Transcript Honors Unit 2b: Nuclear

Nuclide which is unstable.
It emits radiation & changes
into another kind of atom.
Radioactive Nuclide
An atom with a specific
number of protons and a
specific number of neutrons.
14C
6
12C
6
14N
7
are all nuclides
Nuclide
Two atoms with the same
atomic number but different
mass numbers.
Isotopes
A change in the identity of a
nucleus as a result of a
change in the number of its
protons.
Transmutation Reaction
As stability , energy .
Relationship between
stability and energy
Attractive force between all nucleons.
Holds the nucleus together.
But it is a very short-range force.
Nuclear Strong Force
Occur between like charges.
Occur between protons in the nucleus.
Longer-range force.
Electrostatic repulsive
forces
- Can be assessed by neutron to proton
ratio.
- A certain number of neutrons are
needed to increase the strong nuclear
force (the attractive force) enough to
hold the nucleus together.
- Small atoms, a stable N/P ratio is 1:1
- Large atoms: 1.5:1
Stability of nuclide
All the elements with atomic
number > 83 (or beyond
Bismuth)
That’s all nuclides  84!
Which elements are
unstable?
Alpha, Beta, Gamma
Separated by electric or
magnetic fields.
Opposites attract.
 Rays are pure
energy. No charge
so they are not
deflected by an
electric field.
Types of Radiation
Alpha radiation. Shielding
can be paper or cloth.
Least penetration power
Gamma radiation. Requires
lead/concrete shielding.
Most penetration power
Symbol for alpha radiation
Same as the nucleus of a helium atom
Mass = 4 amu
Charge = +2
4
2He
4
or 2
Symbol for beta particle
Fast moving electron originating
from nucleus
Mass = “zero”
Charge = -1
0
-1e
or
0
-1
or - or 
Symbol for positron.
Mass = “zero.”
Charge = +1.
Positive electron
0
+1e
or
0
+1
or +
Symbol for gamma radiation.
Pure Energy
0 mass
0 charge
0
0
or 
Symbol for neutron
1
0n
or n
Symbol for proton
1
1H
or
1
1p
Have mass numbers & atomic
numbers
Describes changes in the nucleus of
an atom
Nuclear Equations
Unstable nucleus emits an alpha
particle.
Atomic #  by 2. Mass #  by 4.
Alpha Decay
Alpha Decay
Atomic #  by 2. Mass #  by 4.
220Fr
87
 4 + 216At
2
85
Equation represents natural
transmutation.
1 term on reactant side.
220Fr
87
 4 + 216At
2
85
Balance nuclear equations
using conservation of atomic
number & conservation of
mass number.
220
220Fr
87
87
=
4 + 216
 4 + 216At
2
=
85
2 + 85
Use a nuclide
chart!
For elements 1-20:
If the n/p ratio is too
high, beta emission
happens.
If the n/p ratio is too low,
positron emission happens.
Predicting Decay Modes
Beta Decay
Atomic #  by 1.
Mass # stays the same.
42K
19

0e
-1
+
42Ca
20
Positron Emission
Atomic #  by 1.
Mass # stays the same.
19Ne
10
 0e + 19F
+1
9
Elapsed time
Length of H.L.
# of Half-Lives =
Fraction = 1/2n
where n = # of
half-lives
Fraction
Remaining
1
Half-Life Map
Amount
(mass)
Initial Mass
Elapsed
Time
0
# of Half
Lives
0
½
1 X H.L.
1
¼
2 X H.L.
2
1/8
3 X H.L.
3
1/16
4 X H.L.
4
Same as type of
particle emitted
Decay Mode
Weighted average of the
masses of the naturally
occurring isotopes.
Average Atomic Mass
Cl has 2 isotopes:
25% Cl-37 & 75% Cl-35
1) Convert percent abundances to decimal format
2) Multiply each abundance factor by the
appropriate isotopic mass
3) Sum
4) Do a reality check.
0.25(37) + (0.75)(35) = 9.25 + 26.25 = 35.5
35.5 is in between the high & the low, and it is closer to
the more abundant isotopic mass.
How to calculate the Average
Atomic Mass of Cl
Particle “bullet” hits target
nucleus & new isotope is produced.
2 terms on reactant side.
Artificial Transmutation
Artificial Transmutation
target
32S
16
bullet
1
+ n
0

