1 Nuclear Chemistry Chemistry I – Chapter 4.4 & 25

Science Park HS -- Honors Chemistry Agenda Definition of Radioactivity and emissions Discussion of the three most important types of emissions What do we mean by half-life? Where is Radioactivity encountered? Is Radioactivity dangerous?

Science Park HS -- Honors Chemistry Expectations of Students Be able to state what radioactivity is, where these rays come from, what each ray is made of and state why they are dangerous. Be able to identify 4 pioneer scientists who made important contributions to understanding radioactivity Be able to explain the meaning of “half-life”. Student will be asked to find any sources of Radioactivity in his/her environment

Science Park HS -- Honors Chemistry Early Pioneers in Radioactivity Roentgen: Discoverer of X-rays 1895 Becquerel: Discoverer of Radioactivity 1896 The Curies: Discoverers of Radium and Polonium Rutherford: Discoverer Alpha and Beta rays 1897

5 Nuclear Reactions vs. Normal Chemical Changes Nuclear ReactionsNuclear Reactions “Normal” Chemical Reactions“Normal” Chemical Reactions

6 Mass Defect Some of the mass can be converted into energySome of the mass can be converted into energy Shown by a very famous equation!Shown by a very famous equation! E=mc 2 EnergyMass Speed of light

7 Types of Radiation Alpha (  ) – a positively charged helium isotope Alpha (  ) – a positively charged helium isotope Beta (β) – an electronBeta (β) – an electron Gamma (γ) – pure energy; called a ray rather than a particleGamma (γ) – pure energy; called a ray rather than a particle

8 Penetrating Ability

9 Other Nuclear Particles Neutron Neutron Positron – a positive electron Positron – a positive electron Proton – usually referred to as hydrogen-1Proton – usually referred to as hydrogen-1 Any other elemental isotopeAny other elemental isotope

Only one element has unique names for its isotopes … Deuterium and tritium are used in nuclear reactors and fusion research.

11 Balancing Nuclear Reactions In the reactants and products: Atomic numbers must balance and Mass numbers must balance Use a particle or isotope to fill in the missing protons and neutrons

Science Park HS -- Honors Chemistry Three Common Types of Radioactive Emissions Alpha Beta Gamma

There are also absorption of particles: Science Park HS -- Honors Chemistry

14 Nuclear Reactions Alpha emissionAlpha emission Note that mass number goes down by 4 and atomic number goes down by 2. Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

15 Nuclear Reactions Beta emissionBeta emission Note that mass number is unchanged and atomic number goes up by 1.

16 Other Types of Nuclear Reactions Positron ( 0 +1  ) emission: a positive electron Electron capture: Electron capture: the capture of an electron 207

17 Learning Check What radioactive isotope is produced in the following bombardment of boron? 10 B + 4 He ? + 1 n 5 2 0

18 Write Nuclear Equations! Write the nuclear equation for the beta emitter Co-60.

Science Park HS -- Honors Chemistry The three main decays are Alpha, Beta and Gamma

Half life

21 Kinetics of Radioactive Decay For each duration (half-life), one half of the substance decomposes. For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234 After 3.6 days = 25 grams After 7.2 days = 12.5 grams After 10.8 days = 6.25 grams

What is half life? Half life is the time needed for one half of a radioisotope to decay. Suppose you start with grams of a radioisotope that has a half life of exactly 1 year.

What is half life? How much will be left after 1 year? Suppose you start with grams of a radioisotope that has a half life of exactly 1 year.

What is half life? After one year there will be 50.0 g left. Suppose you start with grams of a radioisotope that has a half life of exactly 1 year. After a second year there will be 25.0 g left.

What is half life? After a third year there will be 12.5 grams left. After one year there will be 50.0 g left. After a second year there will be 25.0 g left. After a fourth year there will be 6.25 grams left.

Half-Life Decay of 20.0 mg of 15 O. What remains after 3 half-lives? After 5 half-lives?

Science Park HS -- Honors Chemistry Another Contribution from Rutherford: Half-life of Radioactive Atoms The half-life of a radioactive substance, is the time required for one half of it to decay.

Half life Question 1. A radioisotope has a half-life of 100 years. How long will it take for the radiation to decrease to 1/16 of its original value? 400 years

Half life Question 2. A radioisotope has an activity of 560 counts per minute. After 16 hours the count rate has dropped to 35 counts per minute. What is the half life of the radioisotope? 4 hours

30 Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4 He and 11 B. Reactions using neutrons are called  reactions because a  ray is usually emitted. Radioisotopes used in medicine are often made by  reactions.

31 Artificial Nuclear Reactions Example of a  reaction is production of radioactive 31 P for use in studies of P uptake in the body P n ---> P + 

32 Transuranium Elements Elements beyond 92 (transuranium) made starting with an  reaction U n  U +  U  Np  Np  Pu  Np  Pu 

Nuclear energy All have enough energy to ionize atoms. Gamma rays are electromagnetic energy. Alpha and beta particles have high kinetic energies. All nuclear decay is accompanied by a release of energy.

Nuclear energy This can result in damage to your body. Ionization occurs when electrons are removed from atoms by  or  radiation. An ion is a “charged atom” or group of atoms. cancer

Nuclear energy Forms of ionizing radiation are: Alpha Beta Gamma X-rays Cosmic rays Ultraviolet light (UV) can cause cancer, but it is not ionizing radiation. Neutrons Positrons

There’s even more! But there is an even greater release of energy when the atom splits apart … Some of the energy that holds the nucleus together is carried away by the alpha, beta and gamma radiation.

37 Nuclear Fission

38 Nuclear Fission Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235 U + neutron) 2. Propagation. 236 U fission releases neutrons that initiate other fissions 3. ___________.

39 Stability of Nuclei Out of > 300 stable isotopes: Even Odd Odd Even Z N P 19 9 F 2 1 H, 6 3 Li, 10 5 B, 14 7 N, Ta

40 Band of Stability and Radioactive Decay

41 Representation of a fission process.

42 Nuclear Fission & POWER Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.Currently about 103 nuclear power plants in the U.S. and about 435 worldwide. 17% of the world’s energy comes from nuclear.17% of the world’s energy comes from nuclear.

43 Figure 19.6: Diagram of a nuclear power plant.

44 Nuclear Fusion Fusion small nuclei combine 2 H + 3 H 4 He + 1 n Occurs in the sun and other stars Energy

45 Nuclear Fusion Fusion Excessive heat can not be contained Attempts at “cold” fusion have FAILED. “Hot” fusion is difficult to contain

46 Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

47 Effects of Radiation

48 Geiger Counter Used to detect radioactive substances Radioactivity of Everyday ObjectsRadioactivity of Everyday Objects

49

50 Radiocarbon Dating Radioactive C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation 14 N + 1 o n ---> 14 C + 1 H The C-14 is oxidized to CO 2, which circulates through the biosphere. When a plant dies, the C-14 is not replenished. But the C-14 continues to decay with t 1/2 = 5730 years. Activity of a sample can be used to date the sample.

51 Nuclear Medicine: Imaging Thyroid imaging using Tc-99m

52 Food Irradiation Food can be irradiated with  rays from 60 Co or 137 Cs.Food can be irradiated with  rays from 60 Co or 137 Cs. Irradiated milk has a shelf life of 3 mo. without refrigeration.Irradiated milk has a shelf life of 3 mo. without refrigeration. USDA has approved irradiation of meats and eggs.USDA has approved irradiation of meats and eggs.

53 Half-Life Decay of 20.0 mg of 15 O. What remains after 3 half-lives? After 5 half-lives?