  The electromagnetic scattering resonances of a non-magnetic object much
smaller than the incident wavelength in vacuum can be either described by the
electroquasistatic approximation of the Maxwell's equations if its permittivity
is negative, or by the magnetoquasistatic approximation if its permittivity is
positive and sufficiently high. Nevertheless, these two approximations fail to
correctly account for the frequency shift and the radiative broadening of the
resonances when the size of the object becomes comparable to the wavelength of
operation. In this manuscript, the radiation corrections to the
electroquasistatic and magnetoquasistatic resonances of arbitrarily-shaped
objects are derived, which only depend on the quasistatic current modes. Then,
closed form expressions of the frequency-shift and the radiative Q-factor of
both plasmonic and dielectric modes of small objects are introduced, where the
dependencies on the material and the size of the object are factorized. In
particular, it is shown that the radiative Q-factor explicitly depends on the
multipolar components of the quasistatic modes.
