NUCLEAR CHEMISTRY By: Stephanie Chen and Stephanie Ng

Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie ( ).One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie ( ). She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

Nuclear Reactions vs. Normal Chemical Changes Nuclear reactions involve the nucleusNuclear reactions involve the nucleus The nucleus opens, and protons and neutrons are rearrangedThe nucleus opens, and protons and neutrons are rearranged The opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energyThe opening of the nucleus releases a tremendous amount of energy that holds the nucleus together – called binding energy “Normal” Chemical Reactions involve electrons, not protons and neutrons“Normal” Chemical Reactions involve electrons, not protons and neutrons

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Types of Radiation Alpha (ά) – a positively charged (+2) helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons Alpha (ά) – a positively charged (+2) helium isotope - we usually ignore the charge because it involves electrons, not protons and neutrons Beta (β) – an electronBeta (β) – an electron Gamma (γ) – pure energy; called a ray rather than a particleGamma (γ) – pure energy; called a ray rather than a particle

Other Nuclear Particles Neutron Neutron Positron – a positive electron Positron – a positive electron Proton – usually referred to as hydrogen-1Proton – usually referred to as hydrogen-1 Any other elemental isotopeAny other elemental isotope

Penetrating Ability

X A Z Mass Number Atomic Number Element Symbol Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons A Z 1p1p 1 1H1H 1 or proton 1n1n 0 neutron 0e0e 00 or electron 0e0e +1 00 or positron 4 He 2 44 2 or  particle

Balancing Nuclear Equations 1.Conserve mass number (A). The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants. 1n1n 0 U Cs Rb n1n = x1 2.Conserve atomic number (Z) or nuclear charge. The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants. 1n1n 0 U Cs Rb n1n = x0 23.1

212 Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212 Po. 4 He 2 44 2 or alpha particle Po 4 He + A X 84 2Z 212 = 4 + AA = = 2 + ZZ = Po 4 He Pb

Nuclear Stability and Radioactive Decay Beta decay 14 C 14 N + 0  K 40 Ca + 0  n 1 p + 0  Decrease # of neutrons by 1 Increase # of protons by 1 Positron decay 11 C 11 B + 0  K 38 Ar + 0  p 1 n + 0  Increase # of neutrons by 1 Decrease # of protons by 1 and have A = 0 and Z =

Electron capture decay Increase # of neutrons by 1 Decrease # of protons by 1 Nuclear Stability and Radioactive Decay 37 Ar + 0 e 37 Cl Fe + 0 e 55 Mn p + 0 e 1 n Alpha decay Decrease # of neutrons by 2 Decrease # of protons by Po 4 He Pb Spontaneous fission 252 Cf In n

Learning Check What radioactive isotope is produced in the following bombardment of boron? 10 B + 4 He 13 N + 1 n

Write Nuclear Equations! Write the nuclear equation for the beta emitter Co Co 0 e+ 60 Ni

Artificial Nuclear Reactions New elements or new isotopes of known elements are produced by bombarding an atom with a subatomic particle such as a proton or neutron -- or even a much heavier particle such as 4 He and 11 B. Reactions using neutrons are called  reactions because a  ray is usually emitted. Radioisotopes used in medicine are often made by  reactions.

Artificial Nuclear Reactions Example of a  reaction is production of radioactive 31 P for use in studies of P uptake in the body P n ---> P + 

Transuranium Elements Elements beyond 92 (transuranium) made starting with an  reaction U n ---> U +  U ---> Np  Np ---> Pu  Np ---> Pu 

Nuclear Stability Certain numbers of neutrons and protons are extra stable n or p = 2, 8, 20, 50, 82 and 126 Like extra stable numbers of electrons in noble gases (e - = 2, 10, 18, 36, 54 and 86) Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons All isotopes of the elements with atomic numbers higher than 83 are radioactive All isotopes of Tc and Pm are radioactive 23.2

Band of Stability and Radioactive Decay

Stability of Nuclei Out of > 300 stable isotopes: Even Odd Odd Even Z N P 19 9 F 2 1 H, 6 3 Li, 10 5 B, 14 7 N, Ta

Half-Life HALF-LIFE is the time that it takes for 1/2 a sample to decompose.HALF-LIFE is the time that it takes for 1/2 a sample to decompose. The rate of a nuclear transformation depends only on the “reactant” concentration.The rate of a nuclear transformation depends only on the “reactant” concentration.

Half-Life Decay of 20.0 mg of 15 O. What remains after 3 half-lives? After 5 half-lives?

Kinetics of Radioactive Decay For each duration (half-life), one half of the substance decomposes. For example: Ra-234 has a half-life of 3.6 days If you start with 50 grams of Ra-234 After 3.6 days > 25 grams After 7.2 days > 12.5 grams After 10.8 days > 6.25 grams

Kinetics of Radioactive Decay N daughter rate = - NN tt rate = N NN tt = N - N = N 0 e (- t) lnN = lnN 0 - t N = the number of atoms at time t N 0 = the number of atoms at time t = 0 is the decay constant (sometimes called k) Ln 2 = t½t½ 23.3 k =

Kinetics of Radioactive Decay [N] = [N] 0 exp(- t) ln[N] = ln[N] 0 - t [N] ln [N] 23.3

Radiocarbon Dating 14 N + 1 n 14 C + 1 H C 14 N + 0  t ½ = 5730 years Uranium-238 Dating 238 U 206 Pb   t ½ = 4.51 x 10 9 years 23.3

Learning Check! The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 31 hours?

Nuclear Fission Fission is the splitting of atoms These are usually very large, so that they are not as stable Fission chain has three general steps: 1. Initiation. Reaction of a single atom starts the chain (e.g., 235 U + neutron) 2. Propagation. 236 U fission releases neutrons that initiate other fissions 3. Termination.

Nuclear Fission

U + 1 n 90 Sr Xe n + Energy Energy = [mass 235 U + mass n – (mass 90 Sr + mass 143 Xe + 3 x mass n )] x c 2 Energy = 3.3 x J per 235 U = 2.0 x J per mole 235 U Combustion of 1 ton of coal = 5 x 10 7 J

Representation of a fission process.

Mass Defect Some of the mass can be converted into energySome of the mass can be converted into energy Shown by a very famous equation!Shown by a very famous equation! E=mc 2 EnergyMass Speed of light

Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons. BE + 19 F 9 1 p n BE = 9 x (p mass) + 10 x (n mass) – 19 F mass E = mc 2 BE (amu) = 9 x x – BE = amu 1 amu = 1.49 x J BE = 2.37 x J binding energy per nucleon = binding energy number of nucleons = 2.37 x J 19 nucleons = 1.25 x J 23.2

Nuclear binding energy per nucleon vs Mass number nuclear binding energy nucleon nuclear stability 23.2

Nuclear Fission 23.5 Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass. Non-critical Critical

Nuclear Fusion Fusion small nuclei combine 2 H + 3 H 4 He + 1 n Occurs in the sun and other stars Energy

Nuclear Fusion Fusion Excessive heat can not be contained Attempts at “cold” fusion have FAILED. “Hot” fusion is difficult to contain