Radioactivity and Nuclear Reactions

1. Radioactivity and Nuclear Reactions Ch 9.1-9.2, 9.4

2. Nucleus and the Strong Force • Protons and neutrons are packed tightly together • Two positives normally repel each other, so why don’t the protons in the nucleus repel? • Strong force = one of four basic forces that causes protons and neutrons to be attracted to each other • 100 times stronger than electric force • Short-range force, so it weakens with distance

3. Small vs Large Nuclei • Protons and neutrons are held together less tightly in large nuclei. Why? • Small nuclei have few protons, so the repulsive force on a proton due to other protons is small • In a large nuclei, the attractive strong force is exerted only by the nearest neighbors. All the protons exert repulsive forces making the repulsive force large.

4. Radioactivity • In many nuclei, the strong force keeps the nucleus together (STABLE) • When it can’t, the nucleus can decay and give off matter and energy in a process of radioactivity • Larger nuclei tend to be unstable – all nuclei containing more than 83 protons are radioactive • All elements with more than 92 protons are synthetic and decay soon after they are created (UNSTABLE)

5. Stable and Unstable Nuclei • Smaller elements neutron to proton ratio is 1:1 to be stable isotopes • Heavier elements neutron to proton ratio is 3:2 to be stable isotopes • Nuclei of any isotopes that differ much from these ratios are unstable, whether heavy or light

6. Nuclear Radiation • When an unstable nucleus decays, particles and energy are emitted from the decaying nucleus • Alpha Particles – (2 p and 2 n lost) massive, comparatively speaking; loses energy quickly; can’t pass through paper; changes the element (transmutation); mass changes; can damage the body • Beta Particles – (n turns into p and emits e) e emitted from n; transmutation changes the element; mass doesn’t change; much faster and penetrating; damage body • Gamma Rays – electromagnetic waves that carry energy; most penetrating form; cause less damage to biological molecules

7. At a glance…

8. Radioactive Half-Life • Some radioisotopes decay in less than a second, while others take millions of years • Half-life: the amount of time it takes for half the nuclei in a sample of the isotope to decay

9. Radioactive Half-Life cont

10. Ch 21.3: Absolute-Age Dating of Rocks • Relative-age dating vs. Absolute-Age Dating • Relative-age dating: compares past geologic events based on the observed order of strata in rock record • Absolute-age dating: determines actual age of a rock, fossil, or other object

11. Radioactive Decay • Radioisotopes are found in igneous and metamorphic rocks, some fossils, and organic remains • Emission of radioactive particles and the resulting change into other elements over time is called radioactive decay • This decay stays constant regardless of the environment, pressure, temperature, or any other physical changes • So, these atomic particles become accurate indicators of the absolute age of an object

12. I love you Mrs. Sjuts! 

13. Radioactive Dating • Fossils and rocks can be dating using radioactive isotopes • Amounts of the radioisotope and its daughter nucleus are measured in a sample • Then, the number of half-lives that need to pass to give the measured amounts of the isotope are calculated • The number of half-lives is the amount of time that has passed since the isotope began to decay AND usually is the same as the age of the object.

14. Carbon Dating • The radioactive isotope C-14 is often used to find the ages of once living objects • It is naturally found in most all living things • An atom of C-14 eventually will decay into N-14 with a half-life of 5,730 years • By measuring the amount of C-14 in a sample and comparing it to the amount of C-12, scientists can determine the approx age of plants and animals that lived within the last 50,000 years

15. Uranium Dating • Some rocks contain uranium, which has two radioactive isotopes with long half-lives, both decaying into isotopes of lead • By comparing the uranium isotope and the daughter nuclei the number of half-lives since the rock was formed can be calculated • U-235  0.7 billion years • U-238  4.5 billion years

16. Ch 9.4 Nuclear Reactions • Nuclear Fission – the process of splitting a nucleus into two nuclei with smaller masses • Chain reaction – ongoing series of fission reactions • Critical mass – the amount of fissionable material required so that each fission reaction produce approximately one more fission reaction • Nuclear Fusion – two nuclei with low masses are combined to form one nucleus of larger mass

18. Nuclear Fission • Large elements need a TON of energy in order to hold their nucleus together. • When the large nucleus is split into smaller nuclei, those smaller nuclei don’t require as much energy to stay together… • So, that leftover energy is released! • Atomic bomb – used in Hiroshima and Nagasaki Fission - Chain ReactionNuclear Fission: Pros and Cons Nuclear Meltdown Cooper Nuclear Station near Brownville, NE Fort Calhoun Nuclear Generating System between Ft. Calhoun and Blair

19. Nuclear Fusion • Need very high temperatures in order to overcome the repulsive forces. Sun's Fusion • Scientists cannot control fusion reactions for the purpose of power. • We can, however, use it to make nuclear weapons. Large ones. Hydorgen Bomb - Fusion

20. Nuclear Decay vs. Nuclear Reactions • Decay happens spontaneously • Reactions are controlled and self-sustaining and release much more energy

21. Nuclear Reaction: Plutonium • Pu-239 Used to make nuclear weapons like the one dropped on Nagasaki in 1945