The Born equation is a means of estimating the Gibbs free energy of solvation of a gaseous ion. It is an electrostatic model that treats the solvent as a continuous dielectric medium (it is thus one member of a class of methods known as continuum solvation methods). It was derived by Max Born. where: * NA = Avogadro constant * z = charge of ion * e = elementary charge, 1.6022×10−19 C * ε0 = permittivity of free space * r0 = effective radius of ion * εr = dielectric constant of the solvent