Nuclear Chemistry
Nuclear Chemistry
• Changes in matter originating in the nucleus of
an atom
• All atoms have• p+ and n0 (nucleons: particles that make up the nucleus)
• p+ number is the atomic number = Z
• p+ + n0 = mass number = A
The Nucleus
• Nuclide, is the nucleus of an atom having a specific atomic
number and atomic mass #
• Isotopes: Elements with the same atomic #, but different
mass # due to gaining or losing of neutrons
• Ex: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
• Radionuclides are unstable and spontaneously emit
particles and electromagnetic radiation (EM)
• Radioisotopes- atoms containing these radionuclides
• Transmutation- process by which one element is
converted to another element by the spontaneous
emission of radiation
Large Nuclei
• Nucleus is held together by strong force
• Large nuclei tend to be unstable because
the force is not strong enough to hold it
together
• These nuclei break apart and decay
• All nuclei with 83 or more protons are
radioactive
• Almost all elements with more than 92
protons don’t exist naturally
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 together
• Neutrons play a key role
stabilizing the nucleus.
• Therefore, the ratio of
neutrons to protons is an
important factor.
Neutron-Proton Ratios
For smaller nuclei
(Z 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.
What is Radioactivity?
• When an unstable nucleus emits one or
more particles or energy
• When these particles are emitted, the
element changes to another isotope or to
a different element
• Nuclear radiation refers to radiation
resulting from nuclear changes
Types of Nuclear
Radiation
• Alpha particles
• Beta particles
• Gamma rays
Alpha Particles
• Symbol is
• Actually particles made of 2 p+ and 2 n0,
same as a He nucleus
• +2 charge, most massive of all nuclear
radiation, mass = 4 amu
• Do not travel far, can be stopped by a
sheet of paper
• Can be very dangerous inside human
body – illness and disease
Alpha Particle Emission
Beta Particles
• Symbol is Greek letter, beta (β)
• High speed negatively charged particles
that come from the nucleus – hmmm…
• Neutron (neutral) actually decays to form
a p+ and an e-, e- is ejected from nucleus
• Travel farther than α, but can be
stopped by 3 mm of Al, or 10 mm
of wood
• Can cause damage inside cells
Beta Decay
mk
M
Gamma rays
• Gamma Radiation, γ
• Not made of matter, no charge or mass
• Waves of high electromagnetic energy (photons)
• Usually emitted from nucleus when alpha or beta
decay occurs
• A nucleus de-excites
by emitting a high
energy gamma ray photon
• High energy, can be
stopped by 60 cm of
Al or 7 cm of Pb
Damage Caused
Penetrating Power
Nuclear Decay
• When unstable nuclei emit alpha or beta particles, what
changes?
– Number of protons or neutrons
– New element or new atomic mass
1. The total of the mass numbers on the reactant side
must be the same as the total of the mass numbers on
the product side
2. The total of the atomic numbers on the reactant side
must be the same as the total of the atomic numbers
on the product side
Change During Alpha Decay
• Nucleus gives up 2 p+ and 2 n0
• All mass numbers add up, all atomic
numbers add up
Change During Beta Decay
• Neutron is changed to a proton and an
electron and emitted
• Increases atomic number due to
conversion of a neutron to a proton and
an electron
• So neutron is lost and proton is gained
Gamma Emission
Loss of a -ray (high-energy radiation that
almost always accompanies the loss of a
nuclear particle)
0
0
238 U
92
→
238
Th
90
+ 4He
2
+ 0 (gamma ray)
Types of Radioactive Decay
Electron Capture
(K-Capture)
• In this reaction a nucleus captures one (1) of its own
atom's inner shell electrons which reduces the
atomic number by one.
• This captured electron joins with a proton in the
nucleus to form a neutron.
• After electron capture, an atom contains one less
proton and one more neutron.
1
1
p
+
0
−1
e
1
0
n
Positron Emission:
• Loss of a positron (a particle that has the same
mass as but opposite charge than an electron)
• It is formed when a proton breaks into a neutron
with mass and no charge and this positron with no
mass and the positive charge.
