The number the half lives will be 3 if the initial mass is 20 g and the remaining mass is 2.5 g.
Half-life, in the radioactivity, is the amount of the time needed for half of a radioactive sample's atomic nuclei to decay the (change spontaneously into the other nuclear species by emitting the particles and energy), or, alternatively, amount of the time needed for the radioactive material's rate of the disintegrations per second to decrease by the half.
mi = mr × 2ⁿ
mi= initial mass
mr= remaining mass after the n periods
n = number of the periods
2ⁿ= mi/ mr
2ⁿ=
20/ 2.5
2ⁿ= 8
n= 3
Thus the number of the half lives will be 3.
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