How many half-lives have passed if the amount of parent isotope is 12.5%?

Three half lives.

Explanation:

Given data:

Number of half lives passed = ?

Amount of parent isotope left = 12.5%

Solution:

At time zero = 100%

At 1st half life = 100%/2 = 50%

At second half life = 50%/2 = 25%

At third half life = 25%/2 = 12.5%

It means three half lives would passed.

the number of protons in the nucleus of a given element. has slight luster and conducts electricity at high temperatures like metal.but like a nonmetal,boron is brittle and does not conduct electricity well at low temperatures. in graphite, carbon is a soft,brittle solid.it is reflective,ductile,or malleable.