melting point of oxides of elements in third period

[sheezle]

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)

[Kaeroll]

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, 2009 by Kaeroll
Incorrect.

[UC]

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.

[Kaeroll]

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

[sheezle]

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.

[jdurg]

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.

[sheezle]

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...