NUCLEAR CHEMISTRY

Introduction The chemistry of the atom is determined by the number and arrangement of the electrons. The properties of the nucleus do not strongly affect the chemical behavior of an atom.

Importance Security: Generating power Smoke detectors Determining explosives in airline luggage Monitoring nuclear technology in other countries Generating power Medical applications – nuclear medicine Dating artifacts

Review Atomic number (Z) – number of protons Mass number (A) – sum of the protons and the neutrons Isotopes or Nuclides– atoms with the same atomic number but different mass numbers, different numbers of neutrons. Nucleons – the particles that make up the nucleus.

Facts about the nucleus Very small Very dense: large density Held together by the nuclear strong force Location of the protons and neutrons Most of the mass of an atom is located

Mass Defect You might expect the mass of an atom to be the same as the sum of it’s parts, protons, neutrons, and electrons. Protons 1.007276 amu Neutrons 1.008665 amu Electrons 0.0005486 amu The difference between the calculated mass and the actual mass is known as mass defect.

What causes the lost mass? According to Albert Einstein, mass and energy can be converted into each other. Some of the mass is lost during the formation of the nucleus. The amount of energy can be caluclated using Einstein’s famous equation.

Nuclear Binding Energy The energy released when a nucleus is formed from nucleons. E = mc2 E is for energy unit: Joules (J)=kg.m2/s2 M is for mass unit: kilograms (kg) C is the speed of light (squared) 3.00 x 108 m/s

Binding Energy per Nucleon The binding energy per nucleon is used to compare the stability of different nuclides. It is the binding energy of the nucleus divided by the number of nucleons that are in the nucleus.

Binding Energy Con’t The higher the binding energy per nucleon, the more tightly packed the nucleons are held together, the more stable the nuclide. Elements with intermediate atomic masses have the greatest binding energies per nucleon and are therefore the most stable. Iron is the most stable isotope.

How does the nucleus stay together? Relationship between the nuclear strong force and the electrostatic forces between protons. Like charges repel each other through electrostatic repulsion The nuclear strong force allows protons to attract each other at very short distances.

Why do atoms want more neutrons than protons? As protons increase in the nucleus so does the electrostatic forces, faster than nuclear forces. More neutrons are required to increase the nuclear force and stabilize the nucleus. > 83 the repulsive forces of protons is so great that no stable nuclides exist.

Magic Numbers Stable nuclides tend to have even numbers of nucleons. 159 have both even protons and neutrons Only 4 have odd numbers of protons and neutrons.

Nuclear Shell Model Nucleons exist in different energy levels, or shells, in the nucleus. The number of nucleons that represent completed nuclear energy levels, 2, 8, 20, 28, 50, 82, and 126 Called magic numbers

Nuclear Reactions Unstable nuclei undergo spontaneous changes that change the number of protons and/or neutrons. Give off large amount of energy by emitting radiation during the process of radioactive decay. Eventually unstable radioisotopes of one element are transformed into stable, non-radioactive, isotopes of a different element.

Nuclear Reactions Total of mass number and atomic number must be equal on both sides of a reaction. When the atomic number changes, the identity of the element changes. A transmutation is a change in the identity of a nucleus as a result of a change in the number of protons.

Types of Radiation Alpha Radiation Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus stripped of it’s electrons, 2 protons and 2 neutrons. Charge +2 Large mass, 4 amu. Low penetration power Shielded by paper or clothing.

Alpha Radiation (Con’t) Occurs in unstable nuclei that has too many protons and too many neutrons. Effect on the nucleus: Mass number is reduced by 4 amu Atomic number is reduced by 2

Beta Radiation A fast moving electron Occurs in an unstable nuclei that has too many neutrons Converts a neutron into a proton and a beta particle An electron that doesn’t belong in the nucleus and therefore gets thrown out.

Beta radiation (Con’t) Charge -1 Mass = 1/1840 or 0.0005486 amu Moderate penetration power (0.4 cm) Shielded by metal foil Effect on nucleus: Mass number remains the same Atomic number increases by 1

Positron emission A positive electron or anti-electron Has the same mass as an electron, 1/1840 or 0.0005486 amu Charge +1 Occurs in unstable nuclei that has too many protons

Positron emission (Con’t) Converts a proton into a neutron Effect on the nucleus: Mass number remains the same Atomic number decreases by 1

Electron Capture Occurs in unstable nuclei that has too many protons: same as positron emission An inner orbiting electron gets captured by the nucleus and is used to convert a proton into a neutron. The effect is the same as for positron emission Mass number remains the same Atomic number decreases by 1

Gamma radiation Is high-energy electromagnetic radiation No charge and no mass: no effect on the nucleus Penetration power is high and only lead and several centimeters of concrete can slow it down Always accompanies another form of radiation

Half-Life No two radioisotopes decay at the same rate. t1/2 is the symbol for half-life Half-life is the time required for half the atoms of a radioactive nuclide to decay. The longer the half-life the more stable the nuclide.

