I'll try and answer this since know one else has answered it. The type of bonding that you will observe in the structure in an atom of Al2S3 is called covalent bonding. Each Aluminum in the structure will be covalently attached to two Sulfur atoms in the structure. Since all elements in the periodic table want to be like the noble gases and have their valence electron shell filled, Aluminum will share two electrons with one of the Sulfurs bound to it to produce a double covalent bond, and it will share one electron with another Sulfer to make another covalent bond. Sulfur's ability to accept two electrons from Aluminum gives Sulfer the two electrons needed to fill its valence electron shell. The other Sulfur that shares one electron with each Aluminum, can also donate a pair of electrons to each Aluminum which fills up Aluminum's P-orbital and makes it like the noble gases. Because of the Sulfer's ability to donate a pair of electrons to one of the Aluminum's at a given time, the compound gives the appearance that all bonds in the compound are covalent double bonds, although one bond will always be a single bond at any given time. The reason why the melting poing of AlS3 is so high is because covalent double bonds require large amounts of heat to break, and are very strong.
I hope this helps.
I apologize, the bonding of Al2S3 is ionic, not covalent.
The type of bonding that you will observe in the structure in an atom of Al2S3 is ionic. The charge on each Al is +3 and the charge on each S is -2. Since each aluminum in the structure will transfer its electrons to the 3 sulfurs, the resulting charge on each aluminum will be +3, and the charge on each sulfer will be -2. The number of electrons, which are negative, outnumber the number of protons by two on each sulfur resulting in a negative charge; and the number of protons, which are positive, outnumber the number of electrons by three on each aluminum. The reason why aluminum donates its electrons to sulfur and sulfur doesn't donate their electrons to sulfur has to do with the electronegativity difference between the two. Sulfur is more electronegative then aluminum, which allows each sulfur to fill its valence electron shell. Each sulfur molecule in Al2S3 wants to fill its valence electron shell to become more like the noble gases, and the addition of two extra electrons allows it to fill its valence electron shell. The formation of the ionic bonds due to the donating and receiving of electrons is an exothermic reaction releasing heat that reaches a temperature (check reference) of 1100 Cº, which is the same temperature needed to melt it. And since that much heat is needed to form the bonds in Al2S3, that much heat is needed to break the bond.
I apologize for the initial post, but I googled a structure of Al2S3 and it showed it as having covalent bonds. When I went back and double checked it, I saw that it wasn't covalent, but it was ionic.