Why carbon not form 4bonds with …

Unfortunately, an answer to this requires a quick dip into orbital theory. You may have heard of s, p and d orbitals; carbon as a main group element of group 14 has the electronic configuration of [He]2s22p2[He]2s22p2, meaning two of its electrons are in an s orbital and two in p orbitals in its atomic ground state. The process by which these orbitals form bonds is simply overlap: If you have two orbitals that can be moved together and these two orbitals overlap, you generate a bonding and an antibonding orbital from these two. (This is a mathematical operation known as linear combination; plugging two orbitals into a linear combination means that exactly two orbitals are mathematically required to come out of the linear combination.) The bonding (or antibonding) orbitals that are generated by this process are labelled σ or π orbitals. Note that these are the Greek letters corresponding to s and p: a σ orbital is basically an s orbital streched in one direction and a π orbital is a p orbital streched in the same manner. Due to a number of constraints that you will learn about soon enough™, a carbon–carbon triple bond C≡CC≡C will consist of one σ bond and two π bonds as shown in the image below. Carbon-carbon triple bond showing the orbitals employed Figure 1: H−C≡C−HH−C≡C−H showing the σ and π orbitals required to make the respective bonds. Image taken from jahschem.wikispaces.org. See how two of the three possible p orbitals on each carbon (figure 1’s left half) overlap