Tuesday, May 6, 2008 Questions on homework Questions on homework Turn in homework Turn in homework Quick Quiz Quick Quiz Continue on Chapter 19 Continue on Chapter 19 Lab tomorrow (dress accordingly) Lab tomorrow (dress accordingly) Homework: Pg #12, 13, 14, 15, 23, 26 Homework: Pg #12, 13, 14, 15, 23, 26

Radioactive Decay Radioactive nuclei spontaneously decompose forming a new nucleus and one or more particles Radioactive nuclei spontaneously decompose forming a new nucleus and one or more particles Cannot be turned on and off Cannot be turned on and off Particles produced determine type of decay Particles produced determine type of decay

Alpha Particle Decay Alpha (  ) particle decay Alpha (  ) particle decay Common for heavy radioactive nuclides Common for heavy radioactive nuclides An alpha particle is really a helium nucleus An alpha particle is really a helium nucleus This is the most massive type of particle produced This is the most massive type of particle produced Can’t penetrate skin, but if ingested does a lot of harm. Can’t penetrate skin, but if ingested does a lot of harm.

Beta Particle Decay Beta (  ) particle decay Beta (  ) particle decay A  particle is another term for an electron A  particle is another term for an electron Beta particles have greater penetrating ability than alpha particles, but are much less massive Beta particles have greater penetrating ability than alpha particles, but are much less massive

Gamma Ray Production Gamma (  ) decay Gamma (  ) decay A  ray is a high-energy photon of light and is not a particle A  ray is a high-energy photon of light and is not a particle This often accompanies decays of other types to release excess energy This often accompanies decays of other types to release excess energy Gamma has the most penetrating ability, but because it has no mass or charge doesn’t interact with cells as readily to damage them Gamma has the most penetrating ability, but because it has no mass or charge doesn’t interact with cells as readily to damage them It primarily takes the atom from an excited energy state to a lower energy state with no change in the identity of the atom It primarily takes the atom from an excited energy state to a lower energy state with no change in the identity of the atom

Types of Radioactive Decay Often a radioactive nucleus can’t achieve a stable (nonradioactive) state through a single decay and so an entire decay series is the result. Often a radioactive nucleus can’t achieve a stable (nonradioactive) state through a single decay and so an entire decay series is the result.

Figure 19.1: The decay series.

Types of Radioactive Decay Alpha (  ) particle decay Alpha (  ) particle decay Common for heavy radioactive nuclides Common for heavy radioactive nuclides Results in the loss of 4 in mass number Results in the loss of 4 in mass number Loss of 2 in atomic number Loss of 2 in atomic number Try to write the equation for the alpha decay of thorium-230. Try to write the equation for the alpha decay of thorium-230.

Types of Radioactive Decay Beta (  ) particle decay Beta (  ) particle decay A  particle is another term for an electron. A  particle is another term for an electron. The net effect of  particle production is to change a neutron to a proton and electron. The proton increases Z, and the electron is released as a high energy particle. The mass # does not change. The net effect of  particle production is to change a neutron to a proton and electron. The proton increases Z, and the electron is released as a high energy particle. The mass # does not change.

Types of Radioactive Decay Try to write the equation for the beta decay of Ac-227. Try to write the equation for the beta decay of Ac-227.

Types of Radioactive Decay Gamma (  ) decay Gamma (  ) decay A  ray is a high-energy photon of light and is not a particle. A  ray is a high-energy photon of light and is not a particle. This often accompanies decays of other types to release excess energy. This often accompanies decays of other types to release excess energy. It produces no net change in A or Z. It produces no net change in A or Z.

Types of Radioactive Decay Positron decay Positron decay A positron is a particle with the same mass as an electron, but the opposite charge. A positron is a particle with the same mass as an electron, but the opposite charge. It has the effect of changing a proton to a neutron. It has the effect of changing a proton to a neutron. The result is no change in mass #, and a decrease in one of the atomic #. The result is no change in mass #, and a decrease in one of the atomic #.

