Why is the aluminium boiling point higher than silicon? And why is silicon's melting point higher than aluminium?

Why is the Aluminium boiling point higher than silicon? And why is silicon's melting point higher than aluminium?

$$\ce{Si}$$ has 4 bonds which are covalent in nature. $$\ce{Al}$$ is metal thus possesses ionic bond.

Where have I not understood the concept?

• The concept itself does not seem to be awfully useful in predicting these differences. Oct 23 '20 at 6:07
• You can't directly compare the melting of aluminum with the melting of silicon. Silicon converts from a nonmrtallic structure to a metallic one when it melts, aluminum is metallic all the time. Oct 23 '20 at 9:34
• For one aluminium does not for ionic bonds with itself; those are metallic bonds.
– A.K.
Oct 23 '20 at 19:30
• BTW, though the BP of Al is 2743 K, it's vapor pressure is fairly high at lower temps... see en.wikipedia.org/wiki/Aluminium . Thus, Al metalization is easily done in vacuo perhaps 1,000 K cooler. So BP is not quite so sharp, in this case. Now gallium is ab bit more extreme... Oct 23 '20 at 20:14
• I'm afraid that the entire premise of the question is wrong. Both the melting point and the boiling point of Si are higher than than those of Al. Al: mp = 660 C, bp = 2519 C. Si: mp = 1414 C, bp = 2900 C. Oct 24 '20 at 4:14

1 Answer

Aluminium is a metal thus formed of metallic bonds. The positive Al cations are regularly arranged in a lattice and surrounded by a sea of free/delocalised negatively charged electrons (therefore metals can conduct electricity). The negative charge of electrons and positive charge of cations have strong attraction forces which require a lot of heat energy to break. However, silicon being a non-metal forms simple covalent bonds with other silicon particles. Covalent substances have weak intermolecular forces between their molecules so they require less energy. However, covalent atoms are hard to break from each other because the pairs of shared electrons are attracted to both positively charged nuclei.

• As u said covalent atoms are harder to break due to mutual sharing of electrons, the silicon would be expected to have a higher boiling point however it doesn't. Aluminium has a high boiling point, but a lower melting point than silicon. I guess this all can be imputed to their structures perhaps. Oct 26 '20 at 3:40
• Where this answer slightly misses: silicon has its diamond-like covalent structure only as a solid. Once melted it becomes metallic like aluminum, so a different comparison applies. Jul 18 '21 at 16:46