RadioactiveDecayMaplesdenNoakesWebsite11放射性衰變maplesden諾克斯網(wǎng)站課件

1、第1頁，共29頁。第2頁，共29頁。Atoms are made of three basic building blocks called protons, neutrons and electrons. In any atom, the number of electrons is equal to the number of protons and so the overall charge of an atom is zero. What are atoms made of?There are two properties of protons, neutrons and electr

2、ons that are especially important: mass and charge.-1almost 001+11electronneutronprotonChargeMassParticle第3頁，共29頁。Particles in the modern model第4頁，共29頁。What makes a carbon atom carbon?The atoms of any particular element always contains the same number of protons. In the periodic table, there are two

3、 numbers found with each element. What do these numbers represent? Carbon atoms always have six protons. Atoms with different numbers of protons must be other elements. For example:all atoms with 1 proton are hydrogen atoms;all atoms with 17 protons are chlorine atoms.Atomic number (or proton number

4、) is the number of protonsMass number is the number of protons + the number of neutrons.第5頁，共29頁。All carbon atoms have the same number of protons, but not all carbon atoms are identical.mass number is differentatomic number is the sameAlthough atoms of the same element always have the same number of

5、 protons, they can have different numbers of neutrons. Atoms that differ in this way are called isotopes. What are isotopes?For example, carbon exists as three different isotopes: carbon-12, carbon-13 and carbon-14:Potassium is another element that exists as three different isotopes: potassium-39, p

6、otassium-40 and potassium-41.第6頁，共29頁。Atomic structure key words第7頁，共29頁。第8頁，共29頁。Types of radioactive decay第9頁，共29頁。An alpha particle consists of two protons and two neutrons. It is the same as a helium nucleus.When an atoms nucleus decays and releases an alpha particle, it loses two protons and tw

7、o neutrons. atomic number decreases by 2mass number decreases by 4What happens during alpha decay?The number of protons has changed, so the decayed atom has changed into a new element.238922349024UTh+第10頁，共29頁。An beta particle consists of a high energy electron, which is emitted by the nucleus of th

8、e decaying atom.When an atoms nucleus decays and releases a beta particle, a neutron turns into a proton, which stays in the nucleus, and a high energy electron, which is emitted. What happens during beta decay?The decayed atom has gained a proton and so has changed into a new element.atomic number

9、increases by 1mass number remains the same146147CN+第11頁，共29頁。Gamma radiation is a form of electromagnetic radiation, not a type of particle. When an atoms nucleus decays and emits gamma radiation, it releases energy in the form of electromagnetic radiation.What happens during gamma decay?Gamma rays

10、are usually emitted with alpha or beta particles. For example, cobalt-60 decays releasing a beta particle. The nickel formed is still not stable and so emits gamma radiation.6028Ni*6028Ni+The nickel does not change into a new element. There is no change to the make up of the nucleus and so a new ele

11、ment is not formed.6027Co+第12頁，共29頁。Radioactive decay true or false?第13頁，共29頁。第14頁，共29頁。Radioactivity cannot be seen, it has no smell and does not make any sound so how can it be detected?Radioactivity can be detected with a Geiger counter, which is a Geiger-Mller (GM) tube connected to a ratemeter.

12、The ratemeter gives a reading in counts per second and a loudspeaker clicks for each particle, or burst of radiation, detected by the GM tube.How can radioactivity be measured?GM tuberatemeterIt can also be used to measure the amount of radiation.第15頁，共29頁。What happens to radioactivity?第16頁，共29頁。Rad

13、ioactive decay is a spontaneous process that cannot be controlled and is not affected by temperature.What is half-life?The half-life of a radioactive element is the time that it takes half the atoms in a sample to decay.For example, the half-life of the isotope iodine-131 is 8 days.However, each rad

14、ioactive element has its own particular decay rate, which is called the half-life.This means that after 8 days half the atoms in a sample of iodine-131 have decayed. 8 days later half the remaining atoms have decayed and so on.第17頁，共29頁。How is half-life calculated?第18頁，共29頁。Half-lives range from mil

15、lionths of a second to millions of years. Uranium-235, which is used in nuclear reactors and nuclear weapons, has a half-life of 710 million years. Why is the use of uranium-235 considered controversial? Xenon-133 is a radioactive isotope used for studying lung function. Why does its half-life of 5.

16、2 days make it suitable for this use?How long are half-lives?Some types of nuclei are more unstable than others and decay at a faster rate.RadioisotopeHalf-lifeboron-12uranium-235radium-2260.02 seconds1602 years710 million years第19頁，共29頁。What is the half-life of carbon-14?第20頁，共29頁。How does carbon d

17、ating work?第21頁，共29頁。What are the problems of using carbon dating?What are some of the problems with using carbon dating to predict the age of a sample? The half-life of carbon-14 is 5,700 years. If the sample is older than 60,000 years, the amount of carbon-14 is too small to measure accurately. In

18、stead, radioactive isotopes with longer half-lives, such as uranium-235 with a half-life of 710 million years, can be used to date older samples.Carbon dating anything that died after the 1940s, when nuclear bombs, nuclear reactors and open-air nuclear tests began, is harder to date precisely due to

19、 contamination from this increased background radioactivity.Samples can become contaminated with materials of a different age which may confuse the readings of carbon-14.第22頁，共29頁。Using half-life to date a sampleHalf-life can be used to do many useful calculations.For example, the half-life of carbo

20、n-14 is 5,700 years. If a fossil bone has a count of 25, and a piece of bone from a living body has a count of 200, how old is the fossil?After one half-life, the count will decrease by half to 100.Three half-lives of carbon-14 have passed, so 3 x 5,700 years makes the fossil 17,100 years old. After the second half-life, the count decreases by half again to 50. After the third half-life, the count decreases to 25.第23頁，共29頁。Using half-life in calculations第24頁，共29頁。第25頁，共29頁。Glossaryatomic number The number of protons in the nucleus of an atom, which is the same for all isotop