0
1
11
6
C
e
11
5
B
+
0
1
e
Nuclear binding energy in He
• The mass of a helium nucleus, 4.0015 amu, is less than
that of 2 protons and 2 neutrons, 4.0320 amu. The
energy equivalent of the 0.0305 amu mass defect is the
nuclear energy that binds the nucleons together.
• When a nucleus is formed from protons & neutrons, some
mass (mass defect) is converted to energy (binding
energy), related by the Einstein equation, E = mc2
Decay Rates
• Half-life = the time in which half a
radioisotope’s nuclei to decay into products
• After 1 half-life, half of the substance is
unchanged
• After 2 half-lives, ¼ of the substance is
unchanged
• Half-life table on p. 871
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.
Half-Life
• Measure how quickly a substance decays
• Can be anywhere between nanoseconds
to billions of years, depending on nuclear
stability
• C-14 is used to find the age of relatively
recent materials
• C-14 is taken in in tiny fractions while
alive in some molecules of CO2
• C-14 goes through beta decay, so ratio is
compared in living and nonliving things
Half-Life Example
• The half-life of iodine-131 is 8 days.
• If you start with 36 grams of I-131, how
much will be left after 24 days?
• 36 g
1 half-life
8 days
18 g
• 18 g
2 half-lives
16 days
9g
• 9g
3 half-lives
24 days
4.5 g
Classwork/Homework
• WS – Nuclear Decay
• Half-Life problems on the back
Nuclear Fission and
Fusion
Fusion
• Energy can be obtained when two lighter
nuclei (elements) fuse together to form a
larger (more stable) nucleus
• Occurs in stars, including our sun, energy is
produced when hydrogen nuclei undergo
fusion and release TREMENDOUS amounts
of energy
Fusion in the Sun
• Multi-step
process where
two different
isotopes of
hydrogen fuse
together to form
a helium nucleus
and energy in
form of gamma
rays
Fission
• When a heavy nucleus splits into more
stable nuclei of smaller mass
• Neutrons and energy are released
• Occurs spontaneously and in atomic
bombs
Fission -> Chain Reaction
• One neutron can split the nucleus of an
atom
• As that nucleus undergoes fission, it
releases more neutrons
• Neutrons released in the transmutation
strike other nuclei, causing their decay
and the production of more neutrons.
• This can cause a chain reaction
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.
• Therefore, there must be a certain minimum
amount of fissionable material present for the
chain reaction to be sustained: Critical Mass.
• Example:
1n 
• 235
U
+
92
0
140 Ba
56
93 Kr
+ 3 10 n + 36
• Why is this a chain reaction?
– Neturons start, and continue the reaction
Uncontrolled chain
reactions
• This principle is used in nuclear bombs
• Two or more masses of U-235 are
contained in bomb surrounded by
powerful explosive
• When detonated, fission chain reaction
occurs releasing LARGE amount of
energy which causes devastation to
environment and life forms
• Fortunately, concentration of U-235 in
nature is too low to start a chain
reaction, most is more stable form of
U-238
Controlled Chain Reaction
• Not all neutrons released in a fission
reaction succeed in triggering fission
reaction
• Materials that absorb neutrons can be
used to slow chain reaction
• Concept is used in nuclear power plants
to generate energy
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.
• Moderator: water used to
slow down fast moving
neutrons
Dangers of Nuclear
Radiation
• Changes structure of
hemoglobin
• Changes structure of
macromolecules in body
– health is affected
• Particles ingested
through food can
damage linings of organs
• Destroys bone marrow
• Lung cancer (Radon
gas)
• Genetic mutation
Beneficial Uses of Nuclear
Radiation
• Smoke alarms –produce alpha particles
to create electric current
• Controlled doses are used to treat some
cancers – beams of gamma rays
• Radioactive tracers
• Nuclear power – much less pollution,
more efficient (waste must be dealt with,
though)