1. NUCLEAR CHEMISTRY Kenneth E. Schnobrich

2. General Facts As we look at the Periodic Table we note both the atomic number (Z) and the atomic mass number (A). In 6 C 12, the 6 represents the The number of protons in the nucleus The number of electrons associated with a neutral atom The difference between A (the atomic mass number) and Z (the atomic number) represents the number of neutrons (N) in the nucleus. 6 C 12 and 6 C 14 would be identified as isotopes of the same element and differ in the number of neutrons in the nucleus.

3. General Facts In the case of 6 C 12 and 6 C 14 they have the same number of protons in the nucleus but 6 C 14 has two addition neutrons in the nucleus. Depending on the proton/neutron ratio, nuclei can be unstable and the nucleus breaks down by a process called radioactivity (nuclear decay). Nuclear decay or half life can occur in fractions of a second or it can take hundreds of years. During this decay several of the common products are – Alpha particles (He nuclei) Beta particles (electrons) Gamma radiation

4. General Facts Alpha particles – have a mass of 4 amu and a relative charge of +2 – they are helium nuclei Beta particles – have a mass of 0 amu and a relative charge of - 1 – they have all the properties of electrons Gamma radiation – high energy photons with a very short wavelength (high frequency) – very destructive Some isotopes occur naturally unstable – natural radioactivity Some unstable isotopes are prepared in the lab – induced radioactivity or artificial radioactivity

5. Separating Radiations Positive Plate (+) Negative Plate (-) Beta Particle Alpha Particle Gamma Particle Radioactive sample

6. General Facts In ordinary chemical reactions it is the electrons in the outer shells that are involved and energy is released or absorbed. In nuclear reactions, the nucleus changes and the amount of energy involved can be hundreds of thousands times greater than a chemical reaction. In Nuclear Fusion and Nuclear Fission the amounts of energy released are even greater than the amount released during a normal decay.

7. General Facts Nuclear Fission – a process where the nucleus of a larger unstable atom breaks down into smaller nuclei and other subatomic particles with a tremendous release of energy (like an atom bomb). Nuclear Fusion – a process where smaller nuclei are fused together to form a larger nucleus with the release of tremendous amounts of energy (like the hydrogen bomb or the reaction on the sun’s surface).

8. What Causes Instability? The nucleus is about 10 -15 m in size, very small volume. In the nucleus the protons exhibit typical electrostatic repulsive forces, there are also very strong attractive forces that hold the protons together in the very small nucleus. The neutrons act like a “nuclear glue” and are responsible for holding the protons in the nucleus together. The stability of the nucleus then depends on the balance between the repulsive electrostatic forces between the protons and the “nuclear glue” forces of the neutrons. The neutron/proton ratio is important for stability.

9. Instability For the lighter elements the neutron/proton ratio is close to 1 which appears to be a very stable arrangement. Any element beyond atomic number 83 (Bismuth) has no stable isotopes, they are all radioactive. It would appear that as the atomic number increases the number of neutrons needed for stability greatly increases. A ratio of 1 up to 1.51 seems to be stable (in 82 Pb 206 the nucleus is stable).

10. Particles Associated with Decay In this table you see some of the particles normally associated with nuclear reactions. Unstable nuclei are generally trying to achieve stability by adjusting the neutron/proton ratio. Table taken from NYS Chemistry Reference Tables

11. Instability The neutron/proton ratio is 1 The neutron/proton ratio is 2 (unstable) **Remember anything above atomic number 83 is unstable

12. Nuclear Equations In writing a nuclear equation, it is important to remember that the sum of the subscripts on the left side of the arrow must equal the sum of the subscripts on the right side. The same can be said for the superscripts on the two sides of the equation. Alpha Decay – an alpha particle is released 92 U 238  90 Th 234 + 2 He 4 Beta Decay – a beta particle is released 53 I 131  54 Xe 131 + -1 e 0

13. Nuclear Equations (cont.) Positron Emission – a positron is released 6 C 11  5 B 11 + 1 e 0 ( 1 p 1  0 n 1 + 1 e 0 ) Electron Capture – an electron is captured from an inner electron cloud surrounding the nucleus 37 Rb 81 + -1 e 0  36 Kr 81 ( 1 p 1 + -1 e 0  0 n 1 ) Gamma radiation – the emission of this highly energetic radiation does not change the mass or the charge and is usually associated with emission of another subatomic particle.

