Chapter 18: Nuclear Chemistry Section 1: Atomic Nuclei & Nuclear Stability

Nuclear Stability Nucleon: proton or neutronNucleon: proton or neutron Nuclide: isotope of a specific element identified by the number of protons and neutrons.Nuclide: isotope of a specific element identified by the number of protons and neutrons. Atomic structure/symbol. A = mass number, Z = atomic number X = symbol of elementAtomic structure/symbol. A = mass number, Z = atomic number X = symbol of element AZXAZXAZXAZX Also written by only indicating mass:Also written by only indicating mass: Li - 7

Strong Nuclear Force & Repulsive Forces How can protons exist so close together in the nucleus when they repel each other? Strong nuclear force: interaction that binds nucleons (p & n) together in the nucleus of a nuclide.Strong nuclear force: interaction that binds nucleons (p & n) together in the nucleus of a nuclide. –Like gravity: every object in universe has mass has attraction toward every other object that has mass. –2 factors effect it: Mass of objects: more mass more attractionMass of objects: more mass more attraction Distance between objects: less distance more attractionDistance between objects: less distance more attraction

Binding Energy –When protons and neutrons form a nucleus energy is released. Always more stable when lower in energy. separate nucleons  nucleus + energy (enormous) –Binding Energy: energy released when nucleons come together. Binding energy is one way to determine nuclear stability. –Mass defect due to the difference between the mass of an atom and the sum of the mass of atom’s protons, neutrons and electrons and is equal to the binding energy.

Nuclear Stability Ratio between protons & neutrons Band of stability. –Stable nucleus: equal or more neutrons to protons Ratio 1:1Elements 1-20 Ratio 1:1.5Elements more neutrons are required Elements 83 and above are radioactive. No # of neutrons will be able to hold the nucleus together. Too man protons that want to repel the nucleus apart. Magic Numbers (protons or neutrons): –2, 8, 20, 28, 50, 82, 126 –Note: Every isotope of elements 84 and above is radioactive. Elements below 84 are said to be stable but many have some radioactive isotopes.

Section 2: Nuclear Change Radioactivity or Radioactive decay: –spontaneous breakdown of unstable nuclei to stable nuclei –release of particles and/or electronmagnetic waves Possible particles that can be released: –Proton –Neutron –Beta (  ): charged electron emitted from a neutron ( 0 e-). –Alpha (  ): helium nucleus ( 4 2 He). –Positron emission: positive particle emitted from a proton to form a neutron ( 0 e+). –Gamma ray (  ): high energy photon. Mass convert to energy.

Characteristics of Nuclear Particles ParticleMassChargeSymbolStopped by Proton p, p +, 1 +1 p, 1 1 HFew sheets of paper Neutron1.0080n, n 0, 1 0 nFew cm of Pb * Beta particle ,  -, 0 -1 e Few sheets of Al foil Positron  +, 0 +1 e Same as e - * Alpha particle ,  +2, 4 2 He Skin or sheet paper *Gamma Ray00 0000 Several cm of Pb

Nuclear decay reactions Complete the following: Must be balanced: left side = right side 222 Rn --> 218 Po + ? 87 Rb --> 87 Sr + ? 243 Am --> ? + 4 He ? + 0 e- --> 82 Rb

Section 3: Uses of Nuclear Chemistry Radioactive decay: process when a nucleus goes through to try to become stable releasing radiation. This can take more than one step U-238 (  )  Th-234 (  )  Pa-234 (  )  U-234 (  )  Th-230 (  )  Ra-226 (  )  Rn-222 (  )  Po-218 (  )  Pb-214 (  )  Bi-214 (  )  Po-214 (  )  Pb-210 (  )  Bi-210 (  )  Po-210 (  )  Pb-206 Each step takes a varying amount of time called a half life.

Half-life Half Life: Time require for half of a radioactive substance to decay. Is constant for specific isotopes (pg659 table 2) Can be used to determine the age of a substance. Half LivesOriginal Sample Left 0100% or 1 150% or 1/2 225% or 1/ % or 1/ % or 1/ % or 1/ or 1/64

Problems Practice: A fossil of an unknown age is found. Scientists determine that the C-14/C-12 ratio in the fossil is 1/32 of the ratio found in living substance today. Calculate the age of the fossil.

Fission Splitting the nucleus of a large atom into two or more fragments, produces additional neutrons and energy (nuclear power plants or atomic bombs). 235 U + 1 n ---> 93 Kr Ba n

Power Plants Chain reaction: change in a single molecule makes many molecules change until stable. Critical mass: minimum mass of a fissionable isotope that provides the number of neutrons needed to sustain a change reaction. Control rods help control the chain reaction.

Pros and Cons of Fission Pros Have technology to produce power Cost effective Decreases U.S. dependence on fossil fuels from foreign countries Little pollution Lowers global warming Less strip mining for coal Cons Reactor can overheat: meltdown allows radiation leaks into environment Radioactive wastes produced and needs storage U - is rare and expensive U - could also be used in nuclear weapons.

Nuclear Fusion Combination of the nuclei of small atoms to form a large nucleus, releases energy (mass loss convert into energy E = mc 2 ). Hydrogen Bomb Process occurs in the cores of stars & sun (high temperature 1 million K and pressure) 4 1 H --> 4 He e+

Pros & Cons of Fusion PROS Hydrogen needed is cheap Little to no radioactive waste Fusion produces more energy than fission CONS We don’t have the technology today to produce electricity using this method

Uses of nuclear chemistry Geologic dating Smoke detectors Detecting art forgeries Nuclear medicine Important to minimize daily exposure of living organisms –Acute radiation exposure: large amounts of radiation over a short period of time. –Chronic radiation exposure: small amounts of radiation over a long time. Power / Energy