Transcript I. Radioactivity

CHAPTER 9
Nuclear
Energy
I. Radioactivity
(pg.284-292)
Radioactive Elements
A. Definitions
 Radioactivity
 Process of unstable nuclei of
elements becoming stable through
emitting particles or releasing energy
away from the atom
 Also called nuclear decay
Definitions
 During nuclear decay, the element
can transform into a different isotope
of the same element or to a different
element completely.
 Transmutation
 process of changing one element into
another element by nuclear decay
Definitions
 Nuclear radiation is the released energy
and matter during nuclear decay.
 This can have both positive and
negative effects for life on earth.
Definitions
 Isotopes – elements that have the same
number of protons but different number
of neutrons in their nuclei.
Isotopes
 Carbon-12, Carbon-13, Carbon-14
Where does this take place?
 Radioactivity (nuclear decay) happens
in the nucleus of the atom.
B. Types of Radiation
 Alpha ()
 helium nucleus
 Beta-minus (-)
 electron
4
2
He
2+
paper
0
-1
1-
plastic
0
lead
 Gamma ()
 high-energy photon
e
Types of Radiation
 Neutron emission (n) 1
0 n
0 charge
C. Nuclear Decay
 Why some nuclei decay…
 to obtain a stable ratio of
neutrons to protons
39
19
K
40
19
K
Stable
Unstable
(radioactive)
C. Nuclear Decay
TRANSMUTATION
 Alpha Emission
238
92
U
Th  He
234
90
4
2
 Beta Emission
131
53
I
131
54
Xe  e
0
-1
Example
 Actinium-217 decays by releasing an
alpha particle. Write the equation for
this decay process and determine what
element is formed.
 Step 1: Write the equation with the
original element on the reactant side
and products on the right side.
Example
 217
89
A
Ac
 Z
4
X
+
2
He
Step 2: Write math equations for the atomic
and mass numbers.
217 = A + 4
89 = Z + 2
Example
 Step 3: Rearrange the equations.
A = 217 – 4
Z = 89 - 2
Step 4:Solve for the unknown value, and
rewrite the equation with all nuclei.
A = 213
Z = 87
Example
 217
89
213
Ac

87
4
Fr +
2
He
This is an example of alpha decay.
D. Half-life
 Half-life (t½)
 time it takes for half of the radioactive
nuclei in a sample to decay
Nuclear Decay
Example Half-lives
20
polonium-194
16
iodine-131
carbon-14
uranium-238
0.7 seconds
10.6 hours
8.04 days
5,370 years
4.5 billion years
Mass of Isotopes (g)
lead-212
18
14
12
10
8
6
4
2
0
0
2
4
6
# of Half-Lives
8
10
Half-life
If we start out with 1 gram of
the parent isotope, after the
passage of 1 half-life, there will
be 0.5 gram of the parent
isotope left.
D. Half-life
 How much of a 20-g sample of sodium-24 would
remain after decaying for 30 hours? Sodium-24
has a half-life of 15 hours.
GIVEN:
WORK:
total time = 30 hours number of half-lives = 2
t1/2 = 15 hours
20 g ÷ 2 = 10 g (1 half-life)
original mass = 20 g 10 g ÷ 2 = 5 g (2 half-lives)
5 g of 24Na would remain.
Nuclear Forces
There are two types of forces in the nucleus.
•Strong nuclear force – helps attract the
protons and neutrons in the nucleus and
keep them together.
•Repulsive force- protons repel each other
because they are the same charge
Nuclear Forces
In stable atoms, the attractive forces are
stronger than the repulsive forces.
A. Fission
 splitting a nucleus into two
or more smaller nuclei
 some mass is converted
to large amounts of
energy
1
0
n
235
92
U
141
56
Ba  Kr  3 n
92
36
1
0
A. Fission
 chain reaction - self-feeding reaction
Fission
 Chain reactions can be controlled and
used to create electricity in nuclear
power plants.
 The minimum amount of a substance
that can undergo a fission reaction and
sustain a chain reaction is called
critical mass.
B. Fusion
 combining of two nuclei to form one
nucleus of larger mass
 produces even more
energy than fission
 occurs naturally in
stars
Fusion
Nuclear Radiation in Life
 Background radiation is nuclear
radiation that is around you from natural
sources like the sun, soil, rocks, and
space.
 A rem or millirem (1 rem =
1000millirems) is the unit for radiation.
Nuclear Radiation in Life
 A safe limit is set at 5000 millirems/year.
 Occupation – X-ray tech, flight
attendant
 Where you live- high elevation, near
rocks
 Activities - smoking
A. Nuclear Power
 Fission Reactors
A. Nuclear Power
 Fusion Reactors (not yet sustainable)
National Spherical
Torus Experiment
Tokamak Fusion Test Reactor
Princeton University
A. Nuclear Power
F
I
S
S
I
O
N
 235U is limited
 danger of meltdown
 toxic waste
 thermal pollution
vs.




F
U
S
I
O
N
Hydrogen is abundant
no danger of meltdown
no toxic waste
not yet sustainable
Other benefits to radiation
 Smoke detectors
 Disease detection
 Ultra sound
 CT scan
 MRI
 Cancer treatment