nuclear chemistry by: stephanie chen and stephanie ng
TRANSCRIPT
NUCLEAR CHEMISTRY
By: Stephanie Chen
and
Stephanie Ng
RadioactivityRadioactivity
• One of the pieces of evidence for the One of the pieces of evidence for the fact that atoms are made of smaller fact that atoms are made of smaller particles came from the work ofparticles came from the work of
Marie CurieMarie Curie (1876-1934).(1876-1934). • She discoveredShe discovered radioactivityradioactivity, ,
the spontaneous disintegration of the spontaneous disintegration of some elements into smaller pieces.some elements into smaller pieces.
Nuclear Reactions vs. Nuclear Reactions vs. Normal Chemical Normal Chemical
ChangesChanges• Nuclear reactions involve the nucleusNuclear reactions involve the nucleus• The nucleus opens, and protons and The nucleus opens, and protons and
neutrons are rearrangedneutrons are rearranged• The opening of the nucleus releases a The opening of the nucleus releases a
tremendous amount of energy that tremendous amount of energy that holds the nucleus together – called holds the nucleus together – called binding energybinding energy
• ““Normal” Chemical Reactions involve Normal” Chemical Reactions involve electronselectrons, not protons and neutrons, not protons and neutrons
23.1
Types of RadiationTypes of Radiation
e01
He42
• Alpha (Alpha (άά) – a positively ) – a positively charged (+2) helium isotopecharged (+2) helium isotope - - we usually ignore the charge because it we usually ignore the charge because it involves electrons, not protons and neutronsinvolves electrons, not protons and neutrons
•Beta (Beta (ββ) – an electron) – an electron
•Gamma (Gamma (γγ) – pure energy; ) – pure energy; called a ray rather than a called a ray rather than a particleparticle
00
Other Nuclear ParticlesOther Nuclear Particles
e01
n10• NeutronNeutron
• Positron – a positive Positron – a positive electronelectron
•Proton – usually referred to Proton – usually referred to as hydrogen-1as hydrogen-1
•Any other elemental isotopeAny other elemental isotope
H11
Penetrating AbilityPenetrating Ability
XAZ
Mass Number
Atomic NumberElement Symbol
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
A
Z
1p11H1or
proton1n0
neutron0e-1
0-1or
electron0e+1
0+1or
positron4He2
42or
particle
1
1
1
0
0
-1
0
+1
4
2
23.1
Balancing Nuclear Equations
1. Conserve mass number (A).
The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants.
1n0U23592 + Cs138
55 Rb9637
1n0+ + 2
235 + 1 = 138 + 96 + 2x1
2. Conserve atomic number (Z) or nuclear charge.
The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants.
1n0U23592 + Cs138
55 Rb9637
1n0+ + 2
92 + 0 = 55 + 37 + 2x023.1
212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po.
4He242oralpha particle -
212Po 4He + AX84 2 Z
212 = 4 + A A = 208
84 = 2 + Z Z = 82
212Po 4He + 208Pb84 2 82
23.1
Nuclear Stability and Radioactive Decay
Beta decay
14C 14N + 0 + 6 7 -1
40K 40Ca + 0 + 19 20 -1
1n 1p + 0 + 0 1 -1
Decrease # of neutrons by 1
Increase # of protons by 1
Positron decay
11C 11B + 0 + 6 5 +1
38K 38Ar + 0 + 19 18 +1
1p 1n + 0 + 1 0 +1
Increase # of neutrons by 1
Decrease # of protons by 1
and have A = 0 and Z = 023.2
Electron capture decay
Increase # of neutrons by 1
Decrease # of protons by 1
Nuclear Stability and Radioactive Decay
37Ar + 0e 37Cl + 18 17-1
55Fe + 0e 55Mn + 26 25-1
1p + 0e 1n + 1 0-1
Alpha decay
Decrease # of neutrons by 2
Decrease # of protons by 2212Po 4He + 208Pb84 2 82
Spontaneous fission
252Cf 2125In + 21n98 49 023.2
Learning Check
What radioactive isotope is produced in the following bombardment of boron?
10B + 4He 13N + 1n
5 2 7 0
Write Nuclear Equations!
Write the nuclear equation for the beta emitter Co-60.
6060CoCo 00ee ++ 6060NiNi2727 -1 -1 2828
Artificial Nuclear Artificial Nuclear ReactionsReactions
New elements or new isotopes of known New elements or new isotopes of known elements are produced by bombarding an elements are produced by bombarding an atom with a subatomic particle such as a atom with a subatomic particle such as a proton or neutron -- or even a much heavier proton or neutron -- or even a much heavier particle such as particle such as 44He and He and 1111B.B.
Reactions using neutrons are called Reactions using neutrons are called
reactions reactions because a because a ray is usually ray is usually emitted.emitted.
Radioisotopes used in medicine are often Radioisotopes used in medicine are often made by made by reactions. reactions.
Artificial Nuclear Artificial Nuclear ReactionsReactions
Example of a Example of a reaction reaction is is
production of radioactive production of radioactive 3131P for use P for use
in studies of P uptake in the body.in studies of P uptake in the body.
