Transcript Nuclear Chemistry PowerPoint

Nuclear Chemistry
A subfield of chemistry dealing
with radioactivity, nuclear
processes and nuclear properties
History of Nuclear Chemistry
• Traditional chemical reactions occur as a result of the
interaction between valence electrons around an atom's
nucleus.
• In 1896, Henri Becquerel expanded the field of chemistry to
include nuclear changes when he discovered that uranium
emitted radiation.
• Soon after Becquerel's discovery, Marie Sklodowska Curie
began studying radioactivity and completed much of the
pioneering work on nuclear changes. Curie found that
radiation was proportional to the amount of radioactive
element present, and she proposed that radiation was a
property of atoms (as opposed to a chemical property of a
compound).
• Marie Curie was the first woman to win a Nobel Prize and the
first person to win two (the first, shared with her husband
Pierre and Becquerel for discovering radioactivity; the second
for discovering the radioactive elements radium and
polonium).
Radiation and Nuclear Reactions
• In 1902, Frederick Soddy proposed the theory that
"radioactivity is the result of a natural change of an isotope
of one element into an isotope of a different element."
• Nuclear reactions involve changes in particles in an atom's
nucleus and thus cause a change in the atom itself. All
elements heavier than bismuth (Bi) (and some lighter)
exhibit natural radioactivity and thus can "decay" into lighter
elements.
• Unlike normal chemical reactions that form molecules,
nuclear reactions result in the transmutation of one element
into a different isotope or a different element altogether
(remember that the number of protons in an atom defines
the element, so a change in protons results in a change in
the atom).
• There are three common types of radiation and nuclear
changes…
Alpha Radiation (α)
• The emission of an alpha particle from an atom's nucleus.
An α particle contains two protons and two neutrons (and is
similar to a He nucleus: ). When an atom emits an a
particle, the atom's atomic mass will decrease by four units
(because two protons and two neutrons are lost) and the
atomic number (z) will decrease by two units. The element
is said to "transmute" into another element that is two z units
smaller. An example of an a transmutation takes place when
uranium decays into the element thorium (Th) by emitting an
alpha particle, as depicted in the following equation:
238
92
U

4
2
He +
234
90
Th
(Note: in nuclear chemistry, element symbols are traditionally
preceded by their atomic weight (upper left) and atomic
number (lower left).
Beta Radiation (β)
• The transmutation of a neutron into a proton and a
electron (followed by the emission of the electron from
the atom's nucleus: -10 e ). When an atom emits a β
particle, the atom's mass will not change (since there is
no change in the total number of nuclear particles),
however the atomic number will increase by one
(because the neutron transmutated into an additional
proton). An example of this is the decay of the isotope of
carbon named carbon-14 into the element nitrogen:
14
6
C
 0e +
-1
14
7
N
Gamma Radiation (γ)
• Involves the emission of electromagnetic energy
(similar to light energy) from an atom's nucleus.
No particles are emitted during gamma
radiation, and thus gamma radiation does not
itself cause the transmutation of atoms, however
γ radiation is often emitted during, and
simultaneous to, α or β radioactive decay. Xrays, emitted during the beta decay of cobalt-60,
are a common example of gamma radiation.
Half-Life
• Radioactive decay proceeds according to a principal called
the half-life. The half-life (T½) is the amount of time
necessary for one-half of the radioactive material to decay.
For example, the radioactive element bismuth (210Bi) can
undergo alpha decay to form the element thallium (206Tl)
with a reaction half-life equal to five days.
• If we begin an experiment starting with 100 g of bismuth in a
sealed lead container, after five days we will have 50 g of
bismuth and 50 g of thallium in the jar. After another five
days (ten from the starting point), one-half of the remaining
bismuth will decay and we will be left with 25 g of bismuth
and 75 g of thallium in the jar. As illustrated, the reaction
proceeds in halfs, with half of whatever is left of the
radioactive element decaying every half-life period.
• The fraction of parent material that remains after radioactive
decay can be calculated using the equation:
• Fraction remaining = 1
2n
( n = # half-lives elapsed)
• The amount of a radioactive material that remains after a
given number of half-lives is therefore:
• Amount remaining = Original amount * Fraction
remaining
• The decay reaction and T½ of a substance are specific to
the isotope of the element undergoing radioactive decay.
For example, Bi210 can undergo a decay to Tl206 with a
T½ of five days. Bi215, by comparison, undergoes b decay
to Po215 with a T½ of 7.6 minutes, and Bi208 undergoes
yet another mode of radioactive decay (called electron
capture) with a T½ of 368,000 years!
Stimulated Nuclear Reactions
• While many elements undergo radioactive
decay naturally, nuclear reactions can also
be stimulated artificially. Although these
reactions also occur naturally, we are most
familiar with them as stimulated reactions.
There are two such types of nuclear
reactions:
Nuclear Fission
• Reactions in which an atom's nucleus splits into smaller
parts, releasing a large amount of energy in the process.
Most commonly this is done by "firing" a neutron at the
nucleus of an atom. The energy of the neutron "bullet"
causes the target element to split into two (or more)
elements that are lighter than the parent atom.
• During the fission of U235, three neutrons are released in
addition to the two daughter atoms. If these released
neutrons collide with nearby U235 nuclei, they can
stimulate the fission of these atoms and start a selfsustaining nuclear chain reaction. This chain reaction is
the basis of nuclear power. As uranium atoms continue to
split, a significant amount of energy is released from the
reaction. The heat released during this reaction is
harvested and used to generate electrical energy.
• Two Types of Chain Reactions
Nuclear Fusion
• Reactions in which two or more elements "fuse" together to
form one larger element, releasing energy in the process. A
good example is the fusion of two "heavy" isotopes of
hydrogen (deuterium: H2 and tritium: H3) into the element
helium.
• Fusion reactions release tremendous amounts of energy
and are commonly referred to as thermonuclear
reactions. Although many people think of the sun as a large
fireball, the sun (and all stars) are actually enormous fusion
reactors. Stars are primarily gigantic balls of hydrogen gas
under tremendous pressure due to gravitational
forces. Hydrogen molecules are fused into helium and
heavier elements inside of stars, releasing energy that we
receive as light and heat.
• Nuclear Fusion Simulation