The half-life of a substance is the time it takes for that substance to decrease to half its initial amount.

Half-life definition

Half-life is the time it takes for a given quantity to decrease to half its initial value. The half-life of radioactive material is the time it takes for half of the material to decay.

The half-life of a given radioactive element is a constant, regardless of the conditions in which it is measured (temperature, pressure), or whether it’s being created as an unstable atomic nucleus splits up into two smaller nuclei (“decay”), or if you’re talking about something that’s already been around forever.

Half-life is the time it takes for a given quantity to decrease to half its initial value. In other words, if you have 100 atoms of some substance, that number will decrease by half every half-life. For example, if you had 100 grams of hydrogen and left them alone for one year, you would end up with 50 grams at the end of that time—since each atom decays into an atom of helium (usually) in a matter of minutes.

The formula for calculating how many half-lives have passed is simple: H = L/2^t where H= number of half-lives and L= initial amount (in this case 100 g). So using our example above we get: H=100g/2^t which simplifies to t=(H*L)/2^n where n is any natural integer greater than 0 but less than or equal to 2*L

Half-life formula

The formula for calculating the half-life of an element is:

T = \frac {ln(2)}{\lambda} = \tau \times ln(2)

How to calculate the half-life

In nuclear physics, the half-life is the time it takes for a radioactive nuclide to decay by half.

The SI unit of half-life is the second; other units including years and days are used in geology and environmental sciences. For example, 25 years is equivalent to 5555/3 days, or 1555/14 days.

The originally defined natural radioactive decay constant was based on a specific isotope that has since changed into another isotope, unrelated by the similarity of mass or composition but having identical chemical properties. Therefore, this value should not be used today unless referring specifically to the original isotope (i.e., uranium).


What does half-life in physics mean?

Half-life, in radioactivity, is the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay.

Why do physicists use half-life?

In nuclear physics, the half-life is a useful measuring stick for how quickly a radioisotope will undergo radioactive decay, or how long a stable isotope will remain intact.

Which element has the longest half-life?

The half-life of xenon-124 — that is, the average time required for a group of xenon-124 atoms to diminish by half — is about 18 sextillion years (1.8 x 10^22 years), roughly 1 trillion times the current age of the universe.