Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why is the melting point of magnesium oxide higher than aluminium oxide? Ask Question. Asked 4 years, 7 months ago. Active 11 months ago. Viewed 13k times. Improve this question. Michael Harding Michael Harding 1 1 gold badge 2 2 silver badges 9 9 bronze badges. Add a comment. Active Oldest Votes. Reference 1. 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. Ionic compounds tend to be high melting, while covalent have fairly low melting points. This is what happens when I post before my morning coffee - I get it all wrong. Ignore my above post. 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. As a result, the MgO structure is more tightly packed than the Na2O structure. Variable electrical conductivity — diamond does not conduct electricity, whereas graphite contains free electrons so it does conduct electricity. Silicon has a very high melting point due to its giant covalent structure; a lot of energy is needed to break the strong covalent bonds throughout the structure.
Diamond has a very high melting point because a large amount of energy is needed to overcome the many strong covalent bonds. The reason for this probably lies in the increase in electronegativity as you go from sodium to magnesium to aluminium.
That would mean that the electronegativity difference between the metal and the oxygen is decreasing. The smaller difference means that the bond won't be so purely ionic.
It is also likely that molten aluminium oxide contains complex ions containing both aluminium and oxygen rather than simple aluminium and oxide ions.
All this means, of course, that you aren't really comparing like with like - so wouldn't necessarily expect a neat trend. The other problems I came across lie with sodium oxide. I have no idea what the truth of this is - although I suspect that the Webelements melting point value is probably for a pressure above atmospheric pressure although it doesn't say so.
None of these conducts electricity in the solid state, but electrolysis is possible if they are molten. They conduct electricity because of the movement and discharge of the ions present. The only important example of this is in the electrolysis of aluminium oxide in the manufacture of aluminium. Whether you can electrolyse molten sodium oxide depends, of course, on whether it actually melts instead of subliming or decomposing under ordinary circumstances.
If it sublimes, you won't get any liquid to electrolyse! Magnesium and aluminium oxides have melting points far too high to be able to electrolyse them in a simple lab. Note: You will find full details of the electrolysis of aluminium oxide during the extraction of aluminium if you follow this link. The electronegativity of the elements increases as you go across the period, and by the time you get to silicon, there isn't enough electronegativity difference between the silicon and the oxygen to form an ionic bond.
Silicon dioxide is a giant covalent structure. Note: If you aren't happy about electronegativity you will find it explained if you follow this link. There are three different crystal forms of silicon dioxide.
The easiest one to remember and draw is based on the diamond structure. Crystalline silicon has the same structure as diamond. To turn it into silicon dioxide, all you need to do is to modify the silicon structure by including some oxygen atoms. Notice that each silicon atom is bridged to its neighbours by an oxygen atom. Don't forget that this is just a tiny part of a giant structure extending in all 3 dimensions. Note: If you want to be fussy, the Si-O-Si bond angles are wrong in this diagram.
In reality the "bridge" from one silicon atom to its neighbour isn't in a straight line, but via a "V" shape similar to the shape around the oxygen atom in a water molecule. It's extremely difficult to draw that convincingly and tidily in a diagram involving this number of atoms. The simplification is perfectly acceptable. If you need help in drawing this structure you will find a suggestion by following this link. Very strong silicon-oxygen covalent bonds have to be broken throughout the structure before melting occurs.
Because you are talking about a different form of bonding, it doesn't make sense to try to compare these values directly with the metallic oxides.
What you can safely say is that because the metallic oxides and silicon dioxide have giant structures, the melting and boiling points are all high.
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