Half-life Variables Variables Ao = original amount A = final amount T = total time elapsed t1/2 = half-life n = number of half-lives

Half-life Equations n = T t1/2 Ao = A * 2n

Half-life Calculations To solve half-life problems first write down all of the data in the problem. Determine which formula you’re going to use. Plug in the values and calculate

Half-life problem Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remains after 57.2 days if you start with 4.0 mg of the isotope? A0 = 4.0 mg A = ? T = 57.2 days n = T / t1/2 t1/2 = 14.3 days A = A0 / 2n

Problem (Con’t) n = T / t1/2 n = 57.2 days / 14.3 days n = 4 half-lives A = A0 / 2n A = 4.0 mg / 24 A = 4.0 mg / 16 A = 0.25 mg

Half-life graphic Picture representation of half-life ½ remain ½ decay

Total time problem The half-life of radon-222 is 3.824 days. After what time will one-fourth of a given amount of radon remain? A = ¼ remain n = T / t1/2 T = ? t1/2 = 3.824 days

Total time problem (Con’t) * We don’t need to know the beginning amount. Looking at the picture representation we see that it needs to go through 2 half-lives in order to have ¼ remaining. n = T / t1/2 2 = T / 3.824 days T = 2 x 3.824 days T = 7.648 days

Decay Series One nuclear reaction is not always enough to produce a stable nuclide. A decay series is a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached. The heaviest nuclide of each decay series is the parent nuclide and the nuclides produced by the decay is called the daughter nuclide.

Artificial Transmutations Artificial radioactive nuclides are radioactive nuclides not found naturally on Earth. They are made by artificial transmutations, bombardment of nuclei with charged and uncharged particles. Neutrons have no charge and no mass and can easily penetrate the nucleus of an atom.

Artificial Transmutations (Con’t) Positively charged alpha particles, protons, and other ions are repelled by the nucleus. A great deal of energy is needed to bombard nuclei with these particles. Energy may be supplied by accelerating these particles in the magnetic or electric field of a particle accelerator.

Artificial Radioactive Nuclides Radioactive isotopes of all the natural elements have been produced. Technetium and Promethium are not natural elements and have been artificially produced and have filled gaps in the periodic table. Transuranium elements are elements with more than 92 protons in their nucleus. All are radioactive and man-made.

Nuclear Radiation Nuclear Radiation can transfer energy form nuclear decay to the electrons of atoms or molecules and cause ionization. A roentgen (R) is a unit used to measure nuclear radiation exposure. A rem (roentgen equivalent, man) is a unit used to measure the dose of any type of ionizing radiation that factors in the effect that the radiation has on human tissue.

Nuclear Exposure Long term exposure to radiation can cause DNA mutations that result in cancer and other genetic defects. Average background radiation exposure in the U.S. is ~0.1 rem per year. The maximum permissible dose of radiation exposure for a person in the general population is 0.5 rem/year.

Radiation Detection Film badges use exposure of film to measure the approximate radiation exposure of people working with radiation. Geiger-Muller Counters are instruments that detect radiation by counting electric pulses carried by gas ionized by radiation. Used to detect beta, x-rays, and gamma radiation

Radiation Detection Radiation can also be detected when it transfers its energy to substances that scintillate, or absorb ionizing radiation and emit visible light. Scintillation counters are instruments that convert scintillations light to an electric signal for detecting radiation.

Nuclear Fission A very heavy nucleus splits into more-stable nuclei of intermediate mass. Releases enormous amounts of energy Occurs spontaneously or when bombarded by particles.

Nuclear Chain Reactions A chain reaction is a reaction in which the material that starts the reaction is also one of the products and can start another reaction. When there isn’t enough starting material left or when the neutrons escape without hitting the nucleus, the reaction stops. A critical mass is needed to sustain the chain reaction. The minimum amount of nuclide that provides the number of neutrons needed to sustain a chain reaction.

Nuclear Reactors Use controlled-fission chain reactions to produce energy and radioactive nuclides. Nuclear power plants convert heat produced by nuclear fission into electrical energy.

Nuclear power plants There are five main components: sheilding, fuel, control rods, moderator, and coolant. Shielding – radiation-absorbing material used to decrease the emission of radiation, especially gamma rays, from nuclear reactors. Control rods – neutron-absorbing rods that help control the reaction by limiting free neutrons. Moderator – used to slow down the fast neutrons produced by fission Uranium-235 is usually the fuel Coolant is simply water which can absorb neutrons to become heavy water, the H2O becomes D2O.

Nuclear Fusion Low-mass nuclei combine to form a heavier, more stable nucleus. Releases more energy per gram of fuel than fission. Occurs at extremely high temperature and pressure. Occurs in our sun and stars that are similar to our sun. Researchers are currently studying ways to contain the reacting plasma that is required for fusion.