why melting point of silicon dioxide is too high compared to carbondioxide?

Answer 1

silicon dioxide is a giant polymeric molecule. It contains SiO4 units linked to each other in a three dimensional network. Hence solid and more melting point. It requires more energy to seperate the covalently bonded Si and O atoms.
It is all due to the inability of the silicon to form Pi bond with oxygen.

But carbon dioxide is a small discrete molecule. There are weak vanderwaals forces of attractions between them. Hence it is a gas and with low melting and boiling points as it is easy to separate the CO2 molecules.

Answer 2

Although both compounds are bonded via covalent bonding, silicon dioxide is a macromolecule while carbon dioxide is made of molecules containing 1 carbon atom and 2 oxygen atoms.

In order to melt silicon dioxide, the covalent bonds holding the tetrahedral structure together have to be broken. As for carbon dioxide, we only have to separate the moldecules that are held together by Van der Waals forces (NOT bonds).

Answer 3

Silicon dioxide forms a giant structure similar to diamond where the silicon atoms are in the same positions as carbon in diamond with the oxygen atoms placed in between the silicon atoms. The structure has a considerable ionic character and so requires a high temperature to break the bonds to make it melt.
Carbon dioxide doesn't have enough attraction between its molecules to form such a structure and so is a gas.