[Rutherford]

Why KI has a bigger melting point than LiI?
Li ion is smaller that KI, so the distance between Li and I is smaller that K and I, so the crystal cell should be stronger and therefore a bigger melting point should arise. Why it isn't that way?

[camptzak]

Why KI has a bigger melting point than LiI?
Li ion is smaller that KI, so the distance between Li and I is smaller that K and I, so the crystal cell should be stronger and therefore a bigger melting point should arise. Why it isn't that way?

which is more electronegative Lithium or Pottasium?

what do you know about ionic bonds?

[Rutherford]

Lithium. About ionic bounds, I know that there are crystal cells and that their strength can be represented as:q₁*q₂/r². r would be smaller for LiI so the crystal cell should be more stable.

[AWK]

Why KI has a bigger melting point than LiI?
Li ion is smaller that KI, so the distance between Li and I is smaller that K and I, so the crystal cell should be stronger and therefore a bigger melting point should arise. Why it isn't that way?

This is not so simple correlation as some texbooks show
http://www.askiitians.com/iit_jee-S_and_P_Block_elements/Halides-of-Alkali-metals
Such correlations work for the same cation but not for the same anion
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CIIBEBYwCA&url=http%3A%2F%2Facademic.pgcc.edu%2F~ssinex%2Fexcelets%2Flattice_energy.xls&ei=yEEcUK_RBcTIhAerqoGIAw&usg=AFQjCNGd__0-NoCk-EmHuD92mQRWtHZFhQ&sig2=-8JQCxWm1xblGaAlz9oM7Q

[bossza500]

I think that KI is bigger than LiI so the force that hold molecules together will be more than LiI's.
I think this is the reason why KI B.P. is bigger than LiI's 

[Rutherford]

Why KI has a bigger melting point than LiI?
Li ion is smaller that KI, so the distance between Li and I is smaller that K and I, so the crystal cell should be stronger and therefore a bigger melting point should arise. Why it isn't that way?

This is not so simple correlation as some texbooks show
http://www.askiitians.com/iit_jee-S_and_P_Block_elements/Halides-of-Alkali-metals
Such correlations work for the same cation but not for the same anion
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CIIBEBYwCA&url=http%3A%2F%2Facademic.pgcc.edu%2F~ssinex%2Fexcelets%2Flattice_energy.xls&ei=yEEcUK_RBcTIhAerqoGIAw&usg=AFQjCNGd__0-NoCk-EmHuD92mQRWtHZFhQ&sig2=-8JQCxWm1xblGaAlz9oM7Q

Okay, I see that bigger energy is released when the crystal cell of LiI is formed, so it is stronger, but because of bigger polarization it has a lower mp. Thanks for the help.