[TGI4]

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?

[Borek]

How does the attraction of point charges change with the distance?

[TGI4]

How does the attraction of point charges change with the distance?

It decreases with the distance?

[Borek]

It decreases with the distance?

And what are distances between charges in both compounds?

[Enthalpy]

I'm not quite convinced by this problem...

The melting temperature would depend on the liquid's properties too, not only on the solid's ones.

I've found only a decomposition temperature for ZnO, which in addition must be badly inaccurate. But ZnS sublimes at 1850°C and 1atm, wrong data.

50K difference means essentially "the same temperature".

Ionic radii are too variable to make safe deductions. At best, I could consider the molar volumes of both compounds.

Do both crystal have the same shape? With identical "ionic radii", some crystals are more compact than others.

Could one assume the same effective charges for ZnS and ZnO?