Chapter 11

1. Chapter 11 Radioactive Elements

2. Accidental Discovery of Nuclear Radiation • In 1896, French scientist Henri Becquerel accidentally left some uranium samples on a photographic plate & found outlines of the substance on the film • at first, he thought the uranium was giving off X-rays because the photographic paper had been exposed to sunlight • but when it worked without the sun, he hypothesized that the uranium had given off some invisible energy that had never been detected before, later called radiation

3. Background Info • it was known that certain substances glowed when exposed to sunlight (fluorescent) • but his experiment worked even when it wasn’t sunny • he determined that the element uranium was the source of nuclear radiation

4. Other Discoveries of Nuclear Radiation • Later, in 1898, Marie Curie & her husband, Pierre, discovered two other radioactive elements they named polonium & radium

6. An element that gives off nuclear radiation is said to be radioactive. Properties include: • nuclear radiation from radioactive elements will alter photographic film • they produce fluorescence • electric charge can be found in the air surrounding radioactive elements • nuclear radiation damages cells in most organisms

7. Fluorescence

8. The Nucleus • contains protons & neutrons held together by strong forces • strong forces are short-range forces that only work over a very short distance; therefore, the bigger the nucleus, the farther apart the protons & neutrons • as long as the protons & neutrons remain together due to the strong force, the nucleus remains stable • the bigger the nucleus, the more unstable it becomes

10. Radioactivity • when the strong force is not large enough to hold the nucleus together tightly, the nucleus can decay & give off matter & energy • this process where the nucleus breaks apart or decays is called radioactivity • all elements with 83 or more protons are radioactive

11. Binding Energy • the energy required to break up the nucleus • if the binding energy within the nucleus is weak, the nucleus is unstable & will break apart or decay

12. Unstable • synthetic elements are unstable & decay quickly after being created in the laboratory • many isotopes of elements (no matter the nucleus size) are also radioactive • they are sometimes called radioisotopes

13. Isotopes • an atom of an element that has the same number of protons, but a different number of neutrons • many elements can have both radioactive & nonradioactive isotopes (see pg. 271 for examples)

15. 3 Types of Nuclear Radiation • alpha radiation (particles) • beta radiation (particles) • gamma radiation (waves of energy)

16. 3 Types of Radiation

17. Alpha Particles • made of 2 protons + 2 neutrons • has an electric charge of +2 • had an atomic mass of 4 • more massive & most electric charge • lose energy more quickly when interacting with matter • least penetrating (cannot pass through a piece of paper) • very damaging to biological molecules within the body

19. Alpha Decay • when an atom loses an alpha particle, it is no longer the same element because the number of protons is different • the new element formed has an atomic number two less than the original radioactive element, and a mass number that is four less (it lost two neutrons too)

22. Transmutation • the process of changing one element to another through nuclear decay • the atomic mass at the beginning of the equation must equal the atomic mass at the end

23. How Smoke Detectors Work • when alpha radiation passes through matter, they exert an electrical force on the electrons • this force pulls the electrons away fro the atoms, giving the atom a positive charge (cation)

24. How Smoke Detectors Work • some smoke detectors will give off alpha particles to ionized the surrounding air • normally, an electrical current can pass through the ionized air to form a circuit • when smoke particles enter the ionized air, they absorbs the ions & electrons, thus breaking the circuit causing the alarm to go off

25. Beta Particles • move much faster than alpha particles & are more penetrating • these particles are stopped by Al foil • when a neutron in an unstable nucleus decays into a proton, it emits an electron called the beta particle • process is known as beta decay • caused by weak forces

27. Beta Decay/Transmutation • now that an extra proton was produced in the beta decay, the atom changes to another element • although the atom of this new element has a different atomic number, it still has the same atomic mass because the atom lost a neutron & gained a proton

28. Beta Decay

29. Gamma Rays • most penetrating form of radiation • not made of protons, electrons, or neutrons • carry electromagnetic waves of energy • have no mass, no charge • move at the speed of light • stopped by thick, dense materials such as lead or concrete

31. Gamma Decay • alpha & beta decay are accompanied by gamma decay, the release of a gamma ray from the nucleus • since there is no mass or charge, the nucleus does not change into a different nucleus • since energy leaves, the nucleus moves to a lower energy state

35. Electromagnetic Spectrum

36. Decay Series • the series of steps by which a radioactive nucleus decays into a nonradioactive nucleus • spontaneous breakdown continues until a stable nucleus is formed

40. Artificial Transmutation • bombard atomic nuclei with neutrons, alpha particles, or other nuclear “bullets” • done in particle accelerators because of the need to hit the target nuclei with enough force with these high-energy particles

42. Particle Accelerator

43. Artificial Transmutation • first done by Ernest Rutherford who discovered the nucleus of the atom • Italian scientist Enrico Fermi was the first to use neutrons instead of charged particles • neutrons cause the nucleus to disintegrate, get trapped inside or pass right through • can be used to produce radioactive isotopes of natural elements (iodine)

46. Radioactive Half-Life • a measure of the time required by the nuclei of an isotope to decay • the half-life of a radioactive isotope is the amount of time it takes for half the nuclei in a sample of the isotope to decay • time varies from isotope to isotope • the nucleus left after the isotope decays is called the daughter nucleus

49. Radioactive Dating • the ages of materials such as rocks & fossils can be dated using radioactive isotopes & their half-lives • different isotopes are useful in dating different types of materials