# Formula Radioactive Decay Activity    Number of atomic nuclei    Decay constant

## Activity

Unit
Activity of a radioactive sample indicates how fast the atoms of the radioactive sample decay.

## Number of atomic nuclei

Unit
Number of atomic nuclei of a radioactive sample that have not yet decayed. The number is given by:$$N(t) ~=~ N_0 \, \mathrm{e}^{-\lambda \, t}$$

## Decay constant

Unit
Decay constant specifies the probability of decay per unit of time. In other words: 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: $$\tau = \frac{1}{\lambda}$$.