Which of the following molecules has two sigma ($$\sigma$$) and two pi ($$\pi$$) bonds?

To determine which molecule has two sigma (σ) and two pi (π) bonds, we need to analyze the bonding in each option. A sigma bond is formed by the head-on overlap of atomic orbitals and is present in all single bonds. A pi bond is formed by the sideway overlap of atomic orbitals and is found in double bonds (one π bond) or triple bonds (two π bonds).

Starting with option A: C₂H₄ (ethylene). The structure is H₂C=CH₂. The carbon-carbon double bond consists of one σ bond and one π bond. Each carbon is bonded to two hydrogen atoms via single bonds, which are σ bonds. So, there are four C-H σ bonds and one C-C σ bond, totaling five σ bonds. The double bond contributes one π bond. Therefore, C₂H₄ has five σ bonds and one π bond, which does not match the requirement of two σ and two π bonds.

Next, option B: N₂F₂ (dinitrogen difluoride). The common structure is F-N=N-F, with a double bond between the nitrogen atoms. The double bond has one σ and one π bond. Each nitrogen-fluorine bond is a single bond, which is a σ bond. So, there are two N-F σ bonds and one N-N σ bond, totaling three σ bonds. The double bond contributes one π bond. Therefore, N₂F₂ has three σ bonds and one π bond, which does not match.

Moving to option C: C₂H₂Cl₂ (dichloroethene). This has isomers, but both isomers have a carbon-carbon double bond. For example, in 1,2-dichloroethene (ClHC=CHCl), the C=C bond has one σ and one π bond. There are two C-Cl single bonds (σ bonds) and two C-H single bonds (σ bonds). So, total σ bonds are one (from C=C) + two (from C-Cl) + two (from C-H) = five σ bonds, and one π bond. The same count applies to the 1,1-dichloroethene isomer (H₂C=CCl₂). Therefore, C₂H₂Cl₂ has five σ bonds and one π bond, which does not match.

Finally, option D: HCN (hydrogen cyanide). The structure is H-C≡N. The carbon-nitrogen triple bond consists of one σ bond and two π bonds. The hydrogen-carbon bond is a single bond, which is a σ bond. So, total σ bonds are one (from H-C) + one (from C≡N) = two σ bonds. The triple bond contributes two π bonds. Therefore, HCN has two σ bonds and two π bonds, matching the requirement.

Hence, the correct answer is Option D.