Transcript nuclear reactions: identification & balancing

Nuclear Reactions
Natural Transmutation

1 term on reactant side


original isotope (naturally radioactive)
2 terms on product side
emitted particle
 new Isotope

Happens all by itself (spontaneous)
Not affected by anything in environment
Natural Transmutation
16N
7

1 term on
reactant side
0e
-1
+
16O
8
2 terms on
product side
Artificial Transmutation


cause to happen: smash particles
into one another
2 terms on reactant side
original Isotope (non-radioactive)
 particle that hits it

• neutron, proton, or -particle

product side: usually 2 terms
Artificial Transmutation
27Al
13
+
original isotope
or
target nucleus
4He
2

30P
15
+ 1n
0
“bullet”
- thing hits isotope
Artificial Transmutation
27Al
13
14N
7
+ 4He  30P + 1n
2
+ 42He 
15
17O
8
75As
+ 4He 
37Cl
17
+ 1n  38
Cl
17
33
2
0
0
+ 11H
78Br
35
+ 1n
0
all these equations
have 2 reactants!
Bombarding with protons or 

protons & -particles have positive charge
and mass
• do some damage when hit target nucleus
• must be accelerated to high speeds to
overcome repulsive forces between nucleus &
particle (both are +)
What is an accelerator?

vacuum chamber (usually long pipe)
• surrounded by vacuum pumps, magnets, radiofrequency cavities, high voltage instruments &
electronic circuits

inside pipe particles are accelerated to very
high speeds then smashed into each other
Fission Reaction

splitting heavy nucleus into 2 lighter nuclei
requires critical mass of fissionable isotope
• controlled: nuclear reactor
• uncontrolled: bomb
Fission

reactant side: 2 terms
 1 heavy isotope (examples: U-235 or Pu-239)
 bombarding particle – usually a neutron
Fission = Division

product side: at least 2 terms
 2 medium-weight isotopes
 1 or more neutrons
 huge amount energy released
Fission
235U
+ 1n 
91Kr
36
+ 142Ba + 31n + energy
235U
92
+ 1n 
72Zn
30
1n + energy
+ 160
Sm
+
4
62
92
0
0
56
0
0
more than 200 different product isotopes
identified from fission of U-235
small amount of mass is converted to
energy according to E = mc2
Fusion

reactant side has 2 small nuclei:
• H + H;

H + He;
He + He
product side:
• 1 nucleus (slightly larger; still small) and
maybe a particle

source of sun’s energy

2 nuclei unite
2H
1
+ 13H  24He + 01n + energy
CERN
27 kilometer ring
•particles travel just below speed
of light
•10 hrs: particles make 400
million revolutions of ring
FermiLab
4 miles in circumference!
Balancing Nuclear
Equations
Nuclear Equations - tasks

identify type (4 types):
• natural transmutation
• artificial transmutation



fission
fusion
balance to find unknown term
Natural Transmutation – ID

1 term on reactant side
• starting isotope

2 terms on product side
• ending isotope & emitted particle

type of particle emitted characteristic
of isotope – Table N
Nuclear Equations

to balance: use conservation of both
atomic number & mass number
• mass number = left superscript
• atomic number = left subscript
Balancing Nuclear Equations
16N
7

0e
-1
+
16O
8
conservation of mass number:
16 = 0 + 16
conservation of atomic number:
7 = -1 + 8
Writing Equations



write equation for decay of Thorium-232
use Table N to find decay mode: α
write initial equation:
232Th  4He + X
90
2
figure out
what element
it turned into
What’s under the hat?
Little cats X, Y, & Z!
Write an equation for the α
decay of Th-232
232
95
Th  4He + YX
what’s X?
2
Z
so Y = 228
232 = 4 + Y
232Th
90

4He
2
+
Y
X
Z
conservation of mass number:
sum mass numbers on left side must
=
sum mass numbers on right side
232Th
90
 4He + 228X
2
90 = 2 + Z
Z
so Z = 88
conservation of atomic number:
sum of atomic numbers on left side must
=
sum of atomic numbers on right side
232Th
 4He +
90
228X
2
88
X = Ra
use PT to find X:
232Th
90
 4He + 228Ra
2
88
Radioactive Decay Series


sometimes 1 transmutation isn’t enough
to achieve stability
some radioisotopes go through several
changes before achieve stability (no
longer radioactive)
radioactive decay series: Th-232 transmuting to Pb-208
βbeta
β+
positron
18F
9
14C
6
 147 N +
 188 O +
0e
+1
0e
-1
How does the mass number or atomic
number change in α,β or γ decay?

go to Table N:
• find isotope that decays by α or β decay
• write equation
• see how mass number (or atomic number) changes

226 Ra
88

4 
2
+X
so X has to be
α decay of Ra-226:
• mass number decreases by 4
• atomic number decreases by 2

222 X
86
So how do you know if an element is
radioactive or not?
Element (atom)
UNSTABLE
STABLE
n/p ratio
>1.5:1
1:1 up to 1.5:1
atomic number
83 and above
≤ 82
radioactive
yes
not
the key is the proton to neutron ratio