Transcript Nuclear Chemistry

Nuclear Chemistry
Outline
• Background/Introduction
• Spontaneous Nuclear Reactions
- Types of Radioactive Decay
- Radioactive Half-Life
• Stimulated Nuclear Reactions
- Nuclear Fission
- Nuclear Fusion
Nuclear Chemistry - Background
• Traditional Chemistry
– Reactions occur due to interactions between valence
electrons (surrounding nucleus)
• Late 1800s – Early 1900s  New Developments
– Discovery that Uranium emits radiation (Henry Becquerel)
– Amount of radiation emitted is proportional to amount of
element present (Marie Curie)
– “Radioactive” substances = radiation-emitting (Curie)
– Radioactivity = a inherent property of certain ATOMS, as
opposed to a chemical property of compounds (Curie)
• Birth of nuclear chemistry
Video: Early (Mis)uses of Radium
Radium
- A radioactive element discovered in 1898 by Curie
- Found to glow in the dark!
- Many (at the time) thought it had health benefits
Radium Toothpaste
For video, see below URL:
http://www.youtube.com/watch?v=uu96STA5BDA
For more complete list of “quack” cures, see below URL:
http://www.orau.org/ptp/collection/quackcures/quackcures.htm
Chemical vs. Nuclear Reactions
Chemical Reactions
Nuclear Reactions
• Loss, gain, sharing of valence electrons • Changes in nucleus
• Formation of compounds
(Recall the different reaction types)
• Transformation of one element
into a different isotope or a
different element altogether
• Affected by temperature, pressure,
presence of other atoms
• Unaffected by temperature,
pressure, presence of other atoms
Atomic Structure Review
Isotopes: variants of atoms of a particular chemical
element, which have differing numbers of neutrons
(e.g. Carbon-12, Carbon-13, Carbon-14, etc.)
Nuclear Stability
• How can a stable nucleus exist?
– Net positive charge in nucleus surrounded by negative
charges (electrons)
– Electrostatic force
• Opposite charges attract
• Like charges repel
– Why doesn’t the nucleus break apart?
• Nuclear Force (1934)
– Force between protons and neutrons than binds nucleus together
within atom
– Very strong (must be!)
Which Elements/Isotopes Are Radioactive?
• Based on the nuclear force and nuclear stability
– More protons in nucleus  more neutrons needed to
bind nucleus together
– Critical Factor = Neutral-to-Proton Ratio
• Neutron-to-Proton ratios of stable nuclei increase with
increasing atomic number
• Unstable neutron-to-proton ratio = RADIOACTIVE
– Nuclei with atomic numbers ≥ 84 = ALWAYS Radioactive
• Very large nuclei!
• Neutron-to-proton ratio always unstable
Spontaneous Nuclear Reactions
Radiation and Radioactive Decay
• Unstable (radioactive) nuclei emit radiation (energy) in
order to become more stable
• Radioactive Decay – occurs when a nucleus spontaneously
decomposes in this way
• 3 Common Types of Nuclear Reactions
– Alpha Radiation
– Beta Radiation
– Gamma Radiation
Alpha (α) Radiation
Alpha Radiation: Nuclear Equation
• Emission of 2 protons and 2 neutrons (as an alpha particle, 42 He )
from radioactive atom’s nucleus
• Atom’s atomic mass decreases by 4 units
• Atom’s atomic number decreases by 2 units (a different element!)
Beta (β-) Negative Radiation
Beta negative Radiation: Nuclear
Equation
• Conversion of a neutron into a proton and an electron, followed by
the emission of the electron (beta particle) from the nucleus
• Atom’s atomic mass does NOT change
• Atom’s atomic number increases by 1 units (a different element!)
Positron emission
• In β+ radiation a positron is emitted.
• A positron is exactly like an electron in mass and charge force
except with a positive charge.
• It is formed when a proton breaks into a neutron with mass and
neutral charge and this positron with no mass and the positive
charge.
• Positron emission is most common in lighter elements with a low
neutron to proton ratio
Gamma (ϒ) Radiation
•
•
•
•
Emission of electromagnetic energy from an atom’s nucleus
No particles emitted
Often occurs during α and/or β decay
Example: X-rays emitted during β-decay of cobalt-60 (above)
Relative Penetrating Power of
Radiation Types
**slide not in notes
• Alpha radiation has low energy and little penetrating power
compared to beta radiation
• Gamma has the most energy and penetrating power
Uranium-238: Radioactive Decay Chain
Practice: WE DO
1. Write a nuclear equation for the β- decay of francium-234
2. Write a nuclear equation for the α-decay of radium-235
3. Write a nuclear equation for the ϒ-decay of uranium-239
Practice: YOU DO
What isotope is present after Po-210 undergoes 2
consecutive alpha decays, followed by a beta
negative decay, followed by another alpha decay.
