One gram of Radium emits 3.7×1010  α- particles per second. Calculate mean life of radium. Given Atomic mass of radium  = 226

Given : Activity = dNdt =λN = 3.7 ×1010 here λ=ln2Th   and  N=massmolar mass×NA=1226×6.023×1023 ∴3.7×1010=0.693×6.023×1023Th×226 ⇒Th=4.990×1010 s  Now, Mean Life= Half lifeln2=4.990×10100.693=7.2 ×1010 s