Units of activity (the curie and the becquerel) also refer to a quantity of radioactive atoms. Because the probability of decay is a fixed physical quantity, for a known number of atoms of a particular radionuclide, a predictable number will decay in a given time.  The number of decays that will occur in one second in one gram of atoms of a particular radionuclide is known as the specific activity of that radionuclide.
The activity of a sample decreases with time because of decay.
The rules of radioactive decay may be used to convert activity to an actual number of atoms. They state that 1 Ci of radioactive atoms would follow the expression
: N (atoms) × λ (s) = 1 Ci = 3.7 × 10 Bq,
and so
: N = 3.7 × 10 Bq / λ,
where λ is the decay constant in s.
Here are some examples, ordered by half-life: