Huge amounts of radioactive 13153I were released in the accident at Chernobyl in

Question

Huge amounts of radioactive 13153I were released in the accident at Chernobyl in 1986. Chemically, iodine goes to the human thyroid. (Doctors can use it for diagnosis and treatment of thyroid problems.) In a normal thyroid, 13153I absorption can cause damage to the thyroid.

Part A

Write down the reaction for the beta decay of 13153I.

Express your answer as a nuclear equation.

Part B

ts half-life is 8.0 d; how long would it take for ingested 13153I to become 6.0 % of the initial value?

Express your answer using two significant figures.

Part C

Absorbing 1 mCi of 13153I can be harmful; what mass of iodine is this?

Express your answer to one significant figure and include the appropriate units.

Explanation / Answer

a) Iodine 131 becomes xenon 131, the atomic mass remains the same but since the nature of the element changed the number of protons must have changed. In fact it increased by one.

In other words a neutron became a proton (mass remains the same, atomic number goes up by one) and a b particle was emitted.

I-131/53 ----> Xe-131/54 + beta particle

(b) We find the number of half-lives from

N/N0 = n;

(0.10) = n, or n log 2 = log 10, which gives n = 3.32.

Thus the elapsed time is

t = nT1/2 = (3.32)(8.0 days) = 27 days.

(c) We find the number of atoms from

Activity = lN;

(1.00 10 3 Ci)(3.7 1010 decays/s Ci) = [0.693/(8.0 days)(86,400 s/day)]N,

which gives N = 3.69 10^13 atoms.

The mass is

m = [(3.69 10^13 atoms)/(6.02 10^23 atoms/mol)](131 g/mol) = 8.0 * 10^-9 g.

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