Question

What are first order half life units?

Answer 1

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.

Answer 2

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.

Explanation:

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.

Examples*:
=> 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.

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*https://en.wikipedia.org/wiki/List_of_radioactive_isotopes_by_half-life#10.E2.88.9224_seconds