31311515P + P + 11
00n ---> n ---> 32321515P + P +
Transuranium ElementsTransuranium Elements
Elements beyond 92 Elements beyond 92 (transuranium)(transuranium) made made
starting with an starting with an reaction reaction
2382389292U + U + 11
00n ---> n ---> 2392399292U + U +
2392399292U U ---> ---> 239239
9393Np + Np + 00-1-1
2392399393Np Np ---> ---> 239239
9494Pu + Pu + 00-1-1
Nuclear Stability
• Certain numbers of neutrons and protons are extra stable
• n or p = 2, 8, 20, 50, 82 and 126
• Like extra stable numbers of electrons in noble gases (e- = 2, 10, 18, 36, 54 and 86)
• Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons
• All isotopes of the elements with atomic numbers higher than 83 are radioactive
• All isotopes of Tc and Pm are radioactive
23.2
Band of Stability Band of Stability and Radioactive and Radioactive DecayDecay
Stability Stability of Nucleiof Nuclei
• Out of > 300 stable isotopes:
EvenEven OddOdd
OddOdd
EvenEven
ZZNN
157157 5252
5050 55
31311515PP
191999FF
2211H, H, 66
33Li, Li, 101055B, B, 1414
77N, N, 1801807373TaTa
Half-LifeHalf-Life
•HALF-LIFEHALF-LIFE is the time that it takes is the time that it takes for 1/2 a sample to decompose.for 1/2 a sample to decompose.
• The rate of a nuclear transformation The rate of a nuclear transformation depends only on the “reactant” depends only on the “reactant” concentration.concentration.
Half-LifeHalf-Life
Decay of 20.0 mg of Decay of 20.0 mg of 1515O. What remains after 3 half-lives? O. What remains after 3 half-lives? After 5 half-lives?After 5 half-lives?
Kinetics of Radioactive Kinetics of Radioactive DecayDecay
For each duration (half-life), one half of the substance
decomposes.
For example: Ra-234 has a half-life of 3.6 days
If you start with 50 grams of Ra-234
After 3.6 days > 25 gramsAfter 3.6 days > 25 grams
After 7.2 days > 12.5 gramsAfter 7.2 days > 12.5 grams
After 10.8 days > 6.25 gramsAfter 10.8 days > 6.25 grams
Kinetics of Radioactive Decay
N daughter
rate = -Nt
rate = N
Nt
= N-
N = N0e(-t) lnN = lnN0 - t
N = the number of atoms at time t
N0 = the number of atoms at time t = 0
is the decay constant (sometimes called k)
Ln 2=
t½
23.3
k =
Kinetics of Radioactive Decay
[N] = [N]0exp(-t) ln[N] = ln[N]0 - t
[N]
ln [
N]
23.3
Radiocarbon Dating
14N + 1n 14C + 1H7 160
14C 14N + 0 + 6 7 -1 t½ = 5730 years
Uranium-238 Dating
238U 206Pb + 8 4 + 6 092 -182 2 t½ = 4.51 x 109 years
23.3
Learning Check!
The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 31 hours?
Nuclear FissionNuclear FissionFission is the splitting of atomsFission is the splitting of atoms
These are usually very large, so that they are not as These are usually very large, so that they are not as
stablestable
Fission chain has three general steps:Fission chain has three general steps:
1.1. Initiation.Initiation. Reaction of a single atom starts the Reaction of a single atom starts the
chain (e.g., chain (e.g., 235235U + neutron)U + neutron)
2.2. PropagationPropagation. . 236236U fission releases neutrons U fission releases neutrons
that initiate other fissionsthat initiate other fissions
3. 3. TerminationTermination. .
Nuclear FissionNuclear Fission
Nuclear Fission
23.5
235U + 1n 90Sr + 143Xe + 31n + Energy92 54380 0
Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2
Energy = 3.3 x 10-11J per 235U
= 2.0 x 1013 J per mole 235U
Combustion of 1 ton of coal = 5 x 107 J
Representation of a fission process.
Mass DefectMass Defect
• Some of the mass can be converted Some of the mass can be converted into energyinto energy
• Shown by a very famous equation!Shown by a very famous equation!
E=mcE=mc22
EnergyEnergy
MassMass
Speed of lightSpeed of light
Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons.
BE + 19F 91p + 101n9 1 0
BE = 9 x (p mass) + 10 x (n mass) – 19F mass
E = mc2
BE (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984
BE = 0.1587 amu 1 amu = 1.49 x 10-10 J
BE = 2.37 x 10-11J
binding energy per nucleon = binding energy
number of nucleons
= 2.37 x 10-11 J19 nucleons
= 1.25 x 10-12 J
23.2
Nuclear binding energy per nucleon vs Mass number
nuclear binding energynucleon
nuclear stability
23.2
Nuclear Fission
23.5
Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions.The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass.
Non-critical
Critical
Nuclear Fusion
Fusion small nuclei combine
2H + 3H 4He + 1n +
1 1 2 0
Occurs in the sun and other stars
Energy
Nuclear Fusion
Fusion • Excessive heat can not be
contained• Attempts at “cold” fusion have
FAILED.• “Hot” fusion is difficult to contain