Quote: The cadmium isotope 109Cd has a half-life of 462 days. A sample begins with 1.0×1012109Cd atoms. How many are left after (a) 29 days, (b) 850 days, and (c) 5400 days?
You know that radioactive decay is a decaying exponential function of the form
I(t) = I
0e
-at, here t is in days, and a is a rate constant to be solved for given the half life.
Half life means the time until half of the original radioactive atoms are present. So let t
h be the half life. Then
1/2 I
0 = I
0 e
-ath 1/2 = e
-ath ln(1/2) = -a t
h a = -ln(1/2)/t
h = -ln(1/2)/462
Now that you have a you can just plug in the time to answer questions a-c
+6251 
