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. 

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.