Abstract: From the structure date of the rare earth ethylsulphates RE(C₂H₅SO₄)₃·9H₂O(RE=La→Lu,except Pm),their efficient valence electrons,efficient valence densities and average coordinatiOn numbers of bonds etc,have been calculated by using the chemical bond theory of complex crystals of dielectric description。The results showed that in the crystals there are l6 kinds of bond which may be classified approximately in six types,namely:RP-O,O-H,S-O,C-O,C-C,and C-H. On the basis of the bond parameters of these bonds the refractive indexes of the crystals vvere thus calculated,the results were in good agreement vvith the experimentals.

Key words: rare earth ethylsulphate, chemical bond, refractive index, polariziability