An interesting example is #"H"-"C"-="C"-"H"#, **acetylene**.

We utilize the convention that the #z# axis points along the *internuclear axis*. Since the #p# orbital that *overlaps* *head-on* with the #s# orbital is *along the* *internuclear axis*, the #2p_z# orbital of carbon will the one that is compatible with the #1s# of hydrogen AND the #2s# of carbon.

The two carbons each have #\mathbf(sp)# **hybridization**, where they mix their #2s# and #2p_z# orbitals together, achieving an #\mathbf(sp)# **hybrid orbital** of #50%# #s# character and #50%# #p# character, with a new, lower energy, between those of the two original atomic orbitals. This can overlap with hydrogen's #1s# orbital.

**That accounts for each ** #\mathbf(sigma)# **bond that each carbon makes with the other carbon and one of the hydrogens; see (a)**.

Then, the remaining #2p_x# and #2p_y# orbitals of each carbon can respectively overlap with the #2p_x# and #2p_y# orbitals of the other carbon (i.e. #2p_y# with #2p_y#). These CANNOT overlap with hydrogen's #1s# orbital.

**These account for the two** #\mathbf(pi)# **bonds; see (b)**.

Overall, we get that each carbon uses **two** #sp# hybrid orbitals by mixing the #2s + 2p_z# orbitals in order to overlap with hydrogen's #1s# orbital and form **a** #\mathbf(sigma)# **bond** with one of the hydrogens and the other carbon, and each carbon uses its #2p_x# and #2p_y# orbitals to generate **two** #\mathbf(pi)# **bonds** with the other carbon.

**The two** #\mathbf(pi)# **bonds and one** #\mathbf(sigma)# **bond made by each carbon with the other carbon accounts for the triple bond**.