[kissoftalons]

Why is the melting point of NaCl so much higher than KBr?

I thought it would have something to do with their electronegativity and the intermolecular forces...but both compounds are ionic with an electroneg of 1.9
What would the other factors be that would affect melting point?

[Blueshawk]

My guess is that it has something to due with the structure, which are both 6 coordinate: octahedral.

It may be how close the Na and Cl atom are to each other in the crystal, vs how close K is to Br.

NaCl also has a lower density than KBr.

NaCl      MP = 801C     D = 2170 kg m-3

KBr        MP = 734C     D =  2750 kg m-3

If anyone else knows of more or why I am curious too.  THX

[jdurg]

From my values (Na=0.9, Cl=3.16, K=0.82, Br=2.96), the electronegativity difference between potassium and bromine is less than that between chlorine and sodium by about 0.12 (whatever the units are).  So the difference in strength may account, along with other things, for why the melting point of KBr is so much lower than that of NaCl.

[Borek]

Assuming both substances are almost perfectly ionic, crystal is hold just by coulomb force. Charges are identical in both cases (+/- e), but the distances are larger in case of KBr. That's probably not all, but could be pretty important.

[kissoftalons]

The atoms being farther apart in KBr would result in a LOWER melting point? That combined with the fact that KBr has a higher density would cause a lower melting point? I'm trying to think of concepts learned in gr 11 that would help explain this...but nothing comes to mind!!  

[kissoftalons]

Apparently, the melting point has something to do with the ionic radii of the two compounds. The smaller the ion the more concentrated the charge. I don't know how this links to the original question regarding why the melting point of NaCl is higher than KBr.

How do you find the ionic radii of a compound?
The information I found is as follows

Na - 190 pm Cl - 79 pm
K - 243 pm br - 94 pm

So the ionic radii of Chlorine is the lowest...it should be the smallest ion...how does that give it a high melting point?

[Borek]

Na - 190 pm Cl - 79 pm
K - 243 pm br - 94 pm

Obviously something is wrong.

K⁺ and Cl^- are isoelectronic, yet K⁺ has two additional protons in the nucleus, thus it must be smaller. That's not reflected in the data you provided.

To melt the salt you have to move ions apart. If they have the same charge, when it will be easier - when they are close, or far away? Think how the coulomb force changes with distance.