill bacteria in food
radiolabelled
organic
products
compounds
• 226Ra (Radium-226) used in
• 238U archaeological dating
Cancer treatment
•
131I
diagnosis and treatment
of thyroid disorders
• 11C Positron emission
tomography (PET scans)
(U-238 to Pb-206 ratio)
•
241Am
(Americium-241)
smoke detectors
• 235U nuclear reactors and
weapons
Activities and Problem set 5
TB ch. 21 – sections
21.1 and 21.4 most
impt for AP exam
POGIL activity on
nuclear chemistry
Lab activity: Paper lab
on nuclear decay
Concept map (Group)
PHET dating game
http://phet.colorado.edu/en/simulation/radioactivedating-game
• Ch 21 Problems TO DO
• all GIST, sample & practice
exercises, Visualizing concepts,
• Ch 21 end of ch. Red
ex:21.7,9,11,13,15,19,23,
25,27,29,31,33,41,43,45,57,
65,70, (21.70 requires
graphing)
(problems are from eText;
most of them are identical
to those in your textbook)