iodine 131 has a half life of 8 days what is the decay rate

This is the fraction of the number of atoms hat decay in a second

This is the fraction of the number of atoms hat decay in a second, this rate is size of the proportionality constant between the population of the radioactive  atoms and the and the rate at which it decays. It can be expressed per seconds, per minutes, per  hour and even per day.

 For example to expressed it per minutes is to devided the decay constant by 60 second

 

 

 To express it in per day . you simply devide it by 24 * 36000