Why does silicon in silicon dioxide bond with 4 oxygen atoms and not 2?
But carbon in carbon dioxide bonds with 2 oxygen atoms but not 4?

Silicon does not form double bonds with oxygen, whereas carbon is capable of forming double bonds with oxygen. While the carbon dioxide molecular structure is linear, the silicon dioxide has an extended, different covalent structure.

Explanation:

If the sizes of the atoms of Silicon (Si) and Carbon (C) are compared to each other, the Si atoms are larger than carbon - which implies that the Si-O bonds will be longer than the C-O bonds. As a result, the p orbitals present on the Si and O atoms aren't very near to each other, in order to get together for the required overlap sideways which could have formed a stable pi bond. Hence, Silicon forms only single covalent bonds with Oxygen in silicon dioxide, in the form of a diamond structure with each Si atom being connected to its four neighbouring atoms through an O atom.

On the other hand, in the case of carbon dioxide, C is perfectly capable of forming double bonds with O. The different p orbitals are brought close together, resulting in a sideways overlap that leads to two pi bonds, twisted at a right angle to each other. As a result, the Carbon in carbon dioxide bonds with 2 oxygen atoms but not 4.

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