Transcript Radioactivity & Unstable Nuclei

Unstable Nuclei &
Radioactive Decay
Radioactivity
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Nucleus of an element spontaneously
emits subatomic particles &
electromagnetic waves.
Nucleus changes into a different
element when it does this.
Original nucleus is called “unstable.”
Process is called “decay” or
“transmutation.”
Rutherford & Radioactivity
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1898 – Rutherford began
experiments with radioactivity.
1899 – discovered alpha and beta
“rays” from uranium.
Types of Radiation
Radioactivity – types
Characteristics of Radiation
See Table O
Name
Alpha
particle
Beta
particle
Gamma
radiation
Positron
emission
Symbol
 or 42He or24
 or 0e or 0 or
-1
-1

 or 00
0
+1
or 0e or +
+1
Mass
(amu)
4
Relative
Charge
+2
0
-1
0
0
0
+1
2 Neutrons &
2 Protons.
Charge = +2
Mass = 4
Beta Particle – fast moving electron.
Radioactive atom: Change occurs in nucleus.
Shielding
Can we predict exactly
when an atom will decay?
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NO!
For large #’s of atoms, we CAN
predict how many will decay on
average in a given amount of time.
Which elements are
radioactive?
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All elements past Bismuth in the periodic
table.
If the atomic number is  83, it’s
radioactive!
Other elements may have radioactive
isotopes.
Stability depends on neutron/proton
ratio.
applet
What’s Going on in the Nucleus?
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Electrostatic repulsions between
protons. Want to fly apart.
But protons & neutrons all attracted
to each other by nuclear strong
force.
So having neutrons helps hold a
nucleus together.
Strong Force
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Works best if the nucleus isn’t too
large.
As the nucleus gets larger, need to
have more neutrons to help
counteract the electrostatic
repulsion between the protons.
Eventually, the nucleus is too large
to be stable.
Balancing Act
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Balance exists between
electrostatic repulsive force &
nuclear strong force.
Certain #’s of protons & neutrons
make a stable nucleus. Other #’s of
protons & neutrons are unstable. So
the atom decays.
Beyond Element 83
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No amount of neutrons can hold a
nucleus together once it has more
than 83 protons.
Elements 84 & above are
radioactive.
Stability and the n/p ratio
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For atoms below atomic number =
20, best neutron/proton ratio  1.
As atomic number , atoms need
more neutrons to be stable.
So n/p ratio for stable atoms
increases to 1.5 for big atoms.
Stability Line
Type of radiation
emitted depends on
position relative to
stable nuclei.
Blue: too many
neutrons.
Yellow: not enough
neutrons for the
protons.
Red/Orange: too
many protons and
neutrons
Natural Radioactivity –
Unstable Nuclei Emit Radiation
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Spontaneous nuclear change to attain
good n/p ratio (high stability, low
energy state).
Form a new kind of atom.
Each isotope or nuclide decays in a
certain manner to get a better n/p
ratio. The decay mode is named for
the particle emitted. See Table N.
Nuclear Equations
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Describe the decay process.
reactant or starting side (left) 
product or ending side (right).
 separates two sides
Nuclear vs. Chemical
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How is a nuclear change different
from a chemical change?
NUCLEAR
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Involve a change
in an atom’s
nucleus.
Radioactive atoms
spontaneously
emit radiation and
change into other
kinds of atoms.
Nuclear reactions
involve 1,000,000
X more energy
than ordinary
chemical rxns.
CHEMICAL
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Involve changes in
the outermost
electrons.
1 or more
substances changed
into new
substances.
Atoms are
rearranged, but
their identities do
not change.
Mass  Energy
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In nuclear reactions, mass is
converted into energy.
– Mass Defect is the difference
between the mass of a nucleus and the
sum of the masses of its constituent
particles.
E =
2
mc
Binding Energy
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Energy released when a nucleus is formed
from its constituent particles.
It is a measure of the stability of an
atom formed:
– The higher the binding energy the more
stable the nucleus.
– The lightest and heaviest elements are the
most unstable (low BE)
– Intermediate elements are the most stable
(highest BE).
Binding Energy
Fe-56
Ni-62