Why carbon

Organic chemistry is the chemistry of carbon compounds, and carbon earns a whole discipline for two reasons.

First, it forms four bonds. Nitrogen forms three, oxygen two — carbon’s four common bonding positions allow extensive chains and branching. Other elements can also form chains, with different bonding patterns.

Second, carbon–carbon bonds are strong and stable. Silicon sits directly below carbon and also forms four bonds, but Si–Si bonds are generally weaker than C–C bonds. Silicon chains (polysilanes) do exist, but their stability and reactions differ; neither carbon nor silicon chains are indestructible.

Notice what the model does that a drawing cannot. On paper methane is drawn as a cross, with 90° angles that do not exist. In three dimensions the four hydrogens push as far apart as they can get, which puts them at 109.5° — the ideal tetrahedral angle; real sp³ bond angles can differ because of substituents and ring constraints.