10: The nucleus, radioactivity and nuclear medicine You only have to know what I talk about in class

Nuclear reactions Emphasis so far on interpretation of chemical behavoir in terms of the electronic structure of cmpds. Form bonds by transfer of e-’s or sharing of e pairs. This chapter are interested in the chemistry of the nucleus.

Nucleus contains protons and neutrons (to a chemist) and most of the mass of the atom. Protons and neutrons collectively called nucleons. Specific nucleus called a nuclide(isotope). A Q Z A=mass no.=no. of protons+no. of neutrons Z= no. of protons in the nucleus = charge on nucleus = atomic no. Q = symbol for element.

Isotopes 12 C 13 C 14 C 1 H 2 H 3 H Some isotopes are stable, others are not. Stable isotope persists indefinitely.

Do not copy Unstable isotopes are radioactive: on a random basis, one atom of a collection suddenly emits a simpler particle and/or energy of very high frequency (energy) and changes into a different nucleus. The energy emitted is capable of breaking chemical bonds.

2 kinds of radioactive isotopes: 1. natural: occur in nature (Becquerel 1896) Every element after Bi is radioacitive. 2. induced: man-made: brought about by particle bombardment of a nucleus (Rutherford in 1919) All elements with atomic nos. higher than 83 are radioactive. All isotopes of 43 Tc and 61 Pm are radioactive.

Particles that are emitted in radioactive decay (natural) 1. alpha particle: 2 4 He or 2 4  very energetic helium nucleus (no e-s) a. very ionizing in matter b. +2 charge 2 4 He 2+ c. low penetrating power, relatively slow moving (10% speed of light)

2. beta emission (rays, particles) : -1 0   0 e stream of high speed electrons emitted by nucleus; move at 90% speed of light How do you get an electron in the nucleus. neutron decays into a proton and an electron a. -1 charge, “zero” mass b. greater penetrating power, less ionizing power than alpha particle

3. gamma ray:  : electromagnetic radiation of very short wavelength; pure energy a. zero charge, zero mass b. very penetrating, highly energetic c. most radioactive decays emit gamma rays as well as other particles

Other particles emitted 4. neutrons 0 1 n a. uncharged b. produce cell damage due to ionizing effects of neutrons colliding with protons in cells in body.

5. protons 1 1 p 1 1 H a. +1 charge 6. positron 1 0 , 1 0 e a. +1 charge, “zero” mass b. antimatter of beta particle

Ionizing radiation Alpha, beta and gamma radiation are ionizing radiations. Leave trail of ions in their wake. Alpha and beta not bad unless injested-- cause skin and eye damage Gamma can penetrate body and cause internal damage.,

10.2: Balancing nuclear reactions 1. In nuclear transformations charge is conserved: sum of atomic nos. of reactants = sum of at. nos. of products 2. Mass is not conserved (E=mc 2 ) but there is no change in the total mass no. (# of nucleons) sum of mass nos. of reactants = sum of mass nos. of products

Let’s balance nuclear reactions 6 14 C  X _  N U  X e 5 11 B  3 7 Li + X

10.24 Write a nuclear reaction to represent radium-226 decaying to radon-222 and X.

Nuclear transmutation--the alchemists’ dream Bombard a nucleus with other particles (high energy) frequently in particle accelerators. In this you produce radioisotopes artificially--create new ones Pd( , p) Ag 10.26: Complete U N  X n

Radioactive decay rates Radioactive decay follows first order kinetics: rate = k[isotope] The half-life of a given isotope is an identifying characteristic of that isotope. Half-life (t 1/2 ): time required for one-half of given quantity of a substance to undergo change and is independent of how much of the isotope you start with. Isotopes have their own half-life.

Carbon-14 has a half-life of 5730 years. How much of a 500g sample will remain after 17,190 years. If a patient is administered 10 ng of technetium-99m (half-life 6 hours) how much will remain 2 days later?

Isotope half-life 238 U 10 9 yr 218 At 1.4s 210 Bi 5 d 60 Co 5.26y 131 I 8.07d (overactive thyroid) 90 Sr 28.1yr

10.3: properties of radioisotopes Nuclear stability and structure: What allows the packing of protons (+ charge) in such a small region as the nucleus? nuclear diameter about 1 x cm; density of nucleus about 2 x g/cm 3. This corresponds to a density of 220,000,000 tons/cm 3

Thought to be neutrons that allow packing of so concentrated a charge in such a small volume. No nucleus of 2 or more protons without neutrons. ( 1 H) Some more facts about nuclear stability: Nuclei with 2,8,20,50,82 protons or neutrons and with 126 neutrons more stable than nuclei with other nos. Binding energy: energy that holds protons, neutrons together in nucleus

Nuclei with even nos. of neutrons and protons are more stable than those with odd nos. protons neutrons No. stable isotopes odd odd 4( 2 H, 14 N) odd even 50 even odd 53 even even 157

Nuclear binding energy Nuclear binding energy is the energy required to break up a nucleus into its component protons and neutrons. This energy represents the conversion of mass to energy that occurs during an exothermic nuclear rxn.

Dating based on radioactive decay Radiocarbon dating:: C-14 is produced in nature by 7 14 N n  6 14 C H and decays by 6 14 C  7 14 N + -1   In living matter the amt of C-14 remains constant. After a plant dies the amt of C-14 decreases. This can be used to date the matter in question.

10.4 Nuclear fission Nuclear fission: heavy nucleus (mass no. 200) divides into form smaller nuclei of intermediate mass and one or more neutrons with release of a large amt of energy

For example: U n  135 I + 97 Y n 139 Ba + 94 Kr n 131 Sn Mo n 139 Xe + 95 Sr n etc Initiated with slow neutrons (speed of air molecules at room temp)

Nuclear chain rxn: self-sustaining sequence of nuclear fission rxns Critical mass: minimum mass of fissionable material required to generate a self- sustaining nuclear chain rxn (chain propagates so more neutrons are generated than are absorbed or lost to outside--rxn proceeds at ever increasing rate--can become runaway)

subcriticalCritical mass

Nuclear reactors Makes use of U-235 fission reaction Slow neutrons split U-235 more efficiently than fast ones: use a moderator to slow down the neutrons produced in the rxn: non-toxic, inexpensive, not turn radioactive with neutron bombardment, fluid (so can be used as coolant) --water works well

Use enriched U-235 (3-4% in U 2 O 3 ) Control rate of rxn by controlling no of neutrons allowed to react: control rods made of Cd or B (absorb neutrons) Reactor: short of critical mass--chain rxn to continue at a slow, usable rate, but not runaway. Reactors: not atomic bomb capability--but dangers from radioactivity.

113 Cd + 1 n  114 Cd + 

Nuclear fusion Combining of 2 lightweight nuclei to form heavier ones--large release of energy Requires high temperatures--thermonuclear rxns

In sun: 1 H + 2 H  3 He 3 He + 3 He  4 He H 1 H + 1 H  2 H  Fusion reactors: fuels cheap, inexhaustible, little dangers from radioactivity Problems: Haven’t figured out how to get more energy back than put in: for fusion need temps ~100 million degrees C and way of containing atoms in small region of space for fusion