Chapter 10 Nuclear Chemistry Section 10.1 Radioactivity Section 10.4 Fission and Fusion

Section 10.1 Radioactivity Nuclear Decay Antoine Henri Becqueral (1852-1908)-discovery of radioactivity Radioactivity-the process in which an unstable atomic nucleus emits charged particles and energy Any atom containing an unstable nucleus is a radioactive isotope aka. radioisotope

Section 10.1 Radioactivity Nuclear Decay Becquerel-used uranium-238 in his experiment Radioisotopes spontaneously change into other isotopes over time. When composition of a radioisotope changes=undergoes nuclear decay Key Concept: During nuclear decay, atoms of one element can change into atoms of a different element altogether.

Section 10.1 Radioactivity Types of Nuclear Radiation Radioactive substances can be detected by measuring the nuclear radiation they give off. Nuclear radiation-charged particles and energy that are emitted from the nuclei of radioisotopes Key Concept: Common types of nuclear radiation include alpha particles, beta particles, and gamma rays.

Section 10.1 Radioactivity Types of Nuclear Radiation: Alpha Decay Uranium-238 decays=emits alpha particles Def.-a positively charged particle made up of two protons and two neutrons (the same as a helium nucleus) Symbol for alpha particle 4/2 He; subscript(2) is the atomic number; superscript(4) is the mass number (#protons + #neutrons) Also notated by Greek letter α

Section 10.1 Radioactivity Types of Nuclear Radiation: Alpha Decay Refers to nuclear decay that releases alpha particles Is an example of a nuclear reaction Nuclear reactions can be expressed as equations. 238/92 U → 234/90 Th + 4/2 He Alpha decay: product isotope has two fewer protons and two fewer neutrons than the reactant isotope Alpha particles: least penetrating and only travel a few centimeters in the air

Section 10.1 Radioactivity Types of Nuclear Radiation: Beta Decay Thorium-234 decays=releases negatively charged radiation (beta particles) Def.-an electron emitted by an unstable nucleus Nuclear equations written: 0/-1 e or β Atomic number is -1 because of negative charge Very little mass (1/1836)=mass number is 0

Section 10.1 Radioactivity Types of Nuclear Radiation: Beta Decay How can an atom’s nucleus, which is positively charged, emit a negatively charged particle? A neutron decomposes into a proton and an electron. The proton stays trapped in the nucleus; the electron is released 234/90 Th → 234/91 Pa + 0/-1 e The product isotope has 1 proton more and 1 neutron fewer than the reactant isotope. Mass numbers of isotopes are the same b/c the beta particle emitted basically has no mass. Beta particles: smaller mass, move at faster speed and penetrate more than alpha particles

Section 10.1 Radioactivity Types of Nuclear Radiation: Gamma Decay Nuclear radiation does not always consist of charged particles. Gamma ray-a penetrating ray of energy emitted by an unstable nucleus Gamma radiation: has no charge and no mass **Travel through space at the speed of light

Section 10.1 Radioactivity Types of Nuclear Radiation: Gamma Decay Gamma Decay: the atomic number and mass number of the atom remain the same **The energy of the nucleus decreases Often accompanies alpha or beta decay. Abbreviated: γ Much more penetrating than alpha or beta particles Takes several centimeters of lead or meters of concrete to stop gamma radiation

Penetrating Powers of Nuclear Radiation Figure 4

Section 10.1

Section 10.1 Radioactivity Effects of Nuclear Radiation We are exposed to nuclear radiation daily. Background radiation-nuclear radiation that occurs naturally in the environment Radioisotopes in air, water, rocks, plants and animals contribute to background radiation. Cosmic rays (streams of charged particles) from outer space Background radiation levels, in general, are low enough to be safe.

Section 10.1 Radioactivity Effects of Nuclear Radiation Nuclear radiation goes over background levels=damage cells and tissues of the body Key Concept: Nuclear radiation can ionize atoms. Cells exposed=bonds holding DNA molecules and proteins together may break=cells may not function properly Alpha particles, Beta particles and Gamma rays are all forms of ionizing radiation.

Section 10.1 Radioactivity Effects of Nuclear Radiation Alpha particles-skin damage similar to a burn; not serious hazard unless the substance emitting the particles is inhaled or eaten Beta particles-damages tissues in the body more than alpha particles Gamma rays-can penetrate deeply into the human body; all organs can be exposed to ionization damage

Section 10.1 Radioactivity Detecting Nuclear Radiation Scientific instruments can measure nuclear radiation. Key Concept: Devices that are used to detect nuclear radiation include Geiger counters and film badges. Geiger counter-uses gas-filled tube to measure ionizing radiation Nuclear radiation enter the tube, ionizes atoms of the gas, ions produce an electric current (measured) >the amount of nuclear radiation, the >the electric current produced in the tube

Section 10.1 Radioactivity Detecting Nuclear Radiation People working near or with radioactive materials wear film badges to monitor their exposure to nuclear radiation. Film badge-piece of photographic film wrapped in paper Film is developed and replaced with a new one periodically Exposure on film shows the amount of radiation the person wearing badge was exposed to

