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Silicon dioxide, also known as silica, is the major component of sand and is used to make glass. Its high melting point (1700°C) makes it a useful material for many different applications.
It has a very large covalent structure which means that it has strong bonds between its atoms and therefore needs a lot of energy to break these bonds. This is why it has such a high melting point!
Unlike carbon dioxide, which is composed of discrete three-atom molecules, silicon dioxide is a megastructure consisting of thousands of covalent bonds between its silicon and oxygen subunits. As a result, the strength of these bonds is much higher than that of van der waals bonding and thus requires more energy to overcome. Therefore, silicon oxide has a higher melting point than sulfur.
Other elements with high melting points include the Group IIIB nitrides and phosphides, which have an 8 electron sum (3+5) and are therefore relatively electronegative. These materials are therefore strongly attracted to each other and require more energy than silicon to break their atomic bonds. Therefore, their melting and boiling points are significantly higher.
The Group IIIB nitrides and the Group IIA carbides have very high melting and boiling points, due to their large electron sums (6+4). This is because the Group IIIB nitrides are the least electronegative metals and so have stronger interactions between their atoms than the Group IIA carbides, which have smaller electron sums (4+6).