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NUCLEAR CHEMISTRY
Unit 15
Overview Nuclear Chemistry
IsotopesNuclear force
Radioactive decayAlpha, beta, gamma
decayPositron emissionElectron capture
Nuclear Stability
Radiometric DatingHalf-life
Nuclear fusion Nuclear fission Nuclear energy
Mass DefectNuclear binding
energy
Nuclear Chemistry Involves the change in the nucleus of an atom Nuclear reactions are everywhere
Produce sunlightCreate elements (synthetic and natural in stars)Radiation therapy (cancer treatment)Generate electricityNuclear weapons
The Nucleus Remember – the nucleus is comprised of
the two nucleons (protons and neutrons) Atomic Number – number of protons Mass Number – number of protons and
neutrons together It is effectively the mass of the atom
Nuclear Symbols
C126
Mass number (p+ + no)
Atomic number(number of p+)
Element symbol
Isotopes
Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms
Example: There are three naturally occurring isotopes of uranium:○ Uranium-234○ Uranium-235○ Uranium-238
Nuclear Force
Strong nuclear forceHolds protons and
neutrons in nucleus very close together
Strongest force known
Nuclear Force Nucleus is not stable when atoms
experience certain ratios of protons to neutrons
Unstable atoms decay and emit radiationRadioactive decay
Elements with more than 83 protons (bismuth) are naturally radioactive
Radioactive Decay
Radionuclides: Radioactive elements During radioactive decay
The makeup of the nucleus changesThe number of protons may change
○ Means that the element has changed
Natural Radioactive Isotopes Radon-222
Comes from decomposition of Uranium rocks2nd leading cause of lung cancerComes up through cracks in basements
Radium-226Some radium salts glow in the darkEarly 1900s used to be used as paint for watches and clocks
(workers licked paint brushes and got cancer – “radium girls”)
Uranium-238Rocks create radon gasUsed in radioactive dating
Potassium-40One of few light radioactive elementsProduces argon that is found in atmosphere
Other Common RadioisotopesIsotope Use
14C Archaeological dating24Na Circulatory system testing for obstruction32P Cancer detection
51Cr Determination of blood volume
59Fe Measurements of red blood cell formation and lifetimes
60Co Cancer treatment131I Measurement of thyroid activity
153Gd Measurement of bone density226Ra Cancer treatment
3H Archaeological dating235U Nuclear reactors and weapons238U Archaeological dating
241Am Smoke detectors
Measuring Radioactivity One can use a device like this Geiger counter to
measure the amount of activity present in a radioactive sample.
The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
Radioactive Decay(3 Most Common Types)
Alpha ( , a He)2 protons, 2 neutrons
Beta ( , b e)High energy electron
Gamma (g)Electromagnetic radiationHigh energy photons
Alpha, Beta, Gamma Radiation
Alpha Decay:
Loss of an -particle (a helium nucleus)
He or 42
U23892 Th
23490 He
42+
42
Beta Emission:
Loss of a -particle (a high energy electron)
0−1 e0
−1or
I13153 Xe
13154 + e
0−1
Gamma Emission: Loss of a -ray High-energy radiation that almost always
accompanies the loss of a nuclear particleNot usually written in nuclear equation
00
0023490
42
23892 ThHeU
Positron Emission:
Loss of a positron (a particle that has the same mass as but opposite charge of an electron)
C116 B
115 + e
01
01 e0
1or
Has a very short life because it is destroyed when it collides with an electron, producing gamma rays:
e + e 01
0-1
00
Positron Emission
A positron can convert a proton to a neutron
p11 n
10 + e
01
Electron Capture
Capture by the nucleus of an electron from the electron cloud surrounding the nucleusAddition of an electron to a proton in the nucleusAs a result, a proton is transformed into a neutron
p11 + e
0−1 n
10
Nuclear Stability
Several factors predict whether a particular nucleus is radioactiveNeutron-to-proton ratioRadioactive seriesMagic NumbersEvens and Odds
Neutron-Proton Ratios The strong nuclear force helps keep the
nucleus from flying apartProtons repel each otherNeutrons help the strength of the nuclear force
As protons increase, neutrons have to counter-act increasing proton-proton repulsionsIn low atomic number elements (1-20) protons and
neutrons are approximately equalIn high atomic number elements number of neutrons
much larger than protons Neutron-proton ratio helps stabilize nucleus
Neutron-Proton Ratios
For smaller nuclei (Atomic Number 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
Neutron-Proton Ratios
Stable Nuclei
The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.
Stable Nuclei Nuclei above this belt
have too many neutrons.
They tend to decay by emitting beta particles.
(If an isotopes mass number is greater than its atomic weight, the same trend will happen example C)16
6
Stable Nuclei Nuclei below the belt
have too many protons. They tend to become
more stable by positron emission or electron capture.
