SP HYBRIDIZATION OF CARBON

In sp hybridization, one 2s and one 2p orbitals of excited carbon intermix to form two sp-hybrid orbitals in linear geometry. Each sp hybrid orbital is occupied by one electron. The remaining pure 2p orbitals ( for our convenience, let us say: 2p_y and 2p_z) orient at right angles to the sp-hybrid orbitals. These are also occupied by one electron each.

The two sp hybrid orbitals form 2 σ-bonds in linear geometry. Thus the bond angle will be about 180^o. The remaining pure 'p' orbitals will form two π-bonds. Thus carbon again forms total four bonds i.e., two σ-bonds and two π-bonds.
E.g., In acetylene molecule, C₂H₂, each carbon undergoes sp hybridization and forms one σ-bond with a hydrogen atom and one σ-bond with another carbon. The two carbon atoms also form two π-bonds with each other due to sidewise overlapping of pure p-orbitals. Thus a triple bond is formed between two carbon atoms in acetylene molecule.

Note that whenever carbon atom undergoes sp hybridization, it forms 2 σ-bonds and 2 π-bonds. It may either form one triple bond as in case of acetylene or two double bonds e.g. allenes.
The following diagram summarizes the bonding pattern by carbon atom in different hybridizations.