# How do you tell which molecule has a higher melting point?

In the instance of comparing $\ce{NaCl}$, and $\ce{Al2O3}$; which one has a higher melting point and why?

I have learned the following inter molecular forces, however the question the chapter is in does not explain about inter molecular forces, and is actually in the next chapter.

• London Dispersion Forces
• Dipole Dipole
• Hydrogen Bonds
• Ionic Crystal Lattice

I can imagine that London Dispersion Forces will prove that $\ce{Al2O3}$ have a higher melting point due to higher complexity; however, the chapter the question is in is about Properties of Chemical Compounds which talks about the trends on the periodic table ect.

Also, how does Ionic Crystal Lattice affect the melting point in this instance?

• Neither NaCl nor Al2O3 consist of molecules, so the first three points are irrelevant. – Ivan Neretin Oct 11 '17 at 4:25
• Generally, for ionic compounds, we can calculate the lattice energy for it using a simple formula. The higher the lattice energy, the higher the melting point. This is because, for an ionic salt, melting means the breaking of ionic bonds. Therefore, this relationship holds. – Tan Yong Boon Oct 11 '17 at 4:51
• @IvanNeretin I thought London dispersion affects Ionic and Covalent molecules? – George Jones Oct 11 '17 at 11:35
• There is no such thing as ionic molecules. – Ivan Neretin Oct 11 '17 at 11:40
• @IvanNeretin London dispersion forces, under the category of van der Waal forces: These are the weakest of the intermolecular forces and exist between all types of molecules, whether ionic or covalent—polar or nonpolar. – George Jones Oct 11 '17 at 11:44

Frankly a lot of these "comparisons" depend on knowing the answer then forming a rational argument to "justify" the assumed "common knowledge."

I just know that NaCl will form a molten salt fairly readily. Alumina is a refractory so it will melt at very high temperatures.

One thing that points to the difference is the solubility in water. NaCl readily dissolves, alumina does not dissolve to any appreciable extent. NaCl has ionic bonds, whereas alumina is more covalently bound. But you have to know the solubilities, nothing in just the compounds' formulas gives that information.

You also have to appreciate that both of these compounds will form 3D lattices not discrete molecules. But again this is a "fact" that we would all readily assume to be true, but the problem statement gives us no justification for such a belief. You just have to know that there are not discrete molecules in either compound.

Thus the explanation is really more of an educated guess based on the nature of chemistry rather than being absolutely logically rigorous.

• I’m pretty sure the deduction OP should have based their reasoning on should come from the charges of the ions involved and thereby the strength of the crystal lattice. – Jan Oct 11 '17 at 9:01
• @Jan yea I guess. I do not think they think we know by common knowledge melting points of compounds at this level, so I'm assuming it must be something else – George Jones Oct 11 '17 at 11:46

There is no simple, general rule for judging the melting points of compounds

The trouble with judging melting points is that solids have a wide range of different chemical structures. Some are covalent solids made from infinite networks of covalent bonds (alumina, graphite) some are essentially ionic salts (sodium chloride, copper sulphate) some are made from neutral molecules (freebase cocaine, candle wax) and so on. It is the structure that really determines the melting point.

Within a given class (say ionic solids) there may be some generalisable rules (alkali metal halides might show some explicable trends based on ion size, for example). But given a formula and no knowledge about the structure there will not be any rules that are helpful.

Alumina is a solid best thought of as an infinite network of covalent bonds; common salt is an ionic solid consisting of sodium and chloride ions. Alumina is a refractory and strong solid because of this structure, with a very high melting point. Common salt has a high melting point but is much lower than alumina. Some ionic salts with large floppy ions are liquid at room temperature.

The forces holding the solid together determine the melting point but you need to know the structure before you you know what the key forces are. Covalent bonds are strong in covalent network solids. Ionic bonds are not as strong but are still far stronger than the forces that hold together discrete molecules in molecular solids like candle wax or menthol. Knowing the structure tells you a lot; knowing the formula doesn't (always) tell you what you need to know.