# Formula Radioactive Decay Law Initial number    Decay constant

## Number

Unit
Number $$N$$ of atomic nuclei not yet decayed at the time $$t$$.

## Initial number

Unit
Number of atomic nuclei of a radioactive sample present at the beginning, that is at time $$t ~=~ 0$$.

## Decay constant

Decay constant indicates the probability of decay per unit of time. The decay constant determines how fast a nuclide (radioactive substance) decays. Different nuclides decay at different rates.

The reciprocal of the decay constant is the lifetime: $$\tau = \frac{1}{\lambda}$$.

## Time

Unit
Time at which there are still $$N$$ non-decayed atomic nuclei.