14. Emissions Nuclide – refers to the specific radioactive element and its mass number, the atomic number can be found on the Periodic table of the Elements. Half-life (t 1/2 ) – refers to the time it take for half of the given radioactive sample to decay and form another nuclide. The half-life is a constant value and is unaffected by variations in condition change such as temperature and pressure. Decay Mode – describes the subatomic particle emitted during the nuclear change for that radioisotope. Nuclide Name – the name of the element and its atomic mass number (A). **Note – there is a key for the half-life at the bottom of the table Table taken from NYS Chemistry Reference Tables

15. 38 Sr 90 Half-life (t 1/2 ) Amount of 38 Sr 90 Years (t 1/2 = 28.8 years)

16. Nuclear Fission Fission can occur when a slow moving neutron strikes strikes a large nucleus causing it to split. U 235, U 233, and Pu 239 are the most frequently radioisotopes used for this process. 0n10n1 56 Ba 142 36 Kr 91 92 U 235 0n10n1 0n10n1 0n10n1 0n10n1

17. Nuclear Fission During the fission process some of the neutrons produced can become involved in splitting other heavy nuclei and can lead to a chain reaction. Subcritical mass – would indicate that there are not enough “effective” neutron/heavy nuclei collisions to sustain a reaction. Critical mass – would indicate that there are enough “effective” neutron/heavy nuclei collisions to sustain a nuclear chain reaction. In the atomic bomb two subcritical masses are forced to come together to produce a critical mass. In a nuclear reactor, control rods are used to absorb neutrons and help control the fission reaction.

18. Nuclear Fusion In the fusion reaction requires a great deal of energy to actually fuse two smaller nuclei. The products of fusion usually are not radioactive like those of fission. Temperatures as high as 4 x 10 7 K are required for the fusion process. 1H21H2 1H31H3 2 He 4 + 0 n 1

19. Nuclear Fusion Since extremely high temperatures are required to initiate the fusion reaction it is sometimes called a thermonuclear reaction. Other small nuclei can be used in the fusion process. Thermonuclear reactions occur on the surface of the sun and provide both the light and heat necessary to sustain life on earth. Fusion reactions are not a practical source of energy at this point in our technological development because of the high energy needed for the fusion reaction. The hydrogen bomb is a thermonuclear device it literally uses a fission reaction to initiate the fusion reaction.

20. Transmutation In a transmutation reaction we can bombard a nucleus with a selected subatomic particle to produce a radioisotope. The balancing of the subscripts and superscripts must be observed in a transmutation reaction. 7 N 14 + 2 He 4  8 O 17 + 1 H 1 26 Fe 58 + 0 n 1  26 Fe 59 Devices such as the cyclotron can be used to accelerate the subatomic particles used to bombard the target nuclei.

21. E = mc 2 Using the famous equation above we can calculate the energy released in a nuclear reaction. E – stands for energy m – represents the mass loss (mass defect) in a nulcear reaction c – represents the speed of light (3.00 x 10 8 m/s) Since the value for “c” is so large even a small mass change will produce a tremendous amount of energy.

22. Using Radioisotopes In Medicine – I 131 is used to diagnose thyroid problems P 32 can be used as a tracer for the eyes, liver, and tumors Tc 99 can be used as a tracer in the heart, bones, liver, and the lungs Carbon Dating C 14 levels can be used to date carbon based objects C 14 can also be used to trace metabolic pathways Dating Rocks The ratio of U 238 to Pb 206 can be used to date uranium containing rocks

23. Using Radioisotopes In agriculture – P 31 can be used in fertilizers to trace uptake In medicine – Co 60 can be used in radiation treatments for cancer Food supply – Both Co 60 and Cs 137 can be used to irradiate food supplies to kill bacteria like the anthrax bacilli

24. Radiation Damage due to radiation occurs as a result of exposure to the more harmful forms of radiation such as beta radiation and gamma radiation. Damage can occur in somatic cells (like the skin cells) and it interferes with the growth regulating process, cancer can be one of the results. Damage can also occur in the reproductive cells and result in mutations that can be passed on to offspring.