Chapter 21 Lecture- Nuclear Chemistry

Nuclear Chemistry

The nucleus of an atom
contains
protons (+1charge)
and
neutrons (no charge)

The nucleus is held together
by the strong nuclear force
The strong nuclear force is the strongest
force known
Protons and neutrons are very close
together
They exchange a teeny bit of mass back
and forth.
When disrupted, the mass is converted
to energy according to E=mc2
The mass is tiny. The energy is
immense.

Protons and neutrons experience
the strong nuclear force if close
enough

Because protons repel each other
the nucleus needs a certain proton
to neutron ratio for stability

An unstable atom decays by
emitting radiation

These unstable atoms are radioactive.
Carbon-12 is stable. Carbon-14 is
radioactive.
Carbon-14 is a radioisotope.
There are many naturally occurring
radioisotopes and some that are human-
made.

Radioisotopes decay into
stable isotopes of a
DIFFERENT element
In nuclear reactions, the makeup of the
nucleus changes.
Sometimes the number of protons will
change.
If the number of protons changes, the
element has changed.

The three most common
forms of radioactive decay are
alpha(α), beta(β) and
gamma(γ)

25.1

Alpha particles are the least
penetrating.
Gamma rays are the most
penetrating.

To balance nuclear equations
you must know the symbol for
the emitted particle

You must balance the atomic number (number on
bottom) and the atomic mass (number on top)
Here 222 + 4 = 226 and 86 + 2 = 88 its balanced

The stuff that radiates
An alpha particle is a helium nucleus it
has a +2 charge and a mass of 4amu.

A beta particle is an electron
which is formed when a
neutron becomes a proton

A gamma ray is a high energy
electromagnetic wave.
A gamma ray has no mass or
charge so it is not in a nuclear
equation.

particle Alpha α Beta β Gamma γ
Mass 4amu 0 0
Charge +2 -1 0
Effect Radioisotope Radioisotope Radioisotope
loses two converts a neutron loses energy
protons and two to a proton &
neutrons ejects an electron The element
The ELEMENT The ELEMENT does not
changes changes change
What it is Helium electron High energy
nucleus electromagnetic
radiation
Stop it Paper/skin 1cm/metal foil Lead/concrete
damage High ionization Medium Lowest
ionization ionization

Damage from nuclear
radiation is due to ionization of
living tissue.
Nuclear radiation is called ionizing
radiation because it produces ions from
neutral molecules.

Alpha radiation has a low penetration, but
it is the most damaging to living tissue
because it deposits all its energy along
a short path

Nuclear Fission

Fission separates heavy elements into two lighter
elements.
Uranium-235  Barium-141 + Krypton-92

+3neutrons

A huge amount of energy is produced.
Used by humans as an energy source
The fission bomb was used in WWII

Nuclear fusion

Fusion combines two light nuclei into one
heavier element.

Produces even more energy than fission.
Occurs in the sun.
Requires extremely high temperatures
and pressures.

FUSION = to put together
FISSION = to break apart

The Nucleus

• Remember that the nucleus is comprised of
the two nucleons, protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together
is effectively the mass of the atom.

Isotopes

• Not all atoms of the same element have
the same mass due to different
numbers of neutrons in those atoms.
• There are three naturally occurring
isotopes of uranium:
Uranium-234
Uranium-235
Uranium-238

Radioactivity

• It is not uncommon for some nuclides of
an element to be unstable, or
radioactive.
• We refer to these as radionuclides.
• There are several ways radionuclides
can decay into a different nuclide.

Alpha Decay:

Loss of an α-particle (a helium nucleus)
4
2 He
238 → 234 4
92 U 90 U + 2 He

Beta Decay:

Loss of a β-particle (a high energy electron)

0 0
−1 β or −1 e
131 131 0
53 I →
54 Xe + −1 e

Positron Emission:

Loss of a positron (a particle that has the
same mass as but opposite charge than
an electron)
0
1 e
11 11 0
6 C →
5 B + 1 e

Gamma Emission:

Loss of a γ-ray (high-energy radiation
that almost always accompanies the loss
of a nuclear particle)
0
0 γ

Electron Capture (K-Capture)

Addition of an electron to a proton in the
nucleus
As a result, a proton is transformed into a
neutron.
1 0 1
1 p + −1 e →
0 n

Neutron-Proton Ratios

• Any element with more
than one proton (i.e.,
anything but hydrogen)
will have repulsions
between the protons in
the nucleus.
• A strong nuclear force
helps keep the nucleus
from flying apart.


