Why does aluminium oxide have a high melting and boiling point?
Awwal, Aluminum, although it is technically a metal, is on the right side of the periodic table and borders on being a semimetal. Because it is closer to oxygen on the periodic table than many of the other metals are, bonds between aluminum and oxygen tend to have a lot of covalent character (as opposed to being totally ionic), and this makes them very strong bonds. Since the bonds holding the material together are so strong, it takes a lot of energy to break them, so the melting and boiling points are at a much higher temperature than they are for other substances. Silicon dioxide (or as you may know it more commonly: “quartz”) is exactly the same way. Silicon is right next to aluminum on the periodic table, so when silicon forms bonds with oxygen, they are very strong and tough to break. The melting and boiling points of quartz are also very high.