The decay constant (symbol, λ and units, s⁻¹ or a⁻¹) of a radioactive nuclide is its probability of decay per unit time. The number of parent nuclides P therefore decreases with time t as dP/P dt = −λ. The energies involved in the binding of protons and neutrons by the nuclear forces are ca. 1,000,000 times stronger than those of the electronic and molecular forces. Decay probabilities and λs are insensitive not only to temperature and pressure but also to the strength of the bonds in which the radioactive element is held. The decay constant relates to the half-life of the nuclide T_1/2 through T_1/2 = ln 2/λ.