ch 9 nuclear decay
DESCRIPTION
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 c an change as number of neutrons changes for each isotope. - PowerPoint PPT PresentationTRANSCRIPT
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
He42
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
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
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