2. If two reactant molecules collide with each other, what two reasons might they not combine?

The molecules contain like 54 of water 

They make up examples of physical properties 

Is there a c and d?

The equation for exponential decay, with a half-life of 13.2 hours, elapsed time  in hours,  as the initial amount, and  as the final amount, is    for the final amount, we're interested in what percentage of the final amount is left.  let  be the percentage left, in decimal form.  so the final amount,  , would be equal to  (percent of  , the inital value).    divide both sides by  .    since 36 hours have passed, substitute in     convert 0.1510 to a percent.so 15% of iodine-123 remains after 36 hours.