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).

• AQ

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

• 12C 13C 14C

• 1H 2H 3H

• 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 43Tc and

61Pm are radioactive.

Particles that are emitted in radioactive decay (natural)

• 1. alpha particle: 24He or 2

4 very energetic helium nucleus (no e-s)

• a. very ionizing in matter

• b. +2 charge 24He2+

• c. low penetrating power, relatively slow moving (10% speed of light)

• 2. beta emission (rays, particles) : -10

0estream 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 01n

• a. uncharged

• b. produce cell damage due to ionizing effects of neutrons colliding with protons in cells in body.

• 5. protons 11p 1

1H

• a. +1 charge

• 6. positron 10, 1

0e

• 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=mc2) 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

• 614C X_+ 7

14N

• 92239U X + -1

0e

• 511B 3

7Li + 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

• 46106Pd(, p)47

109Ag

• 10.26: Complete

92238U + 7

14N X + 601n

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 (t1/2): 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

• 238U 109yr

• 218At 1.4s

• 210Bi 5 d

• 60Co 5.26y

• 131I 8.07d (overactive thyroid)

• 90Sr 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 10-12 cm; density of nucleus about 2 x 1014 g/cm3.

• This corresponds to a density of 220,000,000 tons/cm3

• 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. (1H)

• 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 isotopesodd odd 4 (2H, 14N)odd even 50 even odd 53even 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

714N + 0

1n 614C + 1

1H

• and decays by

614C 7

14N + -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: 92235U + 0

1n 135I + 97Y +401n

139Ba + 94Kr + 301n

131Sn + 103Mo + 201n

139Xe + 95Sr + 201n

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)

subcritical Critical 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 U2O3)

• 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.

113Cd + 1n 114Cd +

Nuclear fusion

• Combining of 2 lightweight nuclei to form heavier ones--large release of energy

• Requires high temperatures--thermonuclear rxns

• In sun:1H + 2H 3He3He + 3He 4He + 21H

1H + 1H 2H + 10

• 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