What are first order half life units?

2 Answers
Jun 22, 2017

Well, what is time? It had better be in seconds, minutes, etc.

A half-life is just a certain amount of time for half of something to go away / react. Therefore, it has the same units as time does. This does not at all depend on the order of the reaction with respect to the reactant.

Jun 22, 2017

Half-life units in 'years' & 'seconds' are the most widely used, but actually, whatever you wish ... sec, min, hrs, days, yrs. The application and one's preference could dictate more appropriate units.


Half life is the time for the concentration of substance to decrease to one-half its original concentration. For some reactions and decay processes, the time for 1 half-life can be in very small fractions of time while some are very large time intervals and expressed in powers of 10.

=> for the isotope Nitrogen-10 (N-10), the half-life can be as small as #2xx10^-22sec.# This could be expressed as #2xx10^-12# picoseconds (ps). Both are the same time interval.

=> for the isotope Potassium-40 (K-40), the half-life is reported to be #1.3xx10^9# years. While 'years' is the most widely accepted unit, one could express the half-life time interval in Mega-years by dividing by #1xx10^6# years per Mega-year => #1.3xx10^3# Mega-years. Again, both are the same time intervals.

The point is, although 'years' and 'seconds' are most widely accepted, the 'application' and one's 'preference' can dictate the units for half-life. I'm not sure if there is an accepted rule for this. Maybe someone else can add to this for more clairity.