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Ch 9 Nuclear Decay

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.

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Ch 9 Nuclear Decay

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  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 the element • Mass number (A) tells you number of protons + neutrons • can change as number of neutrons changes for each isotope

  2. 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 #

  3. 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

  4. 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 He 4 2

  5. 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 e 0 -1

  6. 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 g 0 or 0

  7. 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

  8. Why decay? • 2 forces inside the nucleus • Repulsion force: protons in the nucleus trying to stay away from each other (one kind of Coulomb 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

  9. Why decay? • If those two forces are not balanced, the nucleus will emit particles until it becomes more stable

  10. Decay Equations • Decay equations are just like chemical equations • Reactant on left • Products on right • All particles have to balance

  11. 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.

  12. 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

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