I have to find melting points for Zinc sulfide and Zinc oxide and then explain it by comparing their lattice energies. I do not have to calculate lattice energies but I have to find their ionic radii, which is supposed to help me.
I found zinc sulfide to have a melting point of 1850 °C and zinc oxide 1900 °C.
As for the radius, I wasn't sure what to do but I found something that kind of made sense so I applied that logic to this problem: The ionic radius of the zinc(II) ion is 0.74 Å and that of the sulfide ion is 1.70 Å.
Therefore, 0.74/1.70 = 0.44.
For zinc oxide I just the radius of the ions again:0.74/1.40 =0.53
I believe lattice energy typically decreases with increasing radius. Also, higher lattice energies typically result in higher melting points, but that should mean that it is Zinc sulfide that has a higher melting point, not zinc oxide. What have I done wrong?