Question
If you have three substances with three different melting points;
- 445 °C
- 1907 °C
- -7 °C
Which of these would most likely have a metallic structure?
I understand that the third option would most likely be covalent. But between the first and second option, there are many bonds which have a similar melting point to both answers. Which of these would most likely indicate a metallic strucure to be involved.
Now, before we get to the "expected theoretical" answer, I should make one point clear: predicting the solid state just based on the melting point data can not be perfect! Browsing the data on melting points on Wikipedia, you'll find that vanadium has a melting point of $\pu{1910 ^\circ C}$, while zinc has a melting point of $\pu{420^\circ C}$. Both these values are very close to the options (1) and (2), hence, one can very well argue that both the options represent a metallic structure.
But this is definitely not the answer your teacher expects. A theoretical (and again, practically wrong) approach instead is to realize that the values of intermolecular forces increase in the order of molecular solid < metallic solid < ionic solid < covalent networked solid. Thus, the option (3) is probably a molecular solid, option (1) can be either an ionic solid or a covalent networked solid, while option (2) must be the metallic solid as expected.
I understand that the third option would most likely be covalent.
Just get rid that misconception already. Covalently bonded networked solids have higher melting points than several metals and even ionic solids! Diamond melts at $\ce{3800 K}$, while boron melts at $\ce{2349K}$. Our beloved ionic solid sodium chloride melts at a poor $\ce{1073K}$, while all the 3d series metals melt under $\ce{2200K}$! Hence, the assumption, that low melting point indicates a covalent solid, is false.
