2. 2
Mass Defect &Mass Defect &
Nuclear StabilityNuclear Stability
• Because an atom is made of protons, neutronsBecause an atom is made of protons, neutrons
and electrons, you would expect the mass ofand electrons, you would expect the mass of
an atom to be equal to the sum of the massesan atom to be equal to the sum of the masses
of the eof the e--
, p, p++
, and n, and n00
, right??, right??
Neutrons + protons + electrons =Neutrons + protons + electrons =
atomic massatomic mass
……right?right?
3. 3
Mass Defect &Mass Defect &
Nuclear StabilityNuclear Stability
Nay.Nay.
The measured mass will beThe measured mass will be lessless than the combinedthan the combined
masses. This difference is called themasses. This difference is called the Mass DefectMass Defect..
Let’s look atLet’s look at heliumhelium ( He) as an example:( He) as an example:
2 protons: (2 x 1.007276 amu) = 2.014552 amu2 protons: (2 x 1.007276 amu) = 2.014552 amu
2 neutrons: (2 x 1.008665 amu) = 2.017330 amu2 neutrons: (2 x 1.008665 amu) = 2.017330 amu
2 electrons: (2 x 0.005486 amu) = 0.001097 amu2 electrons: (2 x 0.005486 amu) = 0.001097 amu
Total combined mass = 4.032979 amuTotal combined mass = 4.032979 amu
44
22
4. 4
Mass Defect &Mass Defect &
Nuclear StabilityNuclear Stability
But wait…But wait…
The atomic mass of a He atom has been measuredThe atomic mass of a He atom has been measured
to be 4.002602 amu,to be 4.002602 amu, NOTNOT 4.032979 amu.4.032979 amu.
That’s 0.030377 amu LESS than the combined mass.That’s 0.030377 amu LESS than the combined mass.
44
22
Just what the heckJust what the heck
is going on?is going on?
5. 5
Mass Defect &Mass Defect &
Nuclear StabilityNuclear Stability
So what causes the difference in mass?So what causes the difference in mass?
Well, according to Einstein’s famous equation E = mcWell, according to Einstein’s famous equation E = mc22
,,
mass can be converted to energy, and energy to mass.mass can be converted to energy, and energy to mass.
So the mass defect is caused by theSo the mass defect is caused by the conversion of massconversion of mass
to energyto energy upon formation of the nucleus.upon formation of the nucleus.
This is calledThis is called nuclear binding energynuclear binding energy..
Remember, the nucleus is like a mousetrap, and theRemember, the nucleus is like a mousetrap, and the
nuclear binding energy is needed to “set” the trap.nuclear binding energy is needed to “set” the trap.
It is TREMENDOUS potential energy.It is TREMENDOUS potential energy.
6. 6
RadioactivityRadioactivity
• One of the pieces of evidence for theOne of the pieces of evidence for the
fact that atoms are made of smallerfact that atoms are made of smaller
particles came from the work ofparticles came from the work of
________________ (1876-1934).(1876-1934).
• She discoveredShe discovered ________________, the, the
spontaneous disintegration of somespontaneous disintegration of some
elements into smaller pieces.elements into smaller pieces.
7. 7
RadioactivityRadioactivity
• One of the pieces of evidence for theOne of the pieces of evidence for the
fact that atoms are made of smallerfact that atoms are made of smaller
particles came from the work ofparticles came from the work of
________________ (1876-1934).(1876-1934).
• She discoveredShe discovered ________________, the, the
spontaneous disintegration of somespontaneous disintegration of some
elements into smaller pieces.elements into smaller pieces.
Marie CurieMarie Curie
radioactivityradioactivity
8. 8
Nuclear ReactionsNuclear Reactions
vs. Normal Chemicalvs. Normal Chemical
ChangesChanges
• Nuclear reactions involve the nucleusNuclear reactions involve the nucleus
• The nucleus opens, and protons andThe nucleus opens, and protons and
neutrons are rearrangedneutrons are rearranged
• The opening of the nucleus releases aThe opening of the nucleus releases a
tremendous amount of energy that holdstremendous amount of energy that holds
the nucleus together – calledthe nucleus together – called bindingbinding
energyenergy
• ““Normal” Chemical Reactions involveNormal” Chemical Reactions involve
electronselectrons, not protons and neutrons, not protons and neutrons
9. 9
Mass DefectMass Defect
• Some of the mass can be converted intoSome of the mass can be converted into
energyenergy
• Shown by a very famous equation!Shown by a very famous equation!
E=mcE=mc22
EnergyEnergy massmass
speed ofspeed of
lightlight
squaredsquared
10. 10
Types of RadiationTypes of Radiation
e0
1−
He4
2
• Alpha (Alpha (άά) – a positively) – a positively
charged helium isotopecharged helium isotope -- wewe
usually ignore the charge because it involvesusually ignore the charge because it involves
electrons, not protons and neutronselectrons, not protons and neutrons
•Beta (Beta (ββ) – an electron) – an electron
•Gamma (Gamma (γγ) – pure energy;) – pure energy;
called a ray rather than acalled a ray rather than a
particleparticle
γ0
0
11. 11
Other Nuclear ParticlesOther Nuclear Particles
e0
1+
n1
0• NeutronNeutron
• Positron – a positivePositron – a positive
electronelectron
•Proton – usually referred toProton – usually referred to
as hydrogen-1as hydrogen-1
•Any other elemental isotopeAny other elemental isotope
H1
1
13. 13
Balancing Nuclear ReactionsBalancing Nuclear Reactions
•In the reactants (starting materials –
on the left side of an equation) and
products (final products – on the right
side of an equation)
Atomic numbers must balance
and
Mass numbers must balance
•Use a particle or isotope to fill in the
missing protons and neutrons
14. 14
Nuclear ReactionsNuclear Reactions
• Alpha emissionAlpha emission
Note that mass number (A) goes down by 4
and atomic number (Z) goes down by 2.
