In other words, the decay rate is independent of an element's physical state such as surrounding temperature and pressure.
The unstable nucleus then releases radiation in order to gain stability.
For example, the stable element Beryllium usually contains 4 protons and 5 neutrons in its nucleus (this is not considered a very large difference).
In other words, the reaction rate does not depend upon the temperature, pressure, and other physical determinants.
However, like a typical rate law equation, radioactive decay rate can be integrated to link the concentration of a reactant with time.
The rate of decay remains constant throughout the decay process.
There are three ways to show the exponential nature of half-life.
Another example is the element Uranium-238 which has 54 more neutrons than its protons (Atomic umber =92).
This element gains stability by passing through various types of decays (19 steps-- also known as the Uranium series) and is converted into Pb-206 (atomic number 82).
The atomic nucleus which is in the center of the atom is buffered by surrounding electrons and external conditions.
Because of this, the study of decay is independent of the element's environment.
For information on each type of decay, read the page Decay pathways.