Nuclear Chemistry

1. Nuclear Chemistry Chapter 25 What so special?

2. Radioactivity • Discovered accidentally using Uranium salts • Without sunlight, Uranium could fog a photographic plate • The Curies showed the fogging was due to rays emitted by the Uranium atoms • Penetrating rays and particles emitted by radioactive source = radiation

3. Nuclear reactions • Differ from chemical reactions • Chemical = stable electron configs • Electrons just relocate within cmpd; nuclei stay the same • Nuclear= nuclei of unstable isotopes (radioisotopes) gain stability by undergoing changes • Always produces large amounts of energy • Not affected by changes in temp, pressure or catalysts • Cannot be sped up, slowed down or stopped

4. Radioactive decay • Radioisotopes have unstable nuclei • Stability depends on the ratio of protons to neutrons and the overall size of the nucleus • Too many or too few neutrons causes instability • An unstable nucleus releases energy by emitting radiation during process called radioactive decay • Unstable isotopes of one element becomes stable isotopes of a different element • Decay is spontaneous and requires no energy

5. Types of Radiation (3) • Alpha radiation (stopped by paper) • Release alpha particle • He nuclei (2p+ & 2n0 & 0e-) • U-238  Th-234 + He2+ (α particle) • Beta radiation (stopped by wood) • Neutron breaks into p+ & e- • p+ stays in nucleus, e- released • n0 H+ (proton) + e- (β particle) • C-14 (radioactive) N-14 (stable) +e-(β)

6. Figure 25.2

7. Gamma Radiation (3rd) • High energy photon • No mass • No electrical charge • Often emitted with alpha particle • Thorium-230  Radon-226 + α + γray • Does not alter the atomic number or mass number • Extremely penetrating and dangerous • Can be almost completely stopped by several m of concrete or cm of lead

8. Figure 25.3

10. Nuclear stability • Low atomic # (<20) 1:1 (n0 : p+) • Up to 1.5:1 for heavy elements • The n:p determines the type of decay • Too many n0 • n0 turns into p+ and emits β • beta emission • ↓ n0, ↑p+,↑ e- • Too few n0 • p+ becomes a n0 by nucleus engulphing an e- • Called electron capture • ↑ n0, ↓ p+, ↓ e- • OR a p+ changes to a n0 (Positron emission) • ↑n0, ↓ p+ • Positron = particle with mass of e- but + charge

11. Figure 25.4 red line = Band of Stability

12. Generalizations • All elements > 83 are radioactive • Have too many n0 AND p+ to be stable • Most undergo α emission • Inc n0 : p+ • Mass # - 4, at # - 2 • Mass is not conserved • Very small amount of mass is converted into energy and released during radioactive decay • (hence photographic plate fogging)

13. Half-Life (t1/2) • Time required for ½ of nuclei of a radioisotope sample to decay • So, after each half-life, half the existing radioactive atoms have decayed into atoms of a new element • Some are billions of years long, others fractions of a second

14. Figure 25.5

15. Table 25.3½ lives and radiation of some naturally occurring radioisotopes

18. Transmutation reactions • The conversion of an atom of one element into the atom of another element • Can occur by radioactive decay • Or when particles bombard the nucleus of an atom • Particles can be p+, n0, or α particles • Remember, α particle = He nucleus

19. Transmutations where? • Occur naturally • N-14  C-14 in upper atmosphere • U-238  (x 14) Pb-206 • In laboratories • First done in 1919 by Rutherford • N-14 + α F-18 (quickly  O-17 + p+) • Lead to discovery of p+ • Chadwick found neutron in 1932 • Be-9 + α C-12 + n0 • Nuclear reactors • Transuranium elements

20. Transuranium Elements • At # > 92 (aka U) • All undergo transmutation • None occur in nature • All are radioactive • All synthesized in nuclear reactors and nuclear accelerators • Accelerators accelerate bombarding particles to very high speeds • Reactors produce beams of low-energy bombarding particles • Hadron Accelerator

21. Examples- fyino, you do not need to know these • U-238 + n0(very slow moving) U-239* • *U-239 is radioactive • U-239  Np-239* + β • *Np-239 is also radioactive and thus unstable • Np-239  Pu-239 + β • Both Np and Pu were synthesized in 1940 in Berkley, Ca

23. Fission of Atomic Nuclei • U-235 and Pu-239 are the only nuclei that can undergo fission • The splitting of a nucleus into smaller fragments as a result of bombardment by slow moving neutrons • Chain rxn when neutrons given off during fission of one nucleus strike another fissionable atom

24. Fun Facts about Fission • Releases HUGE amounts of energy… 1 kg U-235  energy equal to that of 20,000 tons of dyn-o-mite! • In uncontrolled nuclear chain reaction (like an atomic BOMB) the energy is released in fractions of a second! • Can be controlled in nuclear reactors to make use of the energy in small, slowly released amounts

25. Nuclear Fusion fun facts • Fusion = nuclei combine to produce a nucleus of greater mass • Energy released by the sun (Earth’s major source of energy) results from nuclear fusion • fusion releases MORE energy from little nuclei than fission from big nuclei • Catch= fusion only occurs at ridiculously high temps • > 40,000,000°C

26. Fusion to be used under control on earth? • Attempts made to combine H-2 + H-3  He-4 + n0 + ENERGY • Problem = temp • So far, only way to get temp up is to use a fission bomb • like the one used to trigger the controlled fusion reaction that is called a H bomb • Therefore, not a useful idea…

28. Detecting Radiation • Geiger counters • detectsα,β, & γ with audible clicks • Scintillation counters • Uses phosphor-coated surface • Film badges • Made of several layers of photographic film

29. Using radiation • Important in many scientific procedures • Used in agriculture as “tracers” to test effect of pesticides, herbicides, and fertilizers • To diagnose medical problems • I-131 used to identify thyroid disorders • To treat some diseases • Pharmaceuticals sometimes used as radiation therapy • EX: More I-131 than for test = absorbed and emits β & γ rays