(If an isotopes mass number is less than its atomic weight, the same trend will happen example C)11
6
Stable Nuclei
There are no stable nuclei with an atomic number greater than 83.
These nuclei tend to decay by alpha emission.Decreases both protons and neutrons
Radioactive Series
Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.
They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).
Often occur in nature
Magic Numbers Nuclei with 2, 8, 20, 28, 50, or 82 protons
or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.These are called the “Magic Numbers”
Evens and Odds Nuclei with an even number of protons and
neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.
Kinetics of Radioactive Decay
Radioactive decay is a 1st order process Remember this equation:
= t1/2 0.693
k
Radiometric Dating Half life can help determine the age of different
objects
Carbon-14Half life of 5,715 yearsCan determine age of organic materials up to about
50,000 years old
Radiometric Dating Uranium-238
Half life of 4.5×109 yearsUsed to determine age of Earth (measured rocks)
○ Oldest rock found is almost 4.5 billion years old
Nuclear Fusion
Elements can be man-made by bombarding nuclei with particlesAlpha particles accelerated and collided with
nucleusNeutrons bombard nucleus
Bombard nuclei to create transuranium elementsHeavy elements beyond uranium on
periodic table
Particle Accelerators Nuclear transformations can be induced by accelerating a
particle and colliding it with the nuclide These particle accelerators are enormous, having circular
tracks with radii that are miles long
Nuclear Fission The splitting of heavy nuclei
(Fusion is the combination of light nuclei) Process begins by bombarding heavy nucleus
with a neutron 2 main commercial uses
Nuclear WeaponryNuclear Energy
Nuclear Fission About 2 neutrons are produced for each fission
These 2 neutrons cause 2 additional fissions○ Which cause 2 more fissions each
Which cause 2 more fissions each…
This is called a chain reaction
Nuclear Fission
Chain reactions can escalate quicklyIf the reaction is not controlled, it results in a
violent explosion because of the release of too much energy too quickly
Nuclear Energy We can control fission reactions and use it to
create energy
Nuclear Energy Fission reactions are carried out
in nuclear reactorsThe reaction is kept in check by the use of
control rodsThese block the paths of some neutrons,
keeping the system from escalating out of control
The heat generated by the reaction is used to produce steam that turns a turbine connected to a generator
Video: http://www.youtube.com/watch?v=VJfIbBDR3e8
Debates on Nuclear Energy Pros…
Cleaner energy than coal and fossil fuel plants
Doesn’t add to global warming
High amount of electricity can be generated in one plant
Cheaper to run a nuclear facility than a fossil fuel plant
Cons…Nonrenewable source of
energyProduces nuclear waste
that must be stored for thousands of years
Accidents (Chernobyl, Three Mile Island, Fukushima)○ http://www.youtube.com/watch?v=
eGI7VymjSho
Very expensive to build a nuclear facility (about $10 billion per reactor)
Nuclear Energy
We can measure the energy
associated with nuclear reactions
E = mc2
E = energy (J)
m = change in mass (kg) during reaction (mass of products-mass of reactants)
c = speed of light (3.0×108 m/s)
When a system loses mass, it is exothermic (-E)
When a system gains mass, it is endothermic (+E)
Nuclear Energy The mass change in chemical reactions is so small
that we treat them as though mass is conservedEx: Mass change for exothermic process of combustion
of 1 mol of CH4 is -9.9×10-9 grams
Mass change in nuclear reactions is measureableEx: Mass change accompanying decay of 1 mol of
uranium-238 is 50,000 times greater than combustion of CH4
Nuclear Energy (example)
For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.
The change in energy, E, is then
E = (m) c2
E = (−4.6 10−6 kg)(3.00 108 m/s)2
E = −4.1 1011 J
Mass Defect When protons and neutrons form a nucleus, the mass
of the nucleus is less than the sum of the masses of its constituent protons and neutrons
Example: Helium (He) – 2 protons, 2 neutrons
Protons and Neutrons Mass of Nucleus
Mass of 2 protons (2×1.0073 = 2.0146) 4.0015 amu
Mass of 2 neutrons (2×1.0087 = 2.0174)
Total mass = 4.0320 amu
Difference = 4.0320 – 4.0015 = 0.0305 amu (mass defect)
Mass Defect
To measure the energy associated with the mass defect use
E = mc2
Example: Helium (He) – 2 protons, 2 neutrons
E = (5.1×10-29 kg)(3.0×108 m/s)2
E = 4.6×10-12 J
NOTE: 1 gram = 6.022×1023 amu
Nuclear Binding Energy
Energy required to separate a nucleus into its individual nucleons (protons and neutrons)
Also use E = mc2
The larger the binding energy, the more stable the nucleus toward decomposition