Transcript Nuclear Chemistry - Rogue Community College

Chemistry 105
You need:
Textbook
“General, Organic, & Biological Chemistry”
Smith, 2nd edition
Online Study Ctr.:
http://connect.mcgraw-hill.com/class/p_loozen_chem_105_2014
“Laboratory Manual for GOB Chem”
Timberlake, 3rd edition
Online Syllabus & Notes access: http://learn.roguecc.edu/science/ploozen/
For exams bring:
Scantron; pencil/eraser; PT, P.S.*
Nuclear Chemistry
Is this really what the Alchemists
were looking for?
Associated with
brain-scan
technology is
the use of small
amounts of
radioactive
substances.
Isotopes/Nuclides
 An isotope is an atom of an element with a specific atomic
number and a different mass number than another isotope
of that element.
 Carbon-12 and Carbon-14 are isotopes of the same element.
 Stable vs. Unstable isotopes
 Unstable isotopes are also known as radioisotopes or
radionuclides!
 Radioactivity/radioactive decay
Marie Curie,
one of the
pioneers in
the study of
radioactivity,
is the first
person to
have been
awarded two
Nobel Prizes
for scientific
work.
Early experiments: 1896 Becquerel
Radioactivity
 Several types of radiation can be spontaneously
emitted from unstable nuclides. Ernest Rutherford
discovered that naturally radioactive materials
emitted three types of radiation:
 Alpha ()
 Beta
()
 Gamma ()
The effect of an
electromagnetic field
on alpha, beta, and
gamma radiation.
What can you
conclude about the
charge of each type?
Alpha radiation
Alpha radiation: an unstable nucleus emits a particle made of
2 p+ and 2 n˚.
What atomic # change occurs for elements that emit an alpha particle?
Atom giving up alpha particle has atomic # reduced by 2
What atomic mass change occurs?
Atom giving up  particle has atomic mass reduced by 4
Alpha particle (a Helium nucleus)
Alpha radiation has
high ionizing power,
but low penetrating power
Alpha particles travel
1/10th speed of light
Nuclear equation showing
alpha decay of uranium
Note what happens to the atomic mass
and atomic # of the products that are formed
Deducing mass # and atomic #
of daughter nuclide
Atomic # 90 = Thorium
Atomic # 88 = radium
Beta radiation
A beta particle is
simply a
high-energy electron
Travels at 9/10ths
speed of light.
100x more
penetrating power
than alpha
The neutron also
forms a proton
Atomic # increases by 1, Atomic mass is unchanged
Positron emission:
a proton turns into a neutron
atomic number decreased by one
Another type of beta radiation:
positron emission
has intermediate
ionizing power &
intermediate
penetrating power.
Gamma radiation is high-energy
electromagnetic radiation
Gamma radiation has no mass, does not change the element.
Often accompanied by alpha and beta emission,
which do change the element's identity.
Travels at speed of light: has high penetration power!
Radioactive Decay
 Process whereby a radionuclide is transformed
into a nuclide of another element as a result of
radiation emission.
 Parent nuclide --> Daughter nuclide + radiation
 Nuclear equations can be written expressing this
process




Alpha decay
Beta decay
Positron emission
Gamma decay
U-238 --> Th-234 + 
Th-234 --> Pa-234 + 
F-18 --> O-18 + +
Tc-99 --> Tc-99
 Decay series: many radionuclides decay in a series
of steps until a stable nucleus is formed
Decay Series for Uranium-238
In the U-238 decay
series, each nuclide
is unstable except
Pb-206.
**Radon-222 (an
intermediate in this
series) is the major
source of natural
radiation exposure for
the average American.
Rate of Radioactive Decay
 Decay rates are measured using the concept
of Half-Life.
 A half -life (t1/2) is the time required for
one-half of a given quantity of a radioactive
substance to undergo decay!
After each half-life
period, the quantity of
material present at the
beginning of the period is
reduced by half.
Examples of Half-Lives

The first sample of Es-252 discovered in
March 1952, completed its 1st half-life by
my date* of birth:)!.

*actual, not based on Kelvins:-)
Calculations
Amount of radionuclide
undecayed after n half-lives
(
)= (
original amount
of radionuclide
)
x
( )
1
2n
This calculation allows you to determine the amounts
of radioactive material that has decayed, the amount
that remains undecayed and the time elapsed for the
decay process!
Alternative eqn:
A = A0 e- t
A = Current amount of radioactivity
A0 = Original amount of radioactivity
e = base natural log (approximately 2.718)
 = the decay constant = 0.693/t1/2 (where t1/2 = half-life)
t = the amount of time elapsed from A0 to A
Alternative alternative method:
counting!:-)
Example
 The half-life of iodine-131 is
8.0 days. How much of a
0.16 g sample of iodine-131
will remain undecayed after a
period of 32 days?
Example
 Strontium-90 has a
half-life of 28.0 years.
How long will it take
for 94% (15/16) of the
strontium-90 atoms to
undergo decay?