Question

why is carbon is unique?

Answer 1

It easily makes 4 strong bonds, either with more carbon atoms or with other non-metals like oxygen, hydrogen, nitrogen, and phosphorus. This allows long chains of carbon atoms with other atoms hanging off the side, leading to a bewildering range of possible molecules of different shapes, sizes, and chemical properties.

Other non-metals either don't make enough bonds (oxygen makes 2, nitrogen 3, hydrogen 1), or the bonds aren't strong enough to form long chains that are reasonably stable (silicon, phosphorus, etc.). Metals usually don't make covalent bonds at all, so they can't form chains (inter-metallic bonds are far too weak).

Answer 2

Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules.