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NUCLEAR STABILITY
Almost all atoms we’ve encountered
have stable nuclei
– Not radioactive
Radioactive atoms are relatively rare,
which is fortunate since radiation can
be harmful
Radioactive atoms have nuclei that
are disintegrating
So why are some nuclei stable and
others unstable?
The fundamental force that holds
the nucleus together is called
the Strong Nuclear Force.
̵ If the force is strong enough
the nucleus will be stable
One reason the force might not be
strong enough is the number of
protons and neutrons in the nucleus.
– Not all combinations of protons and
neutrons are stable.
– There must be a “magic” number of
each particle to ensure stability
The first 20 stable nuclei follow a
distinct pattern.
– For elements with atomic numbers
between 1 and 20 stable nuclei have
almost equal numbers of protons
and neutrons.
– Beyond 20 protons, nuclei need
increasingly more neutrons than
protons to be stable.
Nuclei are unstable not only if they
contain too few neutrons, but also if
they contain too many.
Radioactive elements/isotopes are
responsible for producing what we
think of as radiation.
There are three different forms of
nuclear radiation
– Alpha Particles emitted
– Beta from nucleus
– Gamma AKA: ionizing radiation
Spontaneous emission of radiation
from an atom is known as
radioactivity.
Alpha Particles
Alpha radiation consists of a stream
of high-energy alpha particles
Consists of 2 protons and 2 neutrons
and is identical to a helium-4 nucleus
Can be represented by the symbol
Alpha Particles
Alpha particles do not have much
penetrating power.
They are able to travel only a few
centimeters through air and are
easily stopped by paper or clothing
Not normally harmful to humans
− Can be very dangerous if source is
ingested
Beta Particles
Beta radiation consists of a stream of
high-speed electrons
A neutron changes into a proton and
an electron
– The proton remains in the nucleus
and the electron is propelled out of
the nucleus
Beta radiation is represented by the
symbol
Beta Particles
The mass number is zero
100 times more penetrating then
alpha radiation
Can damage the skin and tissues
− Ionizing radiation can cause damage
to DNA, which might lead to…
• Apoptosis (cell death)
• Mutation
Gamma Rays
A Gamma ray is highly energetic
light, similar to x-rays
Does not consist of particles
Gamma radiation accompanies
alpha and beta radiation
Much more penetrating than either
of a or b
It is able to penetrate deeply into
solid material, including body tissue
Symbolized by:
Properties of Some Radiations
Alpha Beta Gamma
Property
Radiation Radiation Radiation
Comp- Alpha Beta High-energy
osition Particle Particle photon
4 0
Symbol a, 2 He b, e
-1
Charge 2+ 1- 0
Mass
(amu)
4 1/1837 0
Properties of Some Radiations
Alpha Beta Gamma
Property
Radiation Radiation Radiation
Common Radium- Carbon-14
source 226 Cobalt-60
Appox. 0.05 to 1
energy
5 MeV MeV 1MeV
Low Moderate Very high
Power (0.05mm (4mm body (penetrates body
body tissue) tissue) easily)
Shield- Paper, Lead,
Metal foil
ing clothing concrete
Radioactive Decay
When an atom emits one of these
kinds of radiation, it is said to be
decaying.
An atom may undergo an alpha or
beta decay.
The radiation is called decay because
the nucleus decomposes to form a
new nucleus, called transmutation
The best way to understand a decay
is with a nuclear equation
Alpha Decay Equations
An alpha particle is a particle
composed of 2 protons and 2
neutrons.
With each expulsion of an alpha
particle from the atom’s nucleus the
atom loses 4 units of mass & 2
protons (+2 charged particle)
Any change in #’s of protons changes
the type of atom, this is transmutation.
Beta Decay Equations
If you remember a b is an e- that is
expelled from an atom
This electron is the result of one of the
atom’s neutrons decomposing into a
proton and an electron.
This results in the atom having one
more proton which causes it to
transmutate into a different atom.
Other Nuclear Decay Particles…
There are 2 other types of radioactive
decays observed.
– The antimatter equivalent to a beta
0
particle is called a positron ( +1e )
44
22Ti 44
21 Sc 0
1 e
– The absorption of an electron by a
nucleus, a.k.a. electron capture
44
22 Ti e
0
1
44
21 Sc
Half-Life
Every radioactive isotope has a unique
rate of decay or half-life
– 1 half life is the time required for ½
of nuclei of a radioactive sample
to decay
– After 1 half-life, ½ of the original
nuclei have undergone transmutation
Half lives may be as as short as a
fraction of a second or as long as
billions of years
Half-Life
– Most artificially produced radioactive
isotopes have very short half-lives
– Rapidly decaying isotopes do not
pose long term bio-radiation hazards
to med patients
A simple half-life equation:
n
1
amount left Original amount
2
n thenumber of half - lives
Half-Lifes & Radiation
Radiation
Isotope Half-life emitted
Carbon-14 5730 years b
Potassium-40 1.25x109 b,
Radon-222 3.8 days a
Radium-226 1600 years a,
Thorium-230 75,400yrs a,
Thorium-234 24.1 days b,
Uranium-235 7.0x108yrs a,
Uranium-238 4.46x109yrs a
Half-Life
One radioactive isotope that has a
long half-life is U-238, which decays
through a complex series of radio-
active intermediates to the eventual
stable isotope of Pb-206
– It’s possible to use this method
to date rocks nearly as old as the
solar system
Other Nuclear Reactions
Radioactive decay is only one of
several types of nuclear rxns.
Two other rxns that involves radio-
active particles are fission & fusion
–These rxns deal with the interaction
of nuclear particles not a decay of a
nucleus
Nuclear Fission
In a nuclear fission reaction, a large
nucleus is split into two smaller nuclei
of approximately equal mass
Fission rxns are used to provide what
is commonly called nuclear power.
-carefully controlled fission rxns
In a nuclear reactor, the fission of 4.5g
of U-235 will satisfy a person’s energy
needs for 1 year.
Fission reactors are used for a clean
& efficient source of power.
Nuclear Fission
One fission rxn produces enough
neutrons to start 3 more fission rxns,
which in turn produces the neutrons
needed to start 3 more rxns, and so
on, in a series
called a nuclear
chain reaction.
Nuclear Fusion
In a nuclear fusion rxn, 2 small
nuclei join to form a larger nucleus.
Like a fission rxn, a fusion reaction
converts some of the mass of the
original nuclei into energy- a great
deal of energy
–According to the eqn: E=mc2
Nuclear Fusion
Fusion rxns are hard to initiate and to
control
So far it takes a tremendous amount
of heat to start
Cold fusion is a natural research
opportunity,
–The goal is to
harness the
power of the sun
Using Radiation
Although radiation can be harmful &
should always be handled with care;
it can be used safely & is important
in many procedures
– Radioisotopes called tracers are used
to study chem rxns and molecular
structures
– Also they can be used to study the
inner workings of the body
Using Radiation
One of the reactants is labeled with
a radioisotope and added to the rxn
mixture
After the rxn is complete, the radiation
of the product is measured to
determine the uptake of the tracer
– Using this technique much can be
learned about the reaction
mechanism
Using Radiation
Tracers are used in agricultural
research
– The tracer is introduced to the
substance being tested
– Plants are treated with the radioactive
labeled substance
– Tracer measured to determine the
location of the substance
– Often the tracer is also monitored
in animals that consume the plants,
in water, and in soil.
Using Radiation
Radioisotopes can even
be used to diagnose
diseases
- I-131, is used to detect
thyroid problems
- Tc-99 is used to detect
brain tumors & liver
disorders
- P-32 is used to detect
skin cancer
Tl-207 scan of the
heart
Using Radiation
Radiation has become a routine part
of the treatment of some cancer
- The fast-growing cancer cells are
more susceptible to damage by high-
energy radiation killing the cancer
cells
- Unfortunately if it isn’t used localized
to the cancer cells it can kill healthy
cells as well.