Section 10.4 Fission and Fusion Alternative energy sources may eventually take the place of fossil fuels (oil and coal) Nuclear energy is one that is widely used today. After discovery of radioactivity, found that atomic nuclei have vast amounts of energy Transmutations involved more than just the conversion of one element into another; they also involved conversion of mass into energy. Transmutation-conversion of atoms of one element to atoms of another (Section 10.3)

Section 10.4 Fission and Fusion Nuclear Forces What holds the nucleus of an atom together? Strong nuclear force-the attractive force that bind protons and neutrons together in the nucleus Does not depend on charge: acts among protons, among neutrons, and both protons and neutrons Key Concept: Over very short distances, the strong nuclear force is much greater than the electric forces among protons. Weakens as protons and neutrons get farther apart.

Section 10.4 Fission and Fusion The Effect of Size on Nuclear Force Effect of size of the nucleus on the force is complicated. More protons and neutrons=more possibilities there are for strong nuclear force attractions As size of nucleus increases, distance between protons and neutrons increases **Strong nuclear forces act over short distances=possibilities of many attractions is not realized in a large nucleus Nuclear force felt by proton and neutron in a large nucleus is about the same as the force felt in a small nucleus

Section 10.4 Fission and Fusion Unstable Nuclei Nucleus becomes unstable (ie. Radioactive) when the strong nuclear force can no longer overcome the repulsive electric forces among protons Electric forces increase as nucleus size increases.

Section 10.4 Fission and Fusion Hahn and Strassman (German chemists) 1938 Series of transmutation experiments (conversion of atoms of one element to atoms of another element) Bombarded uranium-235 with high-energy neutrons (produce more massive elements) Instead, produced isotopes of Barium Lise Meitner and Otto Frisch (1939) explained the results of the experiments

Section 10.4 Fission and Fusion Uranium-235 nuclei broken into smaller fragments (nuclear fission) Def.-the splitting of an atomic nucleus into two smaller parts Krypton-91, Barium-142, and Energy Key Concept: In nuclear fission, tremendous amount of energy can be produced from very small amounts of mass.

Nuclear Fission of Uranium-235 Figure 18 Nuclear Fission of Uranium-235

Section 10.4 Fission and Fusion Converting Mass Into Energy When fission occurs, some of the mass of the reactants is lost. Lost mass is converted into energy. **Einstein (30 years before fission) introduced the mass energy equation: E=mc2 Equation described how mass and energy relate E=energy; m=mass; c=speed of light (3.0 x 108 m/s)

Section 10.4 Fission and Fusion Converting Mass Into Energy Small amount of mass = release an enormous amount of energy Large amount of energy = small amount of mass Law of conservation of mass and energy-the total amount of mass and energy remains constant

Section 10.4 Fission and Fusion Triggering a Chain Reaction Nuclear fission can follow a pattern where one reaction leads to a series of others. Def.-neutrons released during the splitting of an initial nucleus trigger a series of nuclear fissions. Speed of chain reactions can vary. Uncontrolled-all of the released neutrons are free to cause other fissions (fast and intense release of energy) Ex. Nuclear weapons

Section 10.4 Fission and Fusion Triggering a Chain Reaction Controlled-some neutrons absorbed by nonfissionable materials (1 new fission for each splitting of an atom) Heat-used to generate electrical energy Disadvantage-radioactive waste Sustain a chain reaction: each nucleus that’s split must produce one neutron that causes the fission of another nucleus Critical mass-smallest possible mass of a fissionable material that can sustain a chain reaction

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Chain Reaction of Uranium-235 Figure 19 Chain Reaction of Uranium-235

Section 10.4 Fission and Fusion Nuclear Energy from Fission Nuclear power plants-20% of electricity in U.S. Controlled fission of uranium-235; done in vessel (fission reactor) Nuclear power plants: no air pollutants Safety and environmental issues: employee protection, radioactive waste (isolated and stored), lose control of fission reactor (core melts b/c cooling system fails {meltdown}); radioactive material may be released, structure housing reactor is not secure=environment can be contaminated

Section 10.4 Fission and Fusion Also releases huge amounts of energy Def.-a process in which the nuclei of two atoms combine to form a larger nucleus Same as fission: a small fraction of the reactant mass is converted into energy Sun and stars powered by fusion of H and He Sun : 600 million tons of H undergo fusion each second; 4 million tons of it is converted to energy

Section 10.4 Fission and Fusion Require extremely high temperatures Sun: temps. can reach 10,000,000 C (matter exists as plasma) Plasma-a state of matter in which atoms have been stripped of their electrons Gas (nuclei and electrons) Future: may provide an efficient and clean source of electricity

Section 10.4 Fission and Fusion Plan: fusion reactors fueled by 2 hydrogen isotopes: deuterium and tritium (produce helium, neutrons, and energy) Problem: Achieving high temperatures required to start the reaction; containing the plasma