Using WORDS, explain each step (as discussed in class) in drawing the valence bond theory diagrams for HCN.

Answer 1

Step 1: Derive the Lewis structure for the molecule.
Start by determining the Lewis structure for HCN, which shows the arrangement of valence electrons around the atoms in the molecule. There is 1 valence electron from hydrogen, 4 from carbon and 5 from nitrogen, hence a total of 10 valence electrons.
C is the central atom and is connected to H and N by a single bond each. This leaves 2 lone pairs on nitrogen, and 2 unpaired electrons on carbon (one on each side). The single electron on carbon forms a triple bond with the nitrogen atom. This results in the Lewis structure of HCN, which is H—C≡N.

Step 2: Determine the atomic orbitals of participating atoms.
The atomic orbitals for the three atoms involved in the bonding are:
- Hydrogen atom: 1s orbital (since it only has one electron shell)
- Carbon atom: 2s, 2px, 2py, and 2pz orbitals (sp hybridization)
- Nitrogen atom: 2s, 2px, 2py, and 2pz orbitals (sp hybridization)

Step 3: Determine the hybridization scheme.
For the carbon atom, it is involved in two σ bonds (one with H, and another with N) and two π bonds (i.e., the triple bond with N). Therefore, it undergoes sp hybridization, forming two sp hybrid orbitals which participate in σ bonding, while the unhybridized 2px and 2py orbitals take part in π bonding.

For the nitrogen atom, it is involved in one σ bond and two π bonds with the carbon atom. Thus, it also undergoes sp hybridization, with two hybrid orbitals being involved in σ bonding and the remaining 2px and 2py orbitals participating in π bonds.

Step 4: Construct the valence bond theory diagram.
- Show the hydrogen atom with its 1s orbital and carbon atom with its two sp hybrid orbital, 2px, and 2py orbitals.
- Show the central carbon atom bonded to the hydrogen atom with a σ bond through an overlap between carbon's sp hybrid orbital and hydrogen's 1s orbital (σ(H1s-Csp)).
- Show the nitrogen atom with its two sp hybrid orbitals, 2px and 2py.
- Show the carbon and nitrogen atoms forming a σ bond between one of the sp hybrid orbitals on the carbon atom and one of the sp hybrid orbitals on the nitrogen atom (σ(Csp-Nsp)).
- Show the formation of two π bonds between the carbon and nitrogen atoms by the parallel overlap of their 2px and 2py orbitals (π(C2px-N2px) and π(C2py-N2py)).

By following these steps, we can visualize the valence bond theory diagram for HCN, which consists of a σ bond between hydrogen and carbon, a σ bond between carbon and nitrogen, and two π bonds between carbon and nitrogen, with both carbon and nitrogen undergoing sp hybridization.