How much of a 1.00 mg sample of iodine remains after 3 half-lives?

0.125 mg.

The mass of a radioactive species decrease by by the end of each half-life.

For this 1.00 mg sample of unknown half-life:

Mass of the sample at the end of the n-th half-life;

.

As a result,

of the sample remains after three half-lives.

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8

level 4 to level 2 : ))