We assume a distribution of clouds optically thick for electron scattering ( OTCM ) which are moderately optically thin for absorption and we consider them as a model of the mean quasar spectrum of Laor et al .
( 1997 ) .
We show that the model is particularly sensitive to the value of the ionization parameter \xi and that for \xi \sim 500 the model well reproduces the optical/UV/X-ray mean quasar spectrum , in agreement with the estimates of the ionization parameter based on the energy of the iron K _ { \alpha } line .
We can not definitively reject synchrotron emission as a source of primary radiation but we favor the model in which the hard X-ray emission is produced by Compton scattering of soft photons in a central hot medium surrounded by cool clouds .
In such a model clouds are located typically at the distance of \sim 12 R _ { Schw } , with the covering factor about 0.88 and the radius of hot plasma is \sim 9 R _ { Schw } .
The model explains optical/UV emission as predominantly due to the dark sides of the clouds and the soft X-ray emission as due to the reflection by the irradiated sides of the clouds .
Therefore , atomic features are expected in these bands although they are hardly present in the observational data .
The kinematical effects connected with the cloud motion affect those features but do not remove them .
The level of the primary emission required to model the mean quasar spectrum is too low to reproduce the equivalent width of the iron K _ { \alpha } line correctly but more detailed computations may resolve this problem .