this isn't a question from homework -- really it is out of curiosity. why is the melting point of Al2O3 lower than the melting point of MgO? i did chance across this problem in a homework packet but was told to "ignore it for now" by my chem teacher because we would learn it later, but i am curious, and would like an answer now!

 

thanks in advance.

btw i have just finished learning about bonding in class. (HL chem)

Off the top of my head, the Al cation is smaller and more charged than the Mg cation, i.e. greater charge density. Therefore in bonding to O, Al has a greater lattice enthalpy, which manifests as a high melting point.

 

(whoops, got this wrong. Was half asleep when I posted. Ignore)

Edited February 24, 200916 yr by Kaeroll
Incorrect.

Off the top of my head, the Al cation is smaller and more charged than the Mg cation, i.e. greater charge density. Therefore in bonding to O, Al has a greater lattice enthalpy, which manifests as a high melting point.

 

In slightly more common terms, the difference in electronegativity between aluminum and oxygen is smaller than between magnesium and oxygen. The [math] Al_2O_3 [/math] therefore has more covalent character than MgO (which is more ionic in character). Ionic compounds tend to be high melting, while covalent have fairly low melting points.

Whoops. This is what happens when I post before my morning coffee - I get it all wrong. Ignore my above post.

hmm, but the difference in elecneg. between sodium and oxygen is larger than the difference in elecneg between magnesium and oxygen, yet MgO has a higher melting point than Na2O as well.

hmm, but the difference in elecneg. between sodium and oxygen is larger than the difference in elecneg between magnesium and oxygen, yet MgO has a higher melting point than Na2O as well.

 

If melting point was based purely on electronegativity, then yes, Na2O would have a higher melting point. But it's more than just that. Melting point is also a factor of ion size. In both cases, the O(2-) ion is the same size. But the Mg(2+) ion is a lot smaller than the Na+ ion. As a result, the MgO structure is more tightly packed than the Na2O structure. Sodium's ionic radius is 1.02 Angstroms while Magnesium's is 0.72 Angstroms, I believe.

 

As a result, the ionic compound of MgO is more tightly drawn to itself than Na2O is.

and Aluminum (+3) would have a smaller ionic radius than both the Na and Mg ions -- but because the diff in elecneg between aluminum and oxygen is smaller than the diff in elecneg between magnesium and oxygen, it ends up having a lower boiling point. is that correct?

 

what are the deciding factors that determines an ionic compound's melting/boiling points? i know in explaining the ionization energy of elements it's first distance, then nuclear charge, then electron-electron repulsion, so i wonder if there's a similar "rubric" to explain m.p and b.p. as well...