Radioactivity(2nd half of Ch 39)

39.5 Radioactive Half-Life

• Radioactive isotopes decay at different rates. • The radioactive decay rate is measured in

terms of a half-life.• The half-life of a substance is the time needed

for half of the radioactive atoms to decay.• Some half-lives are less than a millionth of a

second. But some, like Uranium-238, has a half-life of 4.5 billion years.

39.5 Half-Life cont.

• If half-lives are so long, how do scientists measure them?

• They can measure the rate at which a substance decays, using radiation detectors.

• The half-life of an isotope is related to its rate of disintegration (a half-life can be computed from the rate of disintegration.)

• Stop: answer question 1 on pg 616!

39.6 Transmutation

• When a nucleus emits an alpha or beta particle, a different element is formed.

• The changing of one element to another is called transmutation.

• For example: Uranium has 92 protons. When an alpha particle is ejected during decay, the nucleus is reduced by two protons and two neutrons. The new element is Thorium (atomic number 90).

Transmutation cont.• The Thorium is also radioactive, and decays by

Beta particles. • When a beta particle is ejected, one of the

neutrons spontaneously changes into a proton.• The new element is now Protactinium (atomic

number 91).• Eventually, Uranium decays to harmless and non-

radioactive lead (atomic number 82). See Figure 39.15 pg 619.

• STOP: Answer question 2 on pg 618.

39.7 Artificial Transmutation

• Ernest Rutherford in 1919 was the first to succeed in artificially transmuting an element.

• He bombarded nitrogen nuclei with alpha particles and found oxygen and hydrogen atoms that were not there before.

• Artificial transmutation can be used to convert harmful radioactive waste into nonradioactive elements.

39.8 Carbon Dating

• Taking carbon out to Applebees. (hahaha) • When cosmic rays from outer space hit our upper

atmosphere, transmutation takes place and protons, neutrons and electrons are created.

• The protons combine with electrons to make hydrogen, but the neutrons find their way down to Earth’s lower atmosphere and react with Nitrogen to form Carbon-14 and Hydrogen.

39.8 Carbon Dating cont.• Plants take in some of this radioactive Carbon-14.• Animals eat plants (or eat other animals that eat

plants), so animals get Carbon-14 in them.• Carbon-14 is a beta emitter, and decays back into

nitrogen.• When an animal or plant dies, the amount of

Carbon-14 it has stops being replenished.• The Carbon-14 in the dead plant or animal decays at a

known rate of 5730 years for a half-life.• The ratio of regular Carbon-12 to Carbon-14 can then

be used to determine how long ago the organism lived.• STOP: Answer question 2 on pg 622.

39.9 Uranium Dating

• The dating of older, non-living things is accomplished with uranium instead of Carbon-14.

39.10 Radioactive Tracers

• Small amounts of radioactive elements can be added to things to determine how well that thing is working.

• Examples: it can be added to fertilizer to see how much is being taken up into the plants, medicine to see how much is being absorbed, sewage lines to see where the leak in the underground pipe is, etc.

• See Figure 39.19 pg 624.

39.11 Radiation and You

• Radiation has been around since before humans.

• Radiation is what warms the planet and keeps the core molten.

• Even the helium in a balloon is the product of radioactivity.

39.11 Radiation and You cont.

• Much of the radiation we are exposed to is cosmic radiation from space.

• The atmosphere stops a lot of this radiation.• At higher elevations, radiation is more

intense. For example, in Denver, you get twice the radiation than you do at sea level.

• A couple of flights between NY and San Francisco expose you to as much radiation as a chest X-ray.

39.11 Radiation and You cont.

• Radiation damages cells.• Cells can repair most kinds of molecular

damage if the radiation the cells are exposed to is not too intense.

• But if you are exposed to intense radiation, or moderate radiation over a long period of time, cancer and a shorter life expectancy can result.

Assignment

• Worksheet!