Nuclear Chemistry Chapter 21 Unit 4 Nuclear Chemistry Chapter 21

Background Info A. Definitions: nucleons: protons or neutrons isotopes: ex. U-234 or U-238 nuclide: nucleus with set number of protons and neutrons. radionuclide: radioactive nucleus radioisotopes: atoms that are radioactive

B. Nuclear stability: -stability belt

C. Nuclear Equations & Types of Decay: unstable  electromag C. Nuclear Equations & Types of Decay: unstable  electromag. + particle + atom radioisotope radiation (stable or unstable)

If the atom is unstable, there can be multiple decays or a decay series.

He U He II. Types of Decay A. Alpha Decay: (He-4) + 4 2 238 92  234 90 He 4 2 +

 e I Xe e B. Beta Decay (high energy electron) + or −1 131 53 54  −1 e or I 131 53 Xe 54  + e −1

Beta Decay is: Equal to a neutron changing to a proton n  p + e

C. Gamma Radiation (photons) normally not written in the equation. 

D. Positron Emission: same size, opposite charge of an electron 1 C 11 6  B 5 + e 1

E. Electron Capture (opposite of beta decay) ex E. Electron Capture (opposite of beta decay) ex. Rb-81 + electron  Kr-81 Both positron emission and electron capture are equal to a proton changing into a neutron: positron emission: p  n + positron Electron capture: p 1 + e −1  n

III. Rates of Radioactive Decay Many decay almost instantly (not found in nature) First order kinetic process Half life: time required for ½ to react Each isotope has its own half life Graph p. 840 Not affected by pressure, temp, etc.

A. Dating C-14: because constant rate of decay, can be used to determine age. Half life: 5715 yrs C-14 in atmosphere CO2  plants  animals Ratio of C-12 : C-14 When dead- no longer taking in C…so from ratio at death set and then compare to current, you can find age. As C-14 decays the ratio changes. Atmospheric amts of CO2 fairly constant Check with tree rings & similar ages found

Pb-206 : U-238 U-238  Pb-206 (half life: 4.5 x 10 9 yr) Compare U: Pb ratio Use for geology

B. Calculations based on half lives Rate = kN where k = decay constant N = # of radioactive nuclei activity = rate at which sample decays (units: disintegrations/time, becquerel, curie)

One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

Equation 1 Use if given initial amt, final amt, or time elapsed, and decay constant (or half life) = -kt Nt N0 ln

No = original (bigger #) Nt = amount after time elapsed (smaller #) t = time elapsed k = constant

Equation 2 Use if given half life or decay constant = t1/2 0.693 k t ½ = half life k = rate constant = t1/2 0.693 k

PRACTICE A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archeological sample?

Equation 2 = t1/2 0.693 k

Equation 1 = -kt Nt N0 ln

IV. Biological Effects of Radiation Radiation is all around us

Radiation can: Excite: move e or particles faster known as non-ionizing radiation -usually neutralized by water 2. Ionize: removes an electron known as ionizing radiation – very dangerous

A. Ionizing Radiation Damage depends on: Energy of radiation Exposure time Internal vs. External (gamma and x-ray very harmful because small & can get inside body) Alpha & beta can do more damage if inside body but skin/clothing can block entry because they are larger

Cells most likely damaged: Those that reproduce rapidly Bone marrow, blood, lymph More dangerous for children & pregnant women

B. Radon Gas Generated as uranium decays in rocks & soil Chart on p. 857 Noble gas; no smell, taste, or odor Inhaled & exhaled Short half life…if decays in lungs then an alpha particle (large) is released Becomes Po-218 that also releases an alpha (and has a short half life)

C. Cancer treatment High energy radiation Lots of energy in a little time (short half lives) Administered as: Seeds – coated radioactive material inserted Ingested – (I-131 to thyroid) some chemicals go to specific areas in the body Co-60 shot into tumor Also damages healthy cells - side effects

V. Energy and Nuclear Chemistry E = mc2 Extremely large amounts of energy Fission: (nuclear power & weapons)

Fusion: (reaction on the sun) H-2 + H-3  He + Energy ideal for energy (less harmful products) but requires much energy to get rxn started.