Question

If a rock started with 1,000 atoms of a parent but now contains 250 atoms, how many half lives have passed?

Answer 1

Answer: 2 half lives have passed.

Explanation:

The formula used is;

$a=\frac{a_o}{2^n}$

where,

a = amount of reactant left after n-half lives = 250 atoms

$a_o$ = Initial amount of the reactant = 1000 atoms

n = number of half lives= ?

Putting values in above equation, we get:

$250=\frac{1000}{2^n}$

$2^n=4$

$2^n=2^2$

$n=2$

Therefore, 2 half lives have passed.