Question

Explain the following observations: Carbon has two important oxides which are both gases at room temperature. In contrast, silicon dioxide (silica) is a solid with a very high melting point.

Answer 1

The structure of carbon dioxide

The carbon dioxide is a gas that is it must consist of simple molecules. Carbon can form simple molecules with oxygen because it can form double bonds with the oxygen to form a linear molecule as,

O=C=O.

No any other elements in Group 4 form double bonds with oxygen, so that forces completely different structures on them.

When carbon forms bonds with oxygen, it first promotes one of the electrons in the 2s level into the empty 2p level. This produces 4 unpaired electrons.

the electrons by hybridising the 2s electron and one of the 2p electrons to make two sp¹ hybrid orbitals of equal energy. The other 2p electrons are left alone for the time being.

Oxygen's electronic structure is 1s²2s²2p_x²2p_y¹2p_z¹.

Hybridisation occurs in the oxygen as well. This time, sp²hybrids are formed with the s orbital and two of the p orbitals being rearranged to give 3 orbitals of equal energy - leaving a temporarily unaffected p orbital.

the two of the sp² hybrid orbitals contain lone pairs of electrons.

Then bring them together so that hybrid orbitals overlap end-to-end to form simple covalent bonds. These are properly called sigma bonds,This brings the various p orbitals close enough together that they overlap sideways.

Sideways overlap between the two sets of p orbitals produces two pi bonds

So in order to form a carbon-oxygen double bond, it is necessary for the p orbitals on the carbon and the oxygen to overlap sideways.

The structure of silicon dioxide

Silicon doesn't have double bond with oxygen.

Silicon atoms are bigger than carbon. That means that silicon-oxygen bonds will be longer than carbon-oxygen bonds. the p orbitals on the silicon and the oxygen aren't quite close enough together to allow enough sideways overlap to give a stable pi bond. So, silicon bonds with oxygen in such a way that only single bonds are formed.

There are various different structures for silicon dioxide. The easiest to remember and draw is:

This is based on a diamond structure with each of the silicon atoms being bridged to its other four neighbours via an oxygen atom.

This means that silicon dioxide is a giant covalent structure. The strong bonds in three dimensions make it a hard, high melting solid.