1 mg radium has `2.68 xx 10^(18)` atoms. Its half-life is 1620 years. How many radium atoms will disintegrate from 1 mg of pure radium in 3240 years ?

Explanation

No. of half lives in 3240 years `= (3240)/(1620) = 2`
Amount of radium left after two half lives `= 1 xx ((1)/(2))^(2)`
= 0.25 g
Amount of radium disintegrated = (1 - 0.25) = 0.75 mg
No. of atoms which have disintegrated `=0.75 xx 2.68 xx 10^(18)`
`= 2.01 xx 10^(18)`