Stimulated Nuclear Reactions
Nuclear Fission
• Neutron fired at atom’s nucleus
• Energy of neutron “bullet” causes target element to split into two (or
more) elements that are lighter than the parent atom
– Unpredictable composition of products
– Energy released!
– More neutrons released!
• RESULT?  SEE NEXT SLIDE
Nuclear Fission Chain Reactions
Nuclear Fission Power
(Controlled Nuclear Fission)
Nuclear Fission (Atomic) Bomb
(Uncontrolled Nuclear Fission)
How a nuclear reactor works video
Nuclear Fission (Atomic) Bombs
• Fuel Core (B)
– Two subcritical masses of plutonium
• Each contains not enough fissionable material to sustain nuclear fission
– Neutron source (radioactive isotope) in separate compartment
• Detonation of outer casing of TNT (A)
– Two plutonium masses brought together to form supercritical mass
• Mass now contains enough fissionable material to sustain nuclear fission!
– Neutrons brought to speed necessary to initiate nuclear fission
Nuclear Fusion
• Reactions in which two or more elements “fuse” together to form one larger
element (heaver isotope formed from lighter isotopes)
• Requires extremely high heat (lots of energy!) and pressure
– Analogy: trying to squeeze an unopened can of Coke into a little ball
without spilling any Coke
• Lots of energy is released!
• Example: Fusion of 2 hydrogen isotopes (deuterium and tritium) into helium
Nuclear Fusion (Hydrogen) Bombs
• Central core (B) of trillions of deuterium (H-2) and tritium (H-3) isotopes
-
Surrounded by small atomic (fission) bombs (A)
Detonation of atomic bombs provides energy and pressure for fusion of
hydrogen isotopes into helium
• LOTS of energy released in this process!
-
Atomic bombs (nuclear fission) used to initiate nuclear fusion
Fusion itself  energy release
Release of neutrons accompanying fusion triggers fission of bomb casing (C),
which is made of Uranium!
Nuclear Fusion (Hydrogen) Bombs
• Energy released = over 10 times greater than fission (atomic) bomb
• Has never been used in warfare
• Neutron bomb = Hydrogen bomb without uranium casing
– Less explosion, due to lack of fission of casing
– Large quantity of neutrons propelled outward (neutron radiation) and captured by
nuclei of substances they encounter
– Results: * Neutron radiation induces radioactivity in most substances it encounters
* Massive radiation without massive blast
– Targets: * Potential H-bomb survivors (e.g. tank drivers, stronghold inhabitants, etc.)
* Ballistic missiles (intense neutron flux damages electron components)
Nuclear Fusion in the Sun
• Energy is released from these fusion reactions that we receive
as LIGHT and HEAT!
• NOTE: 3 hydrogen-1 atoms required (total), yet only 2 produced
– The Sun is running out of fuel (hydrogen-1 atoms)!
Controlled Nuclear Fusion on Earth?
• Advantages over nuclear fission
– More energy created
– Very little dangerous radiation released
– Potential Source = Water (and not much of it)!!
• Main Challenge
– Need to find way to bring small sample of deuterium and
tritium to very high temperature and pressure WITHOUT
detonating atomic bombs
– Subject of much current research in physics and chemistry
Half-Life (T1/2) = The amount of time
necessary for one-half of a particular
radioactive sample to decay
Different Elements  Different Half-Lives
Data for most
stable isotope
of element
Half-Life Problems
• Can be completed without the below formulas!
– Conceptual knowledge of half-life is sufficient
Half-Life Formulas
Fraction remaining =
1
n
2
(n = # of half-lives elapsed)
Amount remaining = Original Amount * Fraction Remaining
Practice: WE DO
The half-life of radium-226 is 1600 years. How
many grams of a 0.25 gram sample will remain
after 4800 years?
Practice: YOU DO
1. How many days does it take for 16 g of palladium-103 to
decay to 1.0 g? The half-life of palladium-103 is 17 days.
2. The half-life of thorium-227 is 18.72 days. How many
days are required for 75% of a given amount to decay?