Question

23) The half -life of radium is 1620 years.When will a 20-gam sample of radium be completely gone Explain your reasoning

Answer 1

for radioactive decay...\[N=N_0e^{-at}\]where \[a=\frac{\ln2}T\]
u need N=0 when N_0=20 gm
0=20e^(-at)
e^(-at)=0
as 'a' is finite no. so your t must be infinity...