Transcript Nuclear Chemistry

AP Chemistry Podcast 1.3
Nuclear Chemistry
Nuclear Chemistry
• Nuclear reactions involve changes
that originate in the nucleus of
the atom.
• Chemical changes involve changes
in the electron cloud.
• Uses:
–
60Co-
gamma ray emitter- ionizing
radiation for treatment of cancerous
tumors.
– 201Thallium stress test of heart muscle
– Radiocarbon dating 14C ½ life 5730
years
– Nuclear power ~ 20% of US electricity
production
2
Radioactivity
• Recall that all atoms of the same element have the
same number of protons. The number of neutrons in
the atoms nucleus, however, may be different from
one atom to the next= Isotopes.
Uranium- 234
Uranium-235
Uranium-238
92 protons
92 protons
92 protons
142 neutrons 143 neutrons 146 neutrons
Trace
0.7%
99.3%
• Different isotopes have different abundances
• Different isotopes have different stabilities
3
Patterns of Nuclear Stability
As the atomic number
increases, the neutron to
proton ratio of the stable
nuclei increases. The stable
nuclei are located in the
shaded area of the graph
known as the belt of
stability. The majority of
radioactive nuclei occur
outside this belt.
4
Nuclear Equations
• Radionuclides are unstable nuclei that emit
particles and electromagnetic radiation to
transform into a stable nucleus.
238
92
U
234
90
Th
+
4
2
He
5
Nuclear Equations
• Mass numbers and atomic numbers must be
balanced in all nuclear equations.
6
What product is formed when
thorium-232 undergoes alpha
decay?
7
Types of Radioactive Decay
Alpha decay- nucleus
emits 2 protons and 2
neutrons (He nucleus)
Beta decay- a neutron in
the nucleus decays into
a proton and an
electron, the electron is
emitted
Gamma- high energy, short
wavelength electromagnetic
radiation- accompanies other
radioactive emissions.
8
Types of Radioactive Decay
Electron Capture- capture by the
nucleus of an electron from
the electron cloud
surrounding the nucleus.
Positron- particle with the same
mass as an electron, but an
opposite charge collides
with an electron and
produces gamma radiation.
9
Penetrating Power of Radioactive Decay
10
Radioactive Decay Particles
Particle
Nuclear
Equation
Example
Alpha= 2
Nucleus  4He
226Ra
protons and 2
neutrons
Beta =neutron
2
1n
converts to
0
proton and a high
energy electron
Electron
Capture=
1p
1
 1p + 0e
-1
1
+ 0e 1n
-1
0
222Rn +4He
88
86
131I131Xe
2
+ 0e
53
54
81Rb
+ 0e  81Kr
37
-1
-1
36
electron captured
by nucleus
11
Radioactive Decay Particles
Particle
Positron=proton
converted to a
neutron and an
electron
Gamma=
electromagnetic
radiation
Nuclear
Equation
1p  1n + 0e
1
0
1
Example
11C
6
11B +0e
5
1
Not shown in
equations, but
almost always
accompanies other
decay.
Remember a positron has the same mass as an electron, but the opposite
charge
12
Radioactive Decay Particles
Particle
Effect
Alpha
Decrease atomic mass by ___ and
atomic number by _____.
Atomic number _______________.
Beta
Electron
Capture
Positron
Atomic number _______________.
Atomic number _______________.
Gamma
13
Half Life- the time required for half of any given quantity
of a substance to react / decay. (independent of initial
quantity of atoms)
Half Life Simulation
Number of Th232 atoms in a
sample initially
containing 1
million atoms as
a function of
time. Th-232 has
a half-life of 14
billion years.
14
Half Life Problems
Example:
An isotope of cesium (cesium-137) has a halflife of 30 years. If 1.0 mg of cesium-137
disintegrates over a period of 90 years, how
many mg of cesium-137 would remain?
15
Half Life Problems
1. A 2.5 gram sample of an isotope of strontium-90 was formed in a 1960 explosion of an
atomic bomb at Johnson Island in the Pacific Test Site. The half-life of strontium-90 is 28
years. In what year will only 0.625 grams of this strontium-90 remain?
2. Actinium-226 has a half-life of 29 hours. If 100 mg of actinium-226 disintegrates over a
period of 58 hours, how many mg of actinium-226 will remain?
3. Thallium-201 has a half-life of 73 hours. If 4.0 mg of thallium-201 disintegrates over a
period of 6.0 days and 2 hours, how many mg of thallium-201 will remain?
4. Sodium-25 was to be used in an experiment, but it took 3.0 minutes to get the sodium from
the reactor to the laboratory. If 5.0 mg of sodium-25 was removed from the reactor, how
many mg of sodium-25 were placed in the reaction vessel 3.0 minutes later if the half-life
of sodium-25 is 60 seconds?
5. Selenium-83 has a half-life of 25.0 minutes. How many minutes would it take for a 10.0 mg
sample to decay and have only 1.25 mg of it remain?
16
Uranium-238: an example of an unstable nucleus
decaying to form other unstable nuclei
Uranium-238 is
radioactive,
undergoing alpha
decay. But, the
daughter nuclide is
also radioactive,
undergoing beta
decay, to produce yet
another radioactive
nuclide, which
decays. The atom
goes through a rather
involved sequence of
radioactive decays
(both alpha and beta),
until a stable isotope
(lead-206) is reached.
17
Fission Reaction
Collision of a neutron with a U-235 nucleus can
cause the nucleus to split, creating two smaller
nuclides and three free neutrons. The three
neutrons may travel outward from the fission,
colliding with nearby U-235 nuclei, causing them
to split as well. Each split (fission) is accompanied
18
Fission Chain Reaction
Mousetrap Chain Reaction
Collision of a neutron with a
U-235 nucleus can cause the
nucleus to split, creating two
smaller nuclides and three
free neutrons. The three
neutrons may travel outward
from the fission, colliding with
nearby U-235 nuclei, causing
them to split as well. Each
split (fission) is accompanied
by a large quantity of energy.
If sufficient neutrons are
present, we may achieve a
chain reaction. If only one
neutron were produced with
each fission, no chain
reaction would occur,
because some neutrons
would be lost through the
surface of the uranium 19
sample.
Fission Reaction
20
Fusion Reaction
Tremendous energy needed
to overcome the repulsion
between nuclei. Heat
required for this reaction is
on the order of 40,000,000 K.
The energy from an atomic
bomb could generate this
heat (hydrogen or
thermonuclear weapon).
21