# The half ­life of a radioactive isotope is three hours

The half ­life of a radioactive isotope is three hours. If the initial mass of the isotope were 256 g, the mass of it remaining undecayed after 18 hours would be

1. 16.0 g
2. 12.0 g
3. 8.0 g
4. 4.0 g

t1/2 = 3 hours

n = T/t1/2 = 18/3 = 6

N = No(1/2)n = 256(1/2)6 = 4 gm

The correct option is D.