Nucleons (nuclear particles… protons and
neutrons) are rearranged but conserved
16. 16
Other Types of Nuclear ReactionsOther Types of Nuclear Reactions
Positron (Positron (00
+1+1ββ): a positive electron): a positive electron
Electron capture:Electron capture: the capture of an electron
207 207
19. 19
Artificial Nuclear ReactionsArtificial Nuclear Reactions
New elements or new isotopes of known elementsNew elements or new isotopes of known elements
are produced by bombarding an atom with aare produced by bombarding an atom with a
subatomic particle such as a proton or neutronsubatomic particle such as a proton or neutron
-- or even a much heavier particle such as-- or even a much heavier particle such as 44
HeHe
andand 1111
B.B.
Reactions using neutrons are calledReactions using neutrons are called
γγ reactionsreactions because abecause a γγ ray is usuallyray is usually
emitted.emitted.
Radioisotopes used in medicine are often made byRadioisotopes used in medicine are often made by
γγ reactions.reactions.
20. 20
Artificial Nuclear ReactionsArtificial Nuclear Reactions
Example of aExample of a γγ reactionreaction is productionis production
of radioactiveof radioactive 3131
P for use in studies of PP for use in studies of P
uptake in the body.uptake in the body.
3131
1515P +P + 11
00n --->n ---> 3232
1515P +P + γγ
21. 21
Transuranium ElementsTransuranium Elements
Elements beyond 92Elements beyond 92 (transuranium)(transuranium) mademade
starting with anstarting with an γγ reactionreaction
238238
9292U +U + 11
00n --->n ---> 239239
9292U +U + γγ
239239
9292UU --->---> 239239
9393Np +Np + 00
-1-1ββ
239239
9393NpNp --->---> 239239
9494Pu +Pu + 00
-1-1ββ
23. 23
Nuclear FissionNuclear Fission
Fission is the splitting of atomsFission is the splitting of atoms
These are usually very large, so that they are not as stableThese are usually very large, so that they are not as stable
Fission chain has three general steps:Fission chain has three general steps:
1.1. Initiation.Initiation. Reaction of a single atom starts theReaction of a single atom starts the
chain (e.g.,chain (e.g., 235235
U + neutron)U + neutron)
2.2. PropagationPropagation.. 236236
U fission releases neutrons thatU fission releases neutrons that
initiate other fissionsinitiate other fissions
3.3. ______________________ ..
24. 24
StabilityStability
of Nucleiof Nuclei
• Out of > 300 stable isotopes:
EvenEven OddOdd
OddOdd
EvenEven
ZZ
NN
157157 5252
5050 55
3131
1515PP
1919
99FF 22
11H,H, 66
33Li,Li, 1010
55B,B, 1414
77N,N, 180180
7373TaTa
27. 27
Nuclear Fission & POWERNuclear Fission & POWER
• Currently about 103Currently about 103
nuclear power plants innuclear power plants in
the U.S. and about 435the U.S. and about 435
worldwide.worldwide.
• 17% of the world’s17% of the world’s
energy comes fromenergy comes from
nuclear.nuclear.
31. 31
Half-LifeHalf-Life
• HALF-LIFEHALF-LIFE is the time that it takes foris the time that it takes for
1/2 a sample to decompose.1/2 a sample to decompose.
• The rate of a nuclear transformationThe rate of a nuclear transformation
depends only on the “reactant”depends only on the “reactant”
concentration.concentration.
32. 32
Half-LifeHalf-Life
Decay of 20.0 mg ofDecay of 20.0 mg of 1515
O. What remains after 3 half-lives?O. What remains after 3 half-lives?
After 5 half-lives?After 5 half-lives?
33. 33
Kinetics of Radioactive DecayKinetics of Radioactive Decay
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
34. 34
Learning Check!
The half life of I-123 is 13 hr. How much of
a 64 mg sample of I-123 is left after 39
hours?
38. 38
Radiocarbon DatingRadiocarbon Dating
Radioactive C-14 is formed in the upper atmosphereRadioactive C-14 is formed in the upper atmosphere
by nuclear reactions initiated by neutrons inby nuclear reactions initiated by neutrons in
cosmic radiationcosmic radiation
1414
N +N + 11
oon --->n ---> 1414
C +C + 11
HH
The C-14 is oxidized to COThe C-14 is oxidized to CO22, which circulates, which circulates
through the biosphere.through the biosphere.
When a plant dies, the C-14 is not replenished.When a plant dies, the C-14 is not replenished.
But the C-14 continues to decay with tBut the C-14 continues to decay with t1/21/2 = 5730= 5730
years.years.
Activity of a sample can be used to date the sample.Activity of a sample can be used to date the sample.
40. 40
Food IrradiationFood Irradiation
•Food can be irradiated withFood can be irradiated with γγ rays fromrays from
6060
Co orCo or 137137
Cs.Cs.
•Irradiated milk has a shelf life of 3 mo.Irradiated milk has a shelf life of 3 mo.
without refrigeration.without refrigeration.
•USDA has approved irradiation of meatsUSDA has approved irradiation of meats
and eggs.and eggs.
