Nuclear Chemistry Unit 4

History Wilhelm Conrad Roentgen ( ) Wilhelm Conrad Roentgen ( ) Awarded a Nobel Prize in Physics in 1901 Awarded a Nobel Prize in Physics in 1901 Discovered X-Rays - November 8, 1895 Discovered X-Rays - November 8, 1895 Saw a glowing fluorescent screen on a nearby table and determined that the fluorescence was caused by invisible rays that were able to go through opaque black paper Saw a glowing fluorescent screen on a nearby table and determined that the fluorescence was caused by invisible rays that were able to go through opaque black paper Died of unrelated causes at 77, one of few scientists to always use a lead shield Died of unrelated causes at 77, one of few scientists to always use a lead shield

History Antoine Henri Becquerel ( ) Antoine Henri Becquerel ( ) Nobel Prize in Physics, 1903 Nobel Prize in Physics, 1903 Discovered radioactivity Discovered radioactivity Proved that uranium emitted radiation without an external source of energy such as the sun Proved that uranium emitted radiation without an external source of energy such as the sun Discovered that radiation emitted by uranium shared certain characteristics with X rays but could be deflected by a magnetic field and therefore must consist of charged particles Discovered that radiation emitted by uranium shared certain characteristics with X rays but could be deflected by a magnetic field and therefore must consist of charged particles

History Pierre Curie ( ) Pierre Curie ( ) Nobel Prize in Physics, 1903 Nobel Prize in Physics, 1903 Worked with his wife Marie to investigate the phenomenon of radioactivity in uranium ore Worked with his wife Marie to investigate the phenomenon of radioactivity in uranium ore Died in an accident crossing the street in a rainstorm Died in an accident crossing the street in a rainstorm

History Marie Curie ( ) Marie Curie ( ) Nobel Prize in Physics, 1903 Nobel Prize in Physics, 1903 Nobel Prize in Chemistry, 1911 Nobel Prize in Chemistry, 1911 Discovered the elements Polonium (Po) and Radium (Ra) Discovered the elements Polonium (Po) and Radium (Ra) First person to win two Nobel Prizes First person to win two Nobel Prizes Died of overexposure to radiation Died of overexposure to radiation

Transmutation Involves reactions where the nucleus of the atom is changed Involves reactions where the nucleus of the atom is changed When the atom’s nucleus is changed, radiation is emitted When the atom’s nucleus is changed, radiation is emitted Transmutation: Transmutation: The process when the nucleus changes atomic number and a new element is formed The process when the nucleus changes atomic number and a new element is formed

Radioactive or not? Not all elements are radioactive. Isotopes (also called nuclides) that are not radioactive are called stable isotopes Not all elements are radioactive. Isotopes (also called nuclides) that are not radioactive are called stable isotopes Unstable isotopes: Unstable isotopes: Isotopes that undergo nuclear reactions and emit radioactivity Isotopes that undergo nuclear reactions and emit radioactivity All elements above atomic number 83 are radioactive! All elements above atomic number 83 are radioactive! Also known as radioisotopes or radionuclides Also known as radioisotopes or radionuclides

Unstable Nucleus In the nucleus, there is a lot of + charge, so there should be an electrostatic force pushing all of the protons apart In the nucleus, there is a lot of + charge, so there should be an electrostatic force pushing all of the protons apart This doesn’t happen – why? This doesn’t happen – why? There is a second force that acts on the protons when they are very close together. It is called the strong force There is a second force that acts on the protons when they are very close together. It is called the strong force

Strong Force There is a “magic number” of protons and neutrons that keep the nuclei stable There is a “magic number” of protons and neutrons that keep the nuclei stable When there are an even number of protons and an even number of neutrons, the nucleus is very stable When there are an even number of protons and an even number of neutrons, the nucleus is very stable

Strong Force When there are an even number of protons and an odd number of neutrons, the nucleus is less stable When there are an even number of protons and an odd number of neutrons, the nucleus is less stable

Strong Force When there are an odd number of protons and an odd number of neutrons, the nucleus is very unstable When there are an odd number of protons and an odd number of neutrons, the nucleus is very unstable

Stability Factors The ratio of neutrons to protons also contributes to stability The ratio of neutrons to protons also contributes to stability Maximum stability: Maximum stability: Smaller atoms = Ratio of 1 proton for every 1 neutron (A 1:1 ratio.) Smaller atoms = Ratio of 1 proton for every 1 neutron (A 1:1 ratio.) Larger atoms = More neutrons than protons (the extra mass keeps the atom stable) Larger atoms = More neutrons than protons (the extra mass keeps the atom stable)

Increasing Atomic Number It makes sense that as one adds more protons, it would take more neutrons to help hold the nucleus together It makes sense that as one adds more protons, it would take more neutrons to help hold the nucleus together Remember - all elements above Bismuth (83) are radioactive! Remember - all elements above Bismuth (83) are radioactive! If the atomic number is less than 83, radioactivity will be determined by the number of protons and neutrons If the atomic number is less than 83, radioactivity will be determined by the number of protons and neutrons

Radioactive Decay Process where an unstable nucleus emits particles and/or electromagnetic radiation Process where an unstable nucleus emits particles and/or electromagnetic radiation We say that the nucleus has spontaneously disintegrated to produce a new element We say that the nucleus has spontaneously disintegrated to produce a new element Transmutation occurs naturally Transmutation occurs naturally

Artificial Transmutation Also known as induced radioactivity Also known as induced radioactivity A nucleus alters its identity by reacting with or capturing a neutron or another nucleus A nucleus alters its identity by reacting with or capturing a neutron or another nucleus We say that we have changed or transmuted the nucleus by bombarding it with other particles We say that we have changed or transmuted the nucleus by bombarding it with other particles