32P
15
+
1H
1
Particle “bullet” may be proton or alpha particle.
To react with a nucleus, must overcome + +
repulsive forces by accelerating bullet to high
speeds.
Particle “bullet” may be a neutron. Neutrons have
no charge, so no repulsive forces to overcome. No
acceleration necessary.
Target can be anything from PT.
Artificial Transmutation
Fission is division.
Large nucleus (U-235 or Pu-239) is split into 2
medium sized nuclei by a neutron bullet. Excess
neutrons & a great deal of energy are also
produced.
Fission
Fission
239Pu
94
147Ba + 3 1n
+ 01n  90
Sr
+
38
56
0
Fusion: U for unite and U for sun.
Very small nuclei (H & He) are jammed together.
Huge amounts of energy are released.
Fusion
Fusion
1H
1
+ 21H  3He
2
a)
b)
1n
0
+
235U
92
59Co
27
c)
3He
2
d)
14C
6
142Ba
56

+ 01n 
60Co
27
+
91Kr
36

14N
7
+ -10e
0
fission
Artificial transmutation
+ 11H  4He + 0e
2
+ 3 1n + energy
+1
fusion
Natural transmutation
Identify each of the rxns
The difference between the mass of a
specific atom and the sum of the masses of
its protons, neutrons, & electrons.
Can be expressed in amu or kg.
In nuclear reactions, a small amount of mass
is converted to a huge amount of energy.
Mass Defect, m
The energy released when a nucleus
is formed from its nucleons.
Often expressed per nucleon.
Nuclear Binding Energy
4He
Potential Energy of System
2
+ energy  2 protons + 2 neutrons
Reference level
Separate Nucleons
Yellow arrow shows
the binding energy!
Stable Nucleus
r, distance between nucleons
Potential Well Diagram
Represents potential energy changes during a process
Einstein’s Equation relating
energy and mass!
Recall that to use this equation,
the mass needs to be in kilograms,
not amu’s.
E = mc2 or E =  mc2
1. Count up protons, neutrons, & electrons.
2. Multiply the number of particles X the mass of
the particles.
3. Sum the terms.
4. Subtract the isotopic mass. This is m in amu’s.
5. Convert to kg.
6. Plug into Einstein’s famous equation, E = mc2 or E
= mc2.
7. Divide by the number of nucleons to get BE per
nucleon.
8. Multiply by Avogadro’s number to get binding
energy per nucleon for 1 mole of substance.
STEPS TO CALCULATE
BINDING ENERGY
Curve of Binding Energy
Fe and Ni have the highest binding
energies. The higher the binding energy,
the more energy is released when the
nucleus is formed. So the nucleus is in a
deeper potential well, and it is MORE
stable.
Binding Energy & Stability
Protons and Neutrons
Mass # = # of nucleons
Nucleon
Fuel
Control rods
Containment or shielding
Coolant
Moderator
Parts of a nuclear reactor
Substance that slows down fast neutrons.
Increases the efficiency of the fission
process.
Sometimes the moderator is also the
coolant. Sometimes it is in the fuel rods.
1n
0
+
235U
92
Slow neutrons
work better!

142Ba
56
+
91Kr
36
+ 3 01n + energy
But fast neutrons
come off here!
Moderator
Contain a substance that absorbs
neutrons, removing them from the
reaction. On days with high
electrical demand, the control rods
would be removed from the core.
Control Rods
One of the products is also one of the
reactants
Neutron
products
Neutron
reactant
Chain Reaction
The minimum amount of U-235 or
Pu-239 that will undergo a selfsustaining chain reaction.
Critical Mass
Radioactive Dating:
C-14 to C-12 for organic material.
U-238 to Pb-206 for rocks.
Killing bacteria/spores in food and mail.
Chemical tracers: follow the path of
material in a system. Used to study organic
reaction mechanisms.
Medical uses: I-131, Tc-99
Uses of radioisotopes