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Nuclear Chemistry Review
Isotopes of atoms can be stable or unstable.
Stability of isotopes is based on the number of
protons and neutrons in its nucleus.
Some nuclei are unstable and spontaneously
decay, emitting radiation.
• For small nuclides (atomic numbers 20 or less)
equal numbers of protons and neutrons are
stable.
• For larger numbers, more neutrons are
needed to ensure stability.
Each radioactive isotope has a specific
mode and rate of decay (half life).
• There are 24 nuclides
listed in Table N. Usually
one of these appears on
a question somewhere.
• Remember that they may
not tell you that you can
look it up!
Half-Life Problems
• The key to any half life problem is to figure out
the number of half-lives.
• Often the fraction remaining is given:
• ½ = 1 half life
• ¼ = 2 half lives
• 1/8 = 3 half lives
• 1/16 = 4 half lives
• 1/32 = 5 half lives
How much is left?
How old is it?
How long did it take?
What is the Half-Life?
What is the half-life?
Which Isotope? (What is the half-life?)
Spontaneous decay can involve the release of alpha
particles, beta particles, or gamma radiation from the
nucleus of an unstable isotope.
These emissions differ in mass, charge, and
penetrating power.
• Be able to recognize
and write the symbols
for all of the particles
and radiation emitted
from radioactive
nuclides – they’re all
found on Table O.
Penetrating Power
• The larger the particle, the
smaller the penetrating
power.
• Alphas are the least
dangerous;
• Gamma radiation is the most
dangerous because it is
massless, high energy, and
moving at the speed of light.
Any change in the nucleus of an atom that converts it
from one element to another in called transmutation.
This can occur naturally or can be induced by the
bombardment of the nucleus by high-energy
particles.
• Artificial transmutation equations have two
particles on the left side of the equation.
• One of them is either a neutron or an alpha
particle.
• Natural transmutation has only one particle,
the radioactive nuclide.
• Nuclear transitions can be represented by
equations that include symbols that represent
atomic nuclei (with the mass number and
atomic number), subatomic particles (with
mass number and charge) and/or emissions
such as gamma radiation.
• Be able to distinguish nuclear decay equations
from chemical equations.
• You won’t see mass numbers or atomic
numbers in chemical equations.
• Be able to find some component X in a
transmutation equation.
• The mass numbers and the atomic numbers
on both sides of the equation have to add up.
• For alpha decay, the mass number decreases
by 4 and the atomic number decreases by 2.
• For beta minus decay, the mass number
doesn’t change and the atomic number
increases by one.
• For positron decay (beta plus) the mass
number is unchanged and the atomic number
decreases.
Energy released in a nuclear reaction
(fission/fusion) comes from the fraction
amount of mass converted into energy.
Nuclear changes convert matter into
energy according to E=mc2.
• Fission: Two small atoms (usually hydrogen) make a
larger one (usually helium)
• Fusion: A large atom is split into two smaller atoms by
a neutron. Several neutrons are also produced.
Fission vs Fusion
Energy released during nuclear
reactions is much greater than the
energy released during reactions.
• The mass  energy question is frequently
asked on the regents.
• The mass converted into energy is small, but
the energy produced is enormous.
There are inherent risks associated with
radioactivity and its uses, such as longterm storage and disposal of radioactive
isotopes, and nuclear accidents.
• Often there are reading passages followed by
questions on the free response section.
Radioactive isotopes have many
beneficial uses.
• Radioactive isotopes are used in medicine and
industrial chemistry, e.g., radioactive dating,
tracing chemical and biological processes,
industrial measurement, detection and treatment
of diseases.
• This is another common reading assignment on
the test.
• Medical radioisotopes must have short half-lives,
for obvious reasons.
• Dating rocks requires isotopes with long halflives.
Four Types of Decay Problems
• The key is to find the number of half lives.
• Figuring out the fraction remaining will usually
help you do that:
½ = 1 half life
¼ = 2 half lives 1/8 = 3 half lives
1/16 = 4 half-lives
1/32 = 5 half-lives
• Remember these fractions.
“How much is left?”
• Divide the initial mass in half as many times as
you have half-lives.
“How long did it take?”
• They have to give you the
fraction remaining.
• Once you figure out the
number of half lives,
multiply that number by the
half life (often on Table N)
The Half-Life of an Unfamiliar
Radionuclide
• They must give you the fraction remaining, so
you can figure out the # half-lives. Take the
time elapsed and divide it by that number.
“How much is did you start with?”
• The number of half lifes can be determined by
taking the time elapsed and dividing it by the
half-life.
• Then take the amount remaining and double it
as many times as you have half-lives.