  We present and discuss a "3-dimensional" diagnostic diagram for Seyfert2
galaxies obtained by means of X-ray and [OIII] data on a large sample of
objects (reported in the Appendix). The diagram shows the Kalpha iron line
equivalent width as a function of both the column density derived from the
photoelectric cutoff and the 2-10 keV flux normalized to the [OIII] optical
line flux (the latter corrected for extinction and assumed to be a true
indicator of the source intrinsic luminosity). We find that the hard X-ray
properties of type 2 objects depend on a single parameter, the absorbing column
density along the line of sight,in accordance with the unified model. The
diagram can be used to identify Compton thick sources and to isolate and study
peculiar objects. From this analysis we have obtained a column density
distribution of Seyfert 2 galaxies which is thought to be a good approximation
of the real distribution. A large population of heavily absorbed objects is
discovered, including many Compton thick candidates. Our results indicate that
the mean Log Nh/cm^(-2)in type 2 Seyferts is 23.5 and that as much as 23-30% of
sources have Nh > 10^24 cm^(-2).
