Experimental elastic scattering cross sections in the atomic region 21 less than or equal to Z less than or equal to 82 are measured at an angle of 90 degrees using monoenergetic unpolarised Mo K alpha radiation. The measurements are based on a new method developed to estimate the degree of monochromaticity, the efficiency of the detecting system solid angle correction and some considerations which are necessary in experiments using secondary targets as excitation sources. Unpolarised monoenergetic radiation is produced using an X-ray tube and a secondary target To reflect the data on the current knowledge, the results are compared with theoretical estimates based on form factor formalisms including the anomalous scattering factors and with the recent S-matrix values. Based on the experimental evidence, the present results indicate the influence of solid state environmental effects, the importance of anomalous corrections near thresholds, superiority of S-matrix values over form factor values, correctness of high energy limit values on measured elastic scattering cross sections. However, the comparison of the experimental results with theoretical estimates indicates the importance of numerical calculations based on the multipole expansion of the S-matrix approach and high energy limit values.