Transcript Nuclear Chemistry - Brookwood High School

Nuclear Chemistry
Unstable Nuclei and Radioactive
Decay
Chemical and Nuclear Reactions
Chemical reactions
Nuclear reactions
1. Bonds are broken and formed
on the e- level. p+ and n0 remain
the same
1. Nuclei emit particles and/or
rays
2. Atoms are unchanged, but can
be rearranged
2. Atoms are converted into atoms
of other elements by releasing
energy
3. Involve valence electrons only
3. Can involve protons, neutrons,
and electrons
4. Small energy changes
4. Large energy changes – huge
amounts of energy released
5. Rxn rate influenced by
temperature, pressure,
concentration, and catalysts
5. Rxn rate not normally affected
by temperature, pressure,
concentration, or catalysts
Radioactivity
Chemical reaction involves only an
atom’s electrons – the nucleus remains
unchanged
 Nuclear reaction involves a change in an
atom’s nucleus (p+ or n0 number)
 Radioactivity is when substances
spontaneously emit radiation
 The rays and particles emitted are
radiation

Radioactivity
Radioactive atoms emit radiation because
their nuclei are unstable (gain stability by
losing energy)
 Radioactive decay is when atoms lose
energy by emitting radiation in a
spontaneous process. Atoms keep
decaying until they form stable,
nonradioactive atoms.

Types of Radiation
Alpha, beta, and gamma radiation have
different amounts of electrical charge and
are affected differently by an electric field
 Radioactive source, two plates (+,-),
stream of alpha, beta, and gamma rays

Alpha Radiation
2p+ and 2n0
 Particles deflected to (-) plate
 Extra protons form new element
 Nuclear equation:

 22688Ra
radium-226
→ 22286Rn + 42He
radon-222
alpha particle
Beta Radiation
Fast moving e -1 charge
 Particles deflected to (+) plate
 Nuclear equation:

 146C
carbon-14
→
14
7N
nitrogen-14
+
0
-1β
beta particle
Gamma Radiation




High energy radiation without mass or electrical
charge
Particles are not deflected by electric or
magnetic fields
Unable to form a new atom by themselves since
they are mass-less
They accompany other types of radiation in
reactions
 23892U
Uranium-238
→
234
90Th
thorium-234
+
4
2He
alpha particle
+
200γ
gamma rays
Characteristics of Radiation
Radiation
type
Alpha
Symbol
4
Mass
(amu)
Charge
2He
4
2+
-1β
1/1840
1-
0γ
0
0
Beta
0
Gamma
0
Penetrating Power
Human Body
Nuclear Stability
Elements > atomic number 83 are
naturally radioactive
 Ratio of p+ to n0 determines the stability of
an atom
 Atoms with too many or too few neutrons
are unstable
 Ratio of 1.5 : 1

Fusion Reactions

Nuclear process in which two light nuclei
combine to form a single heavier nucleus.
Examples: thermonuclear weapons and in
future nuclear reactors
Fusion Reactions

The sum of the masses of the product
nuclei is less than the sum of the masses
of the initial fusing nuclei.

E=mc2, explains that the mass that is lost
it converted into energy carried away by
the fusion products.
Fusion and our Universe


Hydrogen isotopes collide in a star and fuse
forming a helium nucleus
Lighter elements fuse and form heavier
elements. These reactions continue until the
nuclei reach iron, the nucleus with the most
binding energy. No more fusion occurs in a star
because it is energetically unfavorable to
produce higher masses. Once a star has
converted a large fraction of its core's mass to
iron, it has almost reached the end of its life.
Fission Reactions
Fission is a nuclear process in which a
heavy nucleus splits into two smaller
nuclei. An example of a fission reaction
that was used in the first atomic bomb and
is still used in nuclear reactors is:
235U
+ n ----> 134Xe + 100Sr + 2n
Fission
Reactions

Fission reactions can produce any combination
of lighter nuclei so long as the number of
protons and neutrons in the products sum up to
those in the initial fissioning nucleus. As with
fusion, a great amount of energy can be
released in fission because for heavy nuclei, the
summed masses of the lighter product nuclei is
less than the mass of the fissioning nucleus
Fission

Fission is a process that has been occurring in
the universe for billions of years. We have not
only used fission to produce energy for nuclear
bombs, but we also use fission peacefully
everyday to produce energy in nuclear power
plants
Fission

Nuclear energy is the most certain future
fuel source that we have. Currently in the
U.S. 107 nuclear reactors are producing
about 17% of our energy requirements.
The U.S. Currently has plans of building
42 more nuclear power plants in the next
20 years.
Downfalls to using Fission and
Fusion as fuel sources…
Radioactive and toxic wastes
 Fear/anxiety of terrorist bombing
 Safe storage of toxic wastes?
 Construction costs more than the energy it
provides
 Coal still used and causing environmental
issues
 Can cause health problems in humans

Practical Applications of Nuclear
Power
Cheap
 Efficient
 Lower air pollution
 Lower the price of electricity
 Uranium is abundant on earth

Half Life


The time it takes for
one-half of a
radioisotope’s nuclei
to decay into its
products
The half-life of
strontium-90 is 29
years
Number
of halflives
Elapsed Amount present
time
0
0
10 g
1
29 yrs
10 x (1/2)= 5
2
58 yrs
10 x
(1/2)(1/2)=2.5
3
87 yrs
10 X
(1/2)(1/2)(1/2)=
1.25
4
116 yrs
10x(1/2)(1/2)(1/2)
(1/2)=.625
Half Life
Amount remaining = (initial amount)(1/2)n
n = the number of half-lives that have passed
Radioactive Decay Practice
Practice Problem #1:
If Gallium-68 has a half-live of 68.3
minutes, how much of a 10.0 mg sample is
left after one half-life? Two half-lives?
Three half-lives?
Radioactive Decay Practice
Practice Problem #2:
If the passing of five half-lives leaves 25.0 mg
of a strontium-90 sample, how much was
present in the beginning?