• Neutrons play a key role
stabilizing the nucleus.
• Therefore, the ratio of
neutrons to protons is an
important factor.


For smaller nuclei
(Z ≤ 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.

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.

Stable Nuclei

• Nuclei below the belt
have too many
protons.
• They tend to
become more stable
by positron emission
or electron capture.

Stable Nuclei

• There are no stable nuclei with an
atomic number greater than 83.
• These nuclei tend to decay by alpha
emission.

Chapter 21 Lecture- Nuclear Chemistry

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).

Some Trends

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.

Some Trends

Nuclei with an even
number of protons
and neutrons tend to
be more stable than
nuclides that have
odd numbers of
these nucleons.

Nuclear Transformations

Nuclear
transformations
can be induced
by accelerating
a particle and
colliding it with
the nuclide.

Particle Accelerators
These particle accelerators are enormous,
having circular tracks with radii that are
miles long.

Kinetics of Radioactive Decay

• Nuclear transmutation is a first-order
process.
• The kinetics of such a process, you will
recall, obey this equation:

Nt
ln = kt
N0
ln[A]t – ln[A]0 = -kt for 1st order kinetics


• The half-life of such a process is:
Not given, but can
0.693 be derived from
= t1/2 previous equation
k
with [A]t equal to
half of [A]0.

• Comparing the amount of a radioactive
nuclide present at a given point in time
with the amount normally present, one
can find the age of an object.

First order processes (including nuclear decay)
always show a constant half-life

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.


A wooden object from an archeological site
is subjected to radiocarbon dating. The
activity of the sample that is due to 14C is
measured to be 11.6 disintegrations per
second. The activity of a carbon sample of
equal mass from fresh wood is 15.2
disintegrations per second. The half-life of
14C is 5715 yr. What is the age of the

archeological sample?

First we need to determine the rate
constant, k, for the process.
0.693
= t1/2
k
0.693
= 5715 yr
k
0.693
=k
5715 yr
1.21 × 10−4 yr−1 = k

Now we can determine t:
Nt
ln = kt
N0
11.6
ln = (1.21 × 10−4 yr−1) t
15.2
ln 0.763 = (1.21 × 10−4 yr−1) t
6310 yr = t

Energy in Nuclear Reactions

• There is a tremendous amount of
energy stored in nuclei.
• Einstein’s famous equation, E = mc2,
relates directly to the calculation of this
energy.


• In the types of chemical reactions we
have encountered previously, the
amount of mass converted to energy
has been minimal.
• However, these energies are many
thousands of times greater in nuclear
reactions.


For example, the mass change for the decay
of 1 mol of uranium-238 is −0.0046 g.
The change in energy, ΔE, is then
ΔE = (Δm) c2
ΔE = (−4.6 × 10−6 kg)(3.00 × 108 m/s)2
ΔE = −4.1 × 1011 J

Nuclear Fission

• How does one tap all that energy?
• Nuclear fission is the type of reaction carried
out in nuclear reactors.

Nuclear Fission

• Bombardment of the radioactive nuclide with
a neutron starts the process.
• Neutrons released in the transmutation strike
other nuclei, causing their decay and the
production of more neutrons.

Nuclear Fission

This process continues in what we call a
nuclear chain reaction.

Nuclear Fission

If there are not enough radioactive nuclides in the
path of the ejected neutrons, the chain reaction
will die out.

Nuclear Fission

Therefore, there must be a certain minimum
amount of fissionable material present for the
chain reaction to be sustained: Critical Mass.

Nuclear Reactors
In nuclear reactors the heat generated by the
reaction is used to produce steam that turns a
turbine connected to a generator.

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 reaching
a dangerous supercritical
mass.

Nuclear Fusion
• Fusion would be a superior
method of generating
power.
 The good news is that the
products of the reaction are
not radioactive.
 The bad news is that in
order to achieve fusion, the
material must be in the
plasma state at several
million kelvins.

Nuclear Fusion
• Tokamak apparati like the
one shown at the right
show promise for carrying
out these reactions.
• They use magnetic fields
to heat the material.

Chapter 21 Lecture- Nuclear Chemistry

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Chapter 21 Lecture- Nuclear Chemistry