Diamond is an insulator, but silicon is a semiconductor. This is because the band gap in Si is smaller than that in C.

There is no Si analogue of graphite; the larger size if the Si atom means that the overlap of the Si p-orbitals is poor, and hence that Si pπ-pπ bonding is poor.

Si forms strong bonds to more electronegative elements, eg. Si-F and Si-O, whereas C forms strong bonds with less electronegative elements, eg. C-H. Hydrocarbons are more stable than silicon hydrides both with respect to decomposition to the elements and combustion to the dioxide and water. Silane (SiH₄) is also spontaneously flammable (it is labile) whereas methane requires sparking to burn (it is inert).

The structure of the simple compounds is determined by the ability of the Si d-orbitals to contribute to bonding. For example, (CH₃)₂0 has a C-O-C angle of 109^o, but (SiH₃)₂O has a Si-O-Si angle of 144^o. This is due to a Si dπ-O pπ bonding interaction leading to a more linear arrangement of the Si-O-Si system. Also, (SiH₃)₃N is planar whereas (CH₃)₃N is pyramidal, and this is due to the same type of interaction, where the overlap is greatest when a planar arrangement is adopted.

Si forms long lived compounds, eg. SiF₆^2-, in which the coordination number of Si is greater than 4, whereas C has coordination numbers which do not exceed 4. Neutral Si compounds such as SiCl₄ are Lewis acids; this is due to the ability of the low lying vacant d-orbitals to act as electron acceptors. The formation of compounds by the Lewis acid-base interaction of the vacant d-orbitals is known as valence shell expansion. 

Multiple bonding (pπ-pπ in addition to ordinary σ-bonds) only occurs in Si at high temperature, or in inert matrices at low temperature. These can be stabilized at room temperature by the use of bulky ligands, eg. R₂Si=SiR₂, where R is a substituted phenyl group. B(E=E)<2B(E-E) for both Si and C, but the double bond in the alkene is kinetically stabilized, eg. the polymerization of ethene requires a catalyst. As a result of this Si forms n single bonds rather than a bond of order n; eg. SiO₂ is a macromolecular solid formed from vertex shared SiO₄ tetrahedra, while CO₂ is a molecular gas.

Silicon (and Germanium) Halides: Silicon and Germanium halides are mild Lewis acids, and the most important is SiCl₄. They react with Lewis base ligands to give five- and six-coordinate complexes. This formation of hypervalent complexes (the valence shell expansion) leads to the much faster hydrolysis of Si/Ge halides compares to CX₄. The water lone pair can bond to the vacant d-orbitals on Si/Ge to form  a six-coordinate species, MX₄(OH₂)₂, followed by elimination of HX to leave the dioxide, MO₂. This mechanism is not possible in CX₄, and hence it is hydrolyzed more slowly.