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Ch 9 Nuclear Decay
• Review from ch 4…
• Atomic number (Z) tells you
number of protons
– Always the same for an element; change
the atomic number and you change the
element
• Mass number (A) tells you
number of protons + neutrons
– can change as number of neutrons
changes for each isotope
Isotopes
• An element with a different number of
neutrons
• Because has same number of protons,
still that element and has all
chem/phys properties
• Write isotopes using atomic # & mass #
Radioactivity
• Elements become unstable over
time…it’s a natural process
• To become more stable, they emit
energy or matter or both
• These matter/energy emissions
are called nuclear radiation
• The process is called nuclear
decay or radioactive decay
Alpha (a) Decay
• When 2 protons and two neutrons
are given off
• Basically the nucleus of a Helium
atom
• Decreases the atomic number by
2 and mass number by 4
4
2
He
Beta (b) Decay
• When an electron is given off from
the nucleus
• A neutron decays into a proton
(which stays) and an electron
which leaves the atom
• Doesn’t change the mass number,
atomic number goes up by 1
0
-1
e
Gamma (g) Radiation
• No mass, so atomic number and
mass number don’t change
• Just a photon of light energy in
the gamma wavelengths
g
or
0
0
g
Mass/energy
• Alpha particles
– Have the most mass and the least energy
– Barely pass through paper
• Beta particles
– Less mass and more energy
– Stopped by 3mm metal foil, 10cm wood
• Gamma radiation
– has no mass and the most energy
– Stopped by 60cm foil or 7 cm lead
– Are most damaging
• All three ionize atoms (steal electrons)
as they move through materials.
This is how the damage is done
Why decay?
• 2 forces inside the nucleus
– Repulsion force: protons in the nucleus
trying to stay away from each other
(Coloumb force)
• Acts over far distances so 1 proton on one side of
the nucleus pushes ones on the other side away
– Strong Nuclear Force: one of 4
fundamental forces holds nucleus
together
• Acts over short distances…only on particle next to
it
• That’s why many more neutrons at higher atomic
number…more strong nuclear force
Why decay?
• If those two
forces are not
balanced, the
nucleus will
emit particles
until it
becomes more
stable
Decay Equations
• Decay equations are just like
chemical equations
– Reactant on left
– Products on right
– All particles have to balance
Half Life
• Nuclei decay at a steady,
measurable rate called the half
life (t1/2 )
• Defined as the time it takes for
only ½ the original to remain
(and ½ to decay)
• It decays into another isotope of the
same element, or into another
element. It doesn’t just disappear.
Half life
• You use the ratio of the original product to
the decayed product to get a % that has
decayed.
• Use that
percent and
the half life
to tell how
old
something is