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National 5 Chemistry

National 5 Chemistry. Nuclear Chemistry. Isotopes. Atoms of the same element (same Z) but different mass number (A). Boron-10 ( 10 B) has 5 p and 5 n: 10 5 B Boron-11 ( 11 B) has 5 p and 6 n: 11 5 B. Radioactivity.

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National 5 Chemistry

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  1. National 5 Chemistry Nuclear Chemistry

  2. Isotopes • Atoms of the same element (same Z) but different mass number (A). • Boron-10 (10B) has 5 p and 5 n: 105B • Boron-11 (11B) has 5 p and 6 n: 115B

  3. Radioactivity • One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). • She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

  4. Types of Radiation

  5. Penetrating Ability

  6. Nuclear Reactions – Alpha Emission • Alpha radiation consists of helium nuclei, 2+ • When a radioactive isotope decays by alpha emission the nucleus loses 2 protons (decreasing the atomic number by 2) and two neutrons (decreasing the mass number by 4).

  7. Nuclear Reactions – Beta Emission • A beta particle is an electron. Since the nucleus does not contain electrons, it is thought that a beta particle is formed when a neutron splits up into a proton and an electron. • The proton stays inside the nucleus, and the electron is shot out of the nucleus as the beta particle. • As the nucleus contain one less neutron and one more proton the atomic number increases by one and the mass number stays the same.

  8. Nuclear Reactions – Gamma Emission • Gamma rays – very high energy waves! • g-rays are sometimes produced after a or b emissions • Radioactive decay generates a new nucleus, but possibly in an unstable configuration of p+, n. E Nuclear energy levels • A drop in energy level emits a g-ray

  9. Origin of the Elements • The Big Bang Theory: In the first moments there were only 2 elements - hydrogen and helium

  10. Origin of the Elements • The heavier elements were formed in the heart of stars by fusing lighter nuclei together to form the heavier elements of the periodic table • The elements were flung across the Universe when stars exploded in massive supernovae “We are not simply in the universe; we are born from it.” (Tyson 1998)

  11. Element Abundance

  12. Stability of Nuclei • H is most abundant element in the universe. • 88.6% of all atoms • He is 11.3% of all atoms • H + He = 99.9% of all atom & 99% of mass of the universe. • This tells us about the origin of the elements, and so does the existence of isotopes.

  13. Half-Life • HALF-LIFE is the time it takes for 1/2 a sample is disappear. The half life ( t1/2 ) of a radioactive isotope is the time taken for the mass or activity of the isotope to halve by radioactive decay. • The rate of a nuclear transformation depends only on the “reactant” concentration. It is a completely random process, and does not depend on mass, pressure, or concentration.

  14. Half-Life

  15. t 1/2 t 1/2 t 1/2 14 14 14 14 C 100g C 50g C 25g C 12.5g 6 6 6 6 Half-Life • The half life of 14C is 5,730 years • 100g of 14C would decay to 12.5g in 3 x t 1/2 , • i.e. 5,730 x 3 years = 17,190 yrs

  16. 100 Mass of Radioactive isotope Remaining /g 50 0 Time /s 100 200 300 400 Half-Life • The time it takes this radioactive isotope to reduce its mass by a half is 100 s. i.e. The mass of the radioactive isotope has changed from 100 g to 50 g. • The half life is therefore 100 s.

  17. Nuclear Fission

  18. Nuclear Fission • In nuclear fission the nuclei of heavier elements break up into two smaller lighter nuclei and release a large output of energy. • 239Pu and 235U are the only important fissionable isotopes. 0.7% of natural uranium contains 235U. • Enrichment of uranium ore produces 2-3% 235U, sufficient for fission.

  19. Nuclear Fission & POWER • There are currently 435 nuclear power plants worldwide. • 17% of the world’s energy comes from nuclear. • Note France and Lithuania.

  20. Nuclear Energy - Dounreay

  21. Nuclear Energy Fuel rods, steel tubes containing either 235U or 235U oxide. The fission process generates heat in these rods. Some reactors use natural U, with 0.7% 235U,others need enriched U fuel, containing 3 % 235U

  22. Nuclear Energy AGR uses C02 gas to transfer heat from the reactor. Other reactors use either water (PWR) or liquid Na.

  23. Nuclear Energy Control rods, contain boron, which absorbs neutrons. Lowering and raising these rods controls the fission process.

  24. Nuclear Energy Moderators, graphite blocks which slow down neutrons enabling them to be more easily captured by the uranium.

  25. 239 0 e n 238 Pu 1 + U + -1 0 93 92 Nuclear Energy- Reprocessing • After several years the fuel becomes less efficient and is replaced. This spent fuel is a mixture of unused uranium, plutonium and waste fission products. • Plutonium is produced when 238U is combined with slow neutrons. • Plutonium does not occur naturally but is capable of fission and is therefore used as an alternative fuel. Fast travelling neutrons are needed, so a moderator is not needed.

  26. Nuclear Energy- Waste • Spent fuel contains both short and long lived radioactive isotopes. • The rods are stored under water to allow them to cool and the short lived isotopes to decay. The spent fuel is sent to Sellafield (reprocessing plant) where the other isotopes are recovered. • Storing As yet, nobody has come up with a safe way of storing this long lived radioactive waste. Ideas include, burial deep underground and encasing in glass.

  27. 1 14 14 1 n N C + H + 0 7 6 1 Radioisotopes and carbon dating • Neutrons from cosmic radiation collide with nitrogen and create a proton and carbon-14 atoms. • The half-life of 14 C is 5730 years.

  28. Radioisotopes and carbon dating

  29. Radioisotopes and carbon dating C14 dating at Oxford University Archaeology Dept.

  30. Radioisotopes and dating rocks • One of the important natural radioactive isotopes is 40K. It has a life of 1.3 x 109 years. 0.012% of all K is made from this isotope. • The constant rate of change between 40K and 40Ar allows for the K/Ar ratio to be used to determine the age of rocks.

  31. Radioisotopes and dating rocks • Rocks can also be dated using 238U, which has a half life of 4.5x109 years. 238U decays to 234Th and then eventually to 206Pb. • The ratio of 238U to 206Pb can be used todates the rock.

  32. Radioisotopes and dating aquifers Dating materials less than 100 years old uses tritium, (formed by cosmic radiation) a beta emitter with a half life of 12 years. calculating the ratio of 1H to 3H is a measure of the age of under ground water known as aquifers.

  33. Nuclear Medicine: Imaging

  34. Nuclear Medicine: Imaging • Technetium-99 is used in more than 85% of the diagnostic scans done in hospitals each year. • Synthesized on-site from Mo-99. • 9942Mo ---> 9943Tc + 0-1b • 9943Tc decays to 9943Tc giving off g ray. • Tc-99 contributes in sites of high activity.

  35. Nuclear Medicine: Imaging Imaging of a heart using Tc-99m before and after exercise.

  36. Food Irradiation • Food can be irradiated with g rays from 60Co or 137Cs. • Irradiated milk has a shelf life of 3 months without refrigeration. • USDA has approved irradiation of meats and eggs, but the process is not (yet) approved in the EU

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