Discuss the shape of acetylene (first member of alkyne).

In acetylene molecule, each carbon atom undergoes sp hybridization. One sp hybrid orbital of one carbon atom overlaps axially with one sp hybrid orbital of the other carbon atom to form sigma (σ;) C – C bond. The remaining sp hybrid orbital of each carbon atom overlaps axially with 1s orbital of hydrogen atom to form sigma (σ;) C – H bonds. The unhybridised) p_y orbitals from the two carbon atoms overlap sidewise to form a  bond. Similarly, the unhybridised 2p_z orbitals from the two carbon atoms overlap sidewise to form one more ; bond.
The two ;  bonds lie in a plane at right angles to each other and each ; bond consists of two ; electron clouds. The electron cloud of one ; bond is considered to lie above and below the line joining the carbon atoms. Thus, the four ; electron clouds overlap and merge to form a single cylindrical cloud about the internuclear axis. 

The – C ≡ C – bond in acetylene consists of one strong a bond (sp -sp) and two weak σ; bonds (p – p). The bond length of carbon-carbon triple bond is 120 pm (1.20 Å).
In acetylene, carbon atoms as well as hydrogen atoms lie along a line and each C – C – H bond angle is 180°. Thus, acetylene has a linear structure.