1 National 5 Chemistry Nuclear Chemistry

Isotopes 2  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 3  One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie ( ).  She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

Types of Radiation 4

Penetrating Ability 5

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).

Nuclear Reactions – Beta Emission 7  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.

Nuclear Reactions – Gamma Emission 8  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

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

10 Origin of the Elements “We are not simply in the universe; we are born from it.” (Tyson 1998)  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

Element Abundance 11

Stability of Nuclei 12  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.

Half-Life 13  HALF-LIFE is the time it takes for 1/2 a sample is disappear. The half life ( t 1/2 ) 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.

Half-Life 14

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

Half-Life 16  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. Time /s Mass of Radioactive isotope Remaining /g

Nuclear Fission 17

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

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

Nuclear Energy - Dounreay 20

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

Nuclear Energy 22 AGR uses C0 2 gas to transfer heat from the reactor. Other reactors use either water (PWR) or liquid Na.

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

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

Nuclear Energy- Reprocessing 25  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 238 U is combined with slow neutrons. 92 n 0 1 U Pu 239 e 0 +  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.

Nuclear Energy- Waste 26  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.

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

Radioisotopes and carbon dating 28

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

Radioisotopes and dating rocks 30  One of the important natural radioactive isotopes is 40 K. It has a life of 1.3 x 10 9 years % of all K is made from this isotope.  The constant rate of change between 40 K and 40 Ar allows for the K/Ar ratio to be used to determine the age of rocks.

Radioisotopes and dating rocks 31  Rocks can also be dated using 238 U, which has a half life of 4.5x10 9 years. 238 U decays to 234 Th and then eventually to 206 Pb.  The ratio of 238 U to 206 Pb can be used to dates the rock.

Radioisotopes and dating aquifers 32 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 1 H to 3 H is a measure of the age of under ground water known as aquifers.

Nuclear Medicine: Imaging 33

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

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

Food Irradiation 36  Food can be irradiated with g rays from 60 Co or 137 Cs.  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