Alpha Decay Radioactive decay where an alpha particle is emitted Radioactive decay where an alpha particle is emitted Alpha is the weakest type of radiation, with the least penetrating power Alpha is the weakest type of radiation, with the least penetrating power A sheet of paper can block alpha particles A sheet of paper can block alpha particles Alpha particles are Helium particles Alpha particles are Helium particles

Alpha Decay, Continued There is always a conservation of mass and charge There is always a conservation of mass and charge Mass number  238 = Mass number  238 = Atomic number  92 = Atomic number  92 =

Beta Decay Radioactive decay where a beta particle is emitted Radioactive decay where a beta particle is emitted Beta particles have more penetrating power than alpha. It would take a thin sheet of aluminum or your hand to block beta particles Beta particles have more penetrating power than alpha. It would take a thin sheet of aluminum or your hand to block beta particles Beta particles are high speed electrons Beta particles are high speed electrons

Beta Decay, Continued There is always a conservation of mass and charge There is always a conservation of mass and charge Mass number  14 = Mass number  14 = Atomic number  6 = 7 + (-1) Atomic number  6 = 7 + (-1)

Gamma Decay Radioactive decay where gamma radiation is emitted Radioactive decay where gamma radiation is emitted Gamma radiation has the greatest penetrating power Gamma radiation has the greatest penetrating power Gamma rays are high energy photons Gamma rays are high energy photons Mass = 0, Charge = 0 Mass = 0, Charge = 0

Other Radiation Neutron Neutron Mass of 1, Charge is neutral Mass of 1, Charge is neutral Proton Proton Mass of 1, Charge is positive Mass of 1, Charge is positive Positron (Opposite of Beta) Positron (Opposite of Beta) Mass of 0, Charge is positive Mass of 0, Charge is positive

Nuclear Symbols

Relative Strength of Radiation

Radiation Review

Separation of Particles Use an electric field to separate a mixture of alpha, beta, and gamma radiation Use an electric field to separate a mixture of alpha, beta, and gamma radiation Alpha is +, so they are attracted to the - plate Alpha is +, so they are attracted to the - plate Beta is -, so they are attracted to the + plate Beta is -, so they are attracted to the + plate Gamma is neutral, so it passes straight through Gamma is neutral, so it passes straight through

Half Life The amount of time needed for one half of the nuclei of a substance to decay The amount of time needed for one half of the nuclei of a substance to decay Any substance that is radioactive will disappear over time as it changes into other substances Any substance that is radioactive will disappear over time as it changes into other substances

Half Life

Example A radioactive substance has a half-life of 20 minutes. If we begin with a 500 g sample, how much of the original sample remains after two hours? A radioactive substance has a half-life of 20 minutes. If we begin with a 500 g sample, how much of the original sample remains after two hours?

Solution The easiest way to attack these questions is to start with the original amount of the sample, then draw arrows representing each half-life The easiest way to attack these questions is to start with the original amount of the sample, then draw arrows representing each half-life Two hours is 120 minutes, so that’s six half-lives: Two hours is 120 minutes, so that’s six half-lives: 500  250  125  62.5     250  125  62.5    At the end of the stated time period, 7.8 g remains At the end of the stated time period, 7.8 g remains

Half Life

Nuclear Reactions Cause transmutation of elements with the release of a large amount of energy Cause transmutation of elements with the release of a large amount of energy These reactions are the source of electric energy at nuclear power plants as well as the energy from the Sun and stars These reactions are the source of electric energy at nuclear power plants as well as the energy from the Sun and stars This immense amount of energy comes from the conversion of matter to energy This immense amount of energy comes from the conversion of matter to energy

Nuclear Reactions The mass of a nucleus is not exactly equal to the sum of the masses of its nucleons The mass of a nucleus is not exactly equal to the sum of the masses of its nucleons This difference in mass means that some nuclear mass is converted to energy This difference in mass means that some nuclear mass is converted to energy

Nuclear Fission

When a neutron strikes a Uranium-235 nucleus it can cause the nucleus to break apart into smaller nuclei When a neutron strikes a Uranium-235 nucleus it can cause the nucleus to break apart into smaller nuclei The fission reaction produces smaller nuclei as well as loose neutrons The fission reaction produces smaller nuclei as well as loose neutrons The loose neutron can strike the smaller nuclei, causing that nuclei to divide The loose neutron can strike the smaller nuclei, causing that nuclei to divide This is known as a nuclear chain reaction This is known as a nuclear chain reaction

Nuclear Fission

Splitting a nucleus into smaller pieces increases overall stability Splitting a nucleus into smaller pieces increases overall stability Fission = division (of the nucleus) Fission = division (of the nucleus) Nuclear fission is used as a source of electricity in nuclear power plants Nuclear fission is used as a source of electricity in nuclear power plants The most common fission reaction is the fission of Uranium-235 The most common fission reaction is the fission of Uranium-235

Nuclear Power When the chain reaction is controlled, the energy can be captured and converted into electricity When the chain reaction is controlled, the energy can be captured and converted into electricity

Nuclear Power Advantages: Not as much fuel needed Not as much fuel needed No pollutants or greenhouse gases released No pollutants or greenhouse gases releasedDisadvantages: Waste material is extremely radioactive, and stays that way for thousands of years Waste material is extremely radioactive, and stays that way for thousands of years They need fuel to operate They need fuel to operate

Nuclear Fusion Nuclei of smaller atoms join together to form a larger atom Nuclei of smaller atoms join together to form a larger atom Converts matter into large amounts of energy Converts matter into large amounts of energy

Nuclear Power Fission Animation Fission Animation Fission Animation Fission Animation Animations Animations Animations Fusion Animation Fusion Animation Fusion Animation Fusion Animation Vision Learning Vision Learning Vision Learning Vision Learning