The natural radioactive decay of 235U is by Alpha emission with a half-life of 704 Million years. Remember that an Alpha particle consists of two protons and two neutrons. What is the radiogenic daughter isotope produced by this decay?
231U
231Th
235Th
238U
206Pb
2. The half-life for the decay of 222Rn gas is about 3.8 days. If we started with one mole of 222Rn gas, how many days (to the nearest 1/10th of a day) would it take until there was only 1/16 of a mole left?
1)
Let the product be X
Balance mass number on both sides
mass of U = mass of He + mass of X
1*235 = 1*4 + M
M = 231
Balance atomic number on both sides
atomic of U = atomic of He + atomic of X
1*92 = 1*2 + A
A = 90
Atomic number 90 is for element Th
So, the product is
231-Th
Answer: 231-Th
2)
After n half life, ratio of mass:
N/No = (1/2)^n
(1/16) = (1/2)^n
(1/2)^4 = (1/2)^n
n = 4
So, it takes 4 half lives
time taken = number of half lives * half life
= 4*3.8 days
= 15.2 days
Answer: 15.2 days
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