Types of Radioactive Decay Try to write the equation for the positron decay of K-38. Try to write the equation for the positron decay of K-38.

Types of Radioactive Decay Electron capture Electron capture Is the process in which one of the inner- orbital electrons is captured by the nucleus. Is the process in which one of the inner- orbital electrons is captured by the nucleus. Has a very similar effect to positron emission. Has a very similar effect to positron emission.

Half-Lives and Radiocarbon Dating

Half Lives Half Life: time required for half of the original sample of nuclei to decay Half Life: time required for half of the original sample of nuclei to decay If a certain radioactive sample contains 1000 nuclei at a given time and 7.5 days later has 500 nuclei, then it has a half life of 7.5 days. If a certain radioactive sample contains 1000 nuclei at a given time and 7.5 days later has 500 nuclei, then it has a half life of 7.5 days. 7.5 days later, it will have days later, it will have 250.

Half Lives A certain type of radioactive nuclide always has the same half-life. A certain type of radioactive nuclide always has the same half-life. Half-lives can be as short as microseconds or as long as billions of years. Half-lives can be as short as microseconds or as long as billions of years. Shorter half-lives indicate greater activity. Shorter half-lives indicate greater activity.

Half Lives Example Example How many half lives does it take for 1 mol to decay to 0.25 mol? How many half lives does it take for 1 mol to decay to 0.25 mol? Two half-lives. Two half-lives.

Radiocarbon dating or carbon-14 dating or carbon-14 dating Carbon-14 decays by  -particle production Carbon-14 decays by  -particle production Carbon-14 is produced in the atmosphere when high-energy neutrons from space (cosmic rays) collide with nitrogen-14 Carbon-14 is produced in the atmosphere when high-energy neutrons from space (cosmic rays) collide with nitrogen-14

Carbon Dating Carbon-14 is continuously produced in this process and it continuously decomposes. Carbon-14 is continuously produced in this process and it continuously decomposes. The two opposite processes have come into balance, so the amount of carbon-14 present in the atmosphere is approximately constant. The two opposite processes have come into balance, so the amount of carbon-14 present in the atmosphere is approximately constant.

Carbon Dating Carbon-14 dating can be used to date wood and cloth artifacts Carbon-14 dating can be used to date wood and cloth artifacts A living plant consumes carbon dioxide in the photosynthesis process and has this isotope in its molecules. A living plant consumes carbon dioxide in the photosynthesis process and has this isotope in its molecules. While it is alive, the C-14 in the plant molecules is equal to that in the atmosphere. While it is alive, the C-14 in the plant molecules is equal to that in the atmosphere. When the tree is cut, there is no more incoming source of C-14 to balance the C-14 that is decaying. When the tree is cut, there is no more incoming source of C-14 to balance the C-14 that is decaying. The C-14 content decreases. The C-14 content decreases.

Carbon Dating Using the known half-life of 5730 years, archaeologists compare the amount in an artifact to the amount currently in trees and figure out the age. Using the known half-life of 5730 years, archaeologists compare the amount in an artifact to the amount currently in trees and figure out the age. A wooden bowl with half as much C-14 as a current living tree would have been through one half life and would therefore be 5730 years old. A wooden bowl with half as much C-14 as a current living tree would have been through one half life and would therefore be 5730 years old.

Carbon Dating Diamonds Diamonds were thought to be only created billions of years ago because no younger diamonds were being found. Diamonds were thought to be only created billions of years ago because no younger diamonds were being found. Found a diamond in Zaire that is 628 million years old- youngest diamond. Found a diamond in Zaire that is 628 million years old- youngest diamond. Diamonds take a long time to surface because they are only formed at depths of 200 km where the high pressures and temperatures favor dense carbon formation. Diamonds take a long time to surface because they are only formed at depths of 200 km where the high pressures and temperatures favor dense carbon formation. Diamonds remain buried until they are brought to the surface by volcanic blasts. Diamonds remain buried until they are brought to the surface by volcanic blasts.