Nuclear Chemistry

Nuclear Chemistry 3221.3.8, 3221.3.9

Nuclear Chemistry The study of the properties and reactions of atomic nuclei Atoms with identical atomic numbers but different mass numbers are called isotopes, and the nucleus of a given isotope is called a nuclide Nuclear reaction: change the nucleus of an atom Radioactivity: phenomena by which an unstable nucleus emits particles and/or electromagnetic radiation spontaneously *All elements greater than Z= 83

More Vocabulary • Nucleon: collective name for protons and neutrons making up the atomic nucleus • Nuclear binding energy: energy released when a nucleus is formed from nucleons • Binding energy per nucleon: the binding energy of the nucleus divided by the number of nucleons it contains

Mass Defect (in Helium atom) 2 protons: (2 x 1.007276 amu) = 2.014552 amu 2 neutrons: (2 x 1.008665 amu) = 2.017330 amu 2 electrons: (2 x .0005486 amu) = 0.001097 amu Total combined mass: 4.032979 amu Actual measured mass: 4.002602 amu Mass defect: 0.030377 amu Mass defect: the difference between the mass of an atom and the sum of the masses of its protons, neutrons and electrons

What causes this loss of mass???? • Albert Einstein postulated that mass could be converted to energy, and energy to mass E = mc2 • The mass defect is caused by the conversion of mass to energy upon the formation of the nucleus • The greater the difference of the masses, the greater the binding energy per nucleon

Nucleons and Nuclear Stability • In atoms with low atomic numbers, the most stable nuclei are those with a neutron-proton ration of approximately 1:1 • Ex. He, C, Cl • In atoms with higher atomic numbers, the most stable nuclei have a ration of 1.5:1 • Ex. Pb-206 (124 neutrons and 82 protons) • In atoms with Z > 83, no stable nuclides exist!!!

Band of Nuclear Stability Stable nuclei tend to have even numbers of nucleons because higher stability occurs when nucleons are paired. Nuclear shell model: nucleons exist in different energy levels, or shells, in the nucleus Magic Numbers: the numbers of nucleons that represent completed nuclear energy levels—2, 8, 20, 28, 50, 82, and 126

Chemical Reaction vs. Nuclear Reaction Chemical Reaction Nuclear Reaction Elements (or isotopes of the same element) are converted from one to another May involve protons, neutrons, electrons, or other elementary particles Reactions are accompanied by absorption or release of tremendous amounts of energy Rates are normally not affected by temperature, pressure, and catalysts • Atoms are rearranged by the breaking and forming of chemical bonds • Only electrons involved in atomic orbitals are involved in the breaking and forming of bonds • Reactions are accompanied by absorption or release of relatively small amounts of energy • Rates of reaction are influenced by temperature, pressure, concentration , and presence of catalyst

Balancing Nuclear reactions • The total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation • When atomic number changes, the identity of the element changes • Transmutation: the change in the identity of a nucleus as a result of a change in the number of its protons

Identify the product that balances the following nuclear reaction:212Po  4He + ____ Mass number: 212 – 4 = 208 Atomic number: 84 – 2 = 82 protons The nuclide has a mass of 208 and Z=82 208Pb 2 84 82

Your turn! 99 2 0 • 253Es + 4He  1n + ___ • 142Pm + ____  142Nd • 187Re + ____  188Re + 1H • 9Be + 4He  ____ + 1n • 22Na + ____  22Ne 61 60 1 75 75 4 2 0 10 11

Radioactive Decay • spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by the emission of particles, electromagnetic radiation, or both • Types: • Alpha particle emission • Beta particle emission • Positron emission • Gamma ray • Electron capture

Alpha (α) radiation • Positively charged with characteristics of helium nuclei (2 protons and neutrons) • Alpha decay is common for heavy radioisotopes • these are fast moving helium atoms. They have high energy, but due to their large mass, they are stopped by just a few inches of air, or a piece of paper. 4He 2

Beta (β) radiation • These are fast moving electrons. Since electrons are might lighter than helium atoms, they are able to penetrate further, through several feet of air, or several millimeters of plastic or less of very light metals. 0β -1

Positron Emission • A particle that has the same mass as an electron, but has a positive charge, and is emitted from the nucleus during some types of radioactive decay 0β +1

Electron capture • An inner orbital electron is captured by the nucleus of its own atom 0e -1

Gamma (γ) radiation • High energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state • Usually occurs immediately following another type of decay, when other types of decay leave the nucleus in an excited state

Writing balanced nuclear equations Alpha emission 242Cm 4He + 238Pu 242Cm  α + 238Pu 238U  4He + 234Th Practice: 96 2 94 96 94 92 2 90

Writing equations (cont’d)

Nuclear Chemistry