How much of a sample of radio-isotope remais after one half life?

Half-life is defined as the time which is taken to reduce the initial amount of isotopes by half.

Hence, after one half-life, half of initial amount of isotopes will remain.

The equation for the half-life, t1/2 = ln 2 / λ, where λ is the decay constant. 

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.