Transcript Nuclear Chemistry

Unit 15
Overview

Nuclear Chemistry

 Isotopes


 Half-life
 Nuclear force

Radioactive decay

 Alpha, beta, gamma

decay
 Positron emission
 Electron capture
Nuclear Stability
Radiometric Dating
Nuclear fusion
Nuclear fission
Nuclear energy
 Mass Defect
 Nuclear binding
energy
Nuclear Chemistry
Involves the change in the nucleus of an atom
 Nuclear reactions are everywhere

 Produce sunlight
 Create elements (synthetic and natural in stars)
 Radiation therapy (cancer treatment)
 Generate electricity
 Nuclear weapons
World Energy Use
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
Mass number
(p+ + no)
12
6
Atomic number
(number of p+)
C
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 force
 Holds 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 radiation

 Radioactive decay

Elements with more
than 83 protons
(bismuth) are
naturally radioactive
Radioactive Decay
Radionuclides: Radioactive elements
 During radioactive decay

 The makeup of the nucleus changes
 The number of protons may change
○ Means that the element has changed
Natural Radioactive Isotopes

Radon-222
 Comes from decomposition of Uranium rocks
 2nd leading cause of lung cancer
 Comes up through cracks in basements

Radium-226
 Some radium salts glow in the dark
 Early 1900s used to be used as paint for watches and
clocks (workers licked paint brushes and got cancer –
“radium girls”)

Uranium-238
 Rocks create radon gas
 Used in radioactive dating

Potassium-40
 One of few light radioactive elements
 Produces argon that is found in atmosphere
Other Common Radioisotopes
Isotope
14C
24Na
32P
51Cr
Use
Archaeological dating
Circulatory system testing for obstruction
Cancer detection
Determination of blood volume
59Fe
Measurements of red blood cell formation and lifetimes
60Co
Cancer treatment
Measurement of thyroid activity
Measurement of bone density
Cancer treatment
Archaeological dating
Nuclear reactors and weapons
Archaeological dating
Smoke detectors
131I
153Gd
226Ra
3H
235U
238U
241Am
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 radiation
 High energy photons
Alpha, Beta, Gamma Radiation
Alpha Decay:
Loss of an a-particle (a helium nucleus)
4
2
238
92
U
He or a

4
2
234
90
4
2
Th + He
Beta Emission:
Loss of a b-particle (a high energy electron)
0
−1
131
53
I
b

or
131
54
0
−1
e
Xe
+
0
−1
e
Gamma Emission:
Loss of a g-ray
 High-energy radiation that almost always
accompanies the loss of a nuclear particle

 Not usually written in nuclear equation
0
0
238
92
g
U  He 
4
2
234
90
Th  g
0
0
Positron Emission:
Loss of a positron (a particle that has the
same mass as but opposite charge of an
electron)
b
or

11
5
0
1
11
6
C
0
1
B
e
+
0
1
e
Has a very short life because it is destroyed when it collides with an
electron, producing gamma rays:
0
1
e + 0-1 e  00 g
Positron Emission

A positron can convert a proton to a
neutron
1
1
p

1
0
n
+
0
1
e
Electron Capture

Capture by the nucleus of an electron from the
electron cloud surrounding the nucleus
 Addition of an electron to a proton in the nucleus
 As a result, a proton is transformed into a neutron
1
1
p
+
0
−1
e

1
0
n
Nuclear Stability

Several factors predict
whether a particular
nucleus is radioactive
 Neutron-to-proton ratio
 Radioactive series
 Magic Numbers
 Evens and Odds
Neutron-Proton Ratios

The strong nuclear force helps keep the
nucleus from flying apart
 Protons repel each other
 Neutrons help the strength of the nuclear force

As protons increase, neutrons have to
counter-act increasing proton-proton
repulsions
 In low atomic number elements (1-20) protons and
neutrons are approximately equal
 In 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.
Neutron-Proton
Ratios
As nuclei get larger, it
takes a greater
number of neutrons
to stabilize the
nucleus.
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 166 C)
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 116 C)
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:

0.693
= t1/2
k
Radiometric Dating

Half life can help determine the age of different
objects

Carbon-14
 Half life of 5,715 years
 Can determine age of organic materials up to about
50,000 years old
Radiometric Dating

Uranium-238
 Half life of 4.5×109 years
 Used 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 particles
 Alpha particles accelerated and collided with
nucleus
 Neutrons bombard nucleus

Bombard nuclei to create transuranium
elements
 Heavy 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 Weaponry
 Nuclear 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 quickly
 If 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 reactors
 The reaction is kept in check by the use of
control rods
 These 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…

Cons…
 Cleaner energy than coal
 Nonrenewable source of
and fossil fuel plants
 Doesn’t add to global
warming
energy
 Produces nuclear waste
that must be stored for
thousands of years
 Accidents (Chernobyl,
Three Mile Island,
Fukushima)
 High amount of electricity
can be generated in one
plant
○ http://www.youtube.com/watch?v
=eGI7VymjSho
 Cheaper to run a nuclear
facility than a fossil fuel
plant
 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
conserved
 Ex: Mass change for exothermic process of combustion
of 1 mol of CH4 is -9.9×10-9 grams

Mass change in nuclear reactions is measureable
 Ex: 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 2 protons (2×1.0073 = 2.0146)
Mass of 2 neutrons (2×1.0087 = 2.0174)
Total mass = 4.0320 amu
Mass of Nucleus
4.0015 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
