How long does it take a 180 g sample of au-198 to decay to 1/8 it’s original mass

8.10 days  

Explanation:

The half-life of gold-198 (2.70 days) is the time it takes for half the gold to decay.  

After one half-life, half (50 %) of the original amount will remain.

After a second half-life, half of that amount (25 %) will remain, and so on.

We can construct a table:

No. of half-lives  Fraction remaining

            1                        

            2                      

            3                      

Thus, the gold will decay to ⅛ of its original mass after three half-lives.

∴ Time required = 3 × 2.70 days = 8.10 days

indium, and thallium have three electrons i think