1. The Atomic Nucleus Chapter 4

2. Cathode Ray Led to the Discovery of Radioactivity Roentgen (1896) discovered a new kind of ray in the cathode ray tube that were not affected by a magnetic field and could pass through opaque materials Fell on a photographic plate that was wrapped up tight to keep light off and the plate became exposed Called them X-rays

3. Becquerel Found Another Kind of Radiation Experimented to see if phosphorescent substances (glow in the dark after being exposed to light) emitted x-rays Uranium exposed the photographic plate without being exposed to light or any other sort of energy So the rays were not x-rays or phosphorescence

4. Marie Curie Found other elements that also emitted radiation Divided uranium ore into fractions that gave off high levels of radiation Called this emission radioactivity Isolated purified samples of 2 new radioactive substances Named them Polonium and Radium

5. 3 Major Products of Radioactivity Alpha, Beta and Gamma Rays Rutherford discovered 2 major forms of radioactivity he identified as alpha rays and beta rays Alpha rays are positively charged particles and are composed of 2 protons and 2 neutrons (nucleus of a helium atom) Beta rays are identical to cathode rays (another name for electron ejected from an atom) Gamma rays were discovered by other investigators. Have no electric charge and no mass. Just extremely energetic form of non- visible light

6. Alpha Particles Do not easily penetrate solid material b/c relatively large and a double positive charge (2+) Huge kinetic energy so can cause significant damage to surface of material (especially living tissue) As they travel through air, pick up electrons to become harmless helium Most helium atoms were once alpha particles ejected from a radioactive element

7. Beta Particles Faster than alpha particles and not easy to stop Can penetrate light materials like paper and clothing and fairly deep into skin where they kill cells Can’t penetrate denser materials like aluminum. Once stopped, become part of material just like any electron

8. Gamma Rays Pure energy like visible light Higher energy than visible light No mass or electric charge so can penetrate most materials Cannot penetrate dense materials such as lead (lead absorbs them) Destroy cells so more harmful to us than alpha or beta rays

9. Radioactivity is Natural Phenomenon Has been around longer than human race Energy from radioactive substances in Earth’s interior heats water that spurts from geyser Most is background radiation that originates form Earth (higher elevation, higher exposure) Cells are able to repair most damage from radiation if not too severe Can cause cells to alter genetic information resulting in mutation Most mutation is insignificant but can cause cancer and if in reproductive cells can cause mutation in offspring

10. Rems are Units of Radiation Ability of radiation to cause harm in rems Lethal dose of radiation begins at 500 rems All radiation from natural sources and medical sources is a fraction of a rem so use millirem (1/1000) of a rem Average person receives 360 millirems/year Human body is significant source of natural radiation from potassium

11. Radon-222 Inert gas arising from uranium deposits Heavier than air so accumulate in basements through cracks in the floor (can use detector) 7000-30000 cases of lung cancer per year from exposure to radon 1/5 of annual exposure comes from non- natural sources (medical testing, TV sets, coal fired power plants, etc.)

12. Radioactive Isotopes Are Useful as Tracers and for Medical Imaging Tracers-radioactive isotopes put into molecules whose location can be traced by radiation they emit Used in farming to follow fertilizer usage Used in industry to test lubricating properties of motor oil Used in medicine to see how a material is distributed through body. Works because only affected by physical and chemical properties, not radioactivity

13. Radioactivity Results from an Imbalance of Forces in Nucleus Strong nuclear force-holds nucleons together even though positively charged Very strong but only over short distance Repulsive force are long ranged so the strong force overcomes the repulsive force of like charged protons Because strong nuclear force decreases over distance, a large nucleus is not as stable as a small one Can fall apart emitting either high-energy particles or gamma rays called radioactive decay

14. Neutrons Act as nuclear cement holding nucleus together – Protons at a distance repel each other so only close protons are attracted by strong nuclear force – Neutrons do not repel so held strongly by strong nuclear force More protons need more neutrons to help balance the repulsive forces Neutrons are not stable when by themselves – Transforms to a proton by emitting an electron (beta radiation) – Needs protons around to keep this from happening

15. When so many more neutrons than protons, neutrons will change to protons Stability decreases because of increased repulsion of protons Pieces of nucleus break away in form of radiation (alpha radiation) Size of nucleus is limited. All atoms with more than 83 protons are radioactive Small atoms that have imbalance in protons and neutrons can also be radioactive

16. Transmutation When radioactive nucleus emits alpha or beta particle, nucleus is changed (atomic number) When emits alpha particle, loses 2 protons and 2 neutrons becoming a new element U-238 has 92 protons, 146 neutrons Becomes thorium with 90 protons, 144 neutrons (change in mass of 4) Thorium emits beta rays leaving 91 protons and 143 neutrons (no change in mass) becoming palladium

17. U-238 decays to Pb-206

18. Shorter ½ Life, Greater Radioactivity Rate of decay of radioactive isotope measured in half-life (time it takes for half of material in sample to decay) Half-lives are constant and not affected by external conditions U-238 has half life of 4.5 billion years Rate of decay can be measured by Geiger counter

19. Isotopic Dating Measures Age of Material Cosmic rays bombard Earth’s atmosphere Cause atoms in upper atmosphere to transmute Results in protons and neutrons spewing out Protons can pick up an electron from an atom in upper atmosphere and become hydrogen atom Neutrons keep going b/c have no charge and can collide with atoms in lower atmosphere causing transmutation

20. C-14 Dating If nitrogen captures a neutron, becomes isotope of carbon by emitting a proton Becomes carbon-14 (radioactive) Has same properties as carbon-12 Forms carbon dioxide which plants consume As long as plant lives, C-14 is replenished When plant dies, C-14 decays at a constant rate Can estimate how old something is based on current level of radioactivity

21. Nuclear Fission Is the Splitting of the Atomic Nucleus While bombarding sample of uranium with neutrons trying to make heavier element, found evidence that they made barium (~half mass of uranium)