Nuclear Chemistry Overview Radioactivity –Alpha, beta, and gamma radiation –Positron emission and K-capture Half-life Fission and nuclear weapons Nuclear power Mass defect Fusion Radiation and humans: benefits and risks

Nuclear Stability Strong nuclear force: holds nuclei together, neutrons involved As Z  more and more neutrons are needed to balance increasing number of protons. Neutron to proton ratio:

Red line: n/p = 1

Nuclear Stability 1.When Z > 83, i.e. bigger than Bi, inherently unstable 2.When Z  20, i.e. Ca and smaller, usually n = p or n = p+1 e.g. 12 C & 13 C 24 Mg & 25 Mg 3. When 20 < Z < 84 > 1

Nuclear Stability Decay routes: 1.Too many neutrons: beta emission 2.Too few neutrons: positron or K-capture 3.Too big: alpha or beta emission

Radioactive Decay Some nuclei are unstable –Decay to attain stability –Release nuclear parts Decay routes: –Alpha emission –Beta emission –Gamma emission –Positron emission –K-capture Historically significant

Alpha Emission Common when Z > 83 He nucleus: 2 protons + 2 neutrons 92 p 146 n UHe + Th 2 p 2 n 90 p 144 n ThHe + Ra still unstable

Problem 1 Determine the products and write reactions for the alpha decay of the following: a) 226 Ra b) 212 Po

Beta Emission n  e + pC  e + N 6 p7 p 8 n7 n Ra  e + Ac 88 p 89 p 142 n141 n

Problem 2 Determine the products and write reactions for the beta decay of the following: a) 20 F b) 231 Th

Gamma Radiation Electromagnetic radiation (high energy light) Accompanies other forms of decay Ionizing radiation—more dangerous than x-rays

Positron Emission and K-capture Either or both occur when too few neutrons. Positron: positively charged electron, antimatter counterpart to an electron K-capture: Capture inner shell electron, same result as positron emission

Half life Way of expressing rates of radioactive decay t ½ = time required for half sample to decay

Half life Shorter half life: more radioactive, less stable nucleus, shorter life Longer half life: less radioactive, more stable nucleus, longer life Half-life activity Radio-dating of artifacts

Problem 3 a) 131 I has a half life of 8.04 days. If you start with a sample of 20.0 mg, how much is left after ~32 days? b)About how long would it take for the sample size to dwindle down to < 1 mg?

Nuclear Fission Binding energy fission fusion

Nuclear Fission U + n  Kr + Ba + 3 n

Nuclear Fission U + n  Br + La + 3 nU + n  Kr + Ba + 3 n

Fission Subcritical: not enough free neutrons to maintain reaction Critical: just enough free neutrons to maintain controlled fission (nuclear power plant) Supercritical: so many neutrons reaction goes out of control (atomic bombs)

Sustainability

Nuclear Power 235 U Natural abundance: < 1% Fuel grade: ~3% Weapons grade: ~7%

Nuclear Weapons Supercritical mass: uncontrolled chain reaction Challenge: how to keep reaction controlled before bomb is dropped

Nuclear Power Plant

Reactor Core

Nuclear Fusion fusion

Nuclear Fusion The sun:

Nuclear Fusion Fusion does not have waste issues common to fission processes. Very high activation energy Thermonuclear reactions (fission) to produce high temperatures required for fusion. Hydrogen bomb:

Radiation and Humans: Risks Calculate your radiation dose –EPA site: – Dose effects > 500 rem: death 100 – 400 rem: sick, likely to survive, risk of future cancers or teratogenic events 20 – 100 decrease in wbc count, depressed immunity, risk of future cancers Smoking ( 210 Pb, 210 Po) up to ~280 mrem/yr

Type of radiation Penetrating ability: Alpha: dead skin (~0.1 cm) Beta: ~1 cm Gamma: deeply penetrating Beneficial Effects: disease diagnostics and treatment

Tissue Affected External: gamma is the most damaging Internal: Alpha (size) Radon gas t ½ = 3.8 days t ½ = 3.11 min  emitter 90 Sr (  ) mimics Ca in the body