ch 9 nuclear decay review from ch 4… atomic number (z) tells you number of protons – always the...
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![Page 1: 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](https://reader030.vdocument.in/reader030/viewer/2022032806/56649f0c5503460f94c1fe8f/html5/thumbnails/1.jpg)
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
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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 #
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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
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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
He42
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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
e0
-1
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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 00or g
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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
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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
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Why decay?• If those two
forces are not balanced, the nucleus will emit particles until it becomes more stable
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Decay Equations• Decay equations are just like
chemical equations– Reactant on left– Products on right– All particles have to balance
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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.
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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