Nuclear Chemistry

I. Terms A. Nuclear reactions involve changes in the nucleus. 1) This is unusual – most chemical reactions only rearrange atoms. B. In nuclear chemistry: (within the scope of this course) 1) Atoms are termed nuclides. 2) Protons and/or neutrons are termed nucleons.

II. The Mass Defect A. Adding the individual masses of the protons, neutrons and electrons in an atom does not equal the atom’s total mass! Helium Example 

II. The Mass Defect B. Energy can exist in many forms and mass-energy is one of them: The “mass defect” ≈ The nuclear binding energy m mass defect  c 2 = E nuclear binding energy

II. The Mass Defect C. Different nuclides have different nuclear binding energies. A higher nuclear binding energy per nucleon means a more “tightly held together” nucleus, which means a more stable, lower energy atom!

II. The Mass Defect Which atoms appear to have the most stable nuclei? 56 Fe, 58 Fe and 62 Ni are the three most stable nuclides

III. Nuclide Stability A. Smaller nuclides (up to about Argon) are most stable if they have a 1:1 ratio of neutrons to protons. B. For later atoms, the stable nuclides start having a 1 ½ neutrons to 1 proton ratio.

III. Nuclide Stability C. “Magic” Numbers Nuclides with 2, 8, 20, 28, 50, 82 or 126 protons or neutrons or total nucleons are exceptionally stable. This leads scientists to believe in a “Nuclear Shell” model, or that nucleons exist at certain energy levels like electrons do. The “Magic” numbers would be like the “Noble Gas” configurations of electrons.

IV. Nuclear Decay A. Many nuclides are unstable, so they attempt to become stable by changing the numbers of protons or neutrons.

This is the decay series for U-238 as it heads toward a stable Pb-206 nuclide

B. Nuclear decay produces three common forms of Radiation Alpha particles – lowest energy, helium nucleii ( 4 2 He) Beta particles – medium energy, high speed electron-type particles ( 0 -1 β) Gamma Rays – high energy electromagnetic waves.

Positrons – particles equal in charge to a proton and equal in mass to an electron ( ). Positrons are a form of anti-matter. Neutrinos, Anti-Neutrions – Ha Ha… go study theoretical physics! C. Nuclear processes can produce other forms of radiation.

Example Problem 1 A. Balance the mass numbers and charge Consider an example: How many protons, neutrons, and electrons are in this Uranium isotope? 92 Protons and 92 Electrons 238 – 92 = 146 Neutrons

V. Nuclear Equations A. Balance the mass numbers and charge Consider an example: Show Uranium Emitting an Alpha particle ( 4 2 He)

Example Problem 2 Identify “x”, “y”, “?”

Example Problem 3 Show Beta Radiation from Iodine-131

Try these on your own: a)Write a nuclear reaction describing the fusion of Aluminum-27 and an alpha particle to yield Phosphorous-30. b) Show Thorium-234 undergoing beta decay.