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Huge amounts of radioactive 131/53 I 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, 131/53I absorption can cause damage to the thyroid.

part a) Write down the reaction for the beta decay of 131/53I.

part b)Its half-life is 8.0 d; how long would it take for ingested 13153I to become 9.5% of the initial value?

part c)Absorbing 1 mCi of 13153I can be harmful; what mass of iodine is this?
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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 � 1013 atoms.

The mass is

m = [(3.69 � 1013 atoms)/(6.02 � 1023 atoms/mol)](131 g/mol) =      8.0 * 10^-9 g.

8.0 * 10^-9 g.
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