Using WORDS, explain each step (as discussed in class) in drawing the valence bond theory diagrams for HCN. You must start at drawing the Lewis structure. Ensure you discuss the choice of the hybrid used and all overlappings, etc. Your grade will be determined by how easy it is to recreate the drawing from your worded descriptions
1. First, draw the Lewis structure of HCN molecule:
- Count the total number of valence electrons: H has 1, C has 4, and N has 5, giving a total of 10 valence electrons.
- Connect the atoms using single bonds: Place H on the left, C in the middle, and N on the right. Connect H-C, and C-N with single bonds. This uses 4 of the 10 valence electrons.
- Complete the octets for the outer atoms (C and N). Carbon needs 2 more electrons, and Nitrogen needs 3 more electrons. Hence add a lone pair to Carbon and one lone pair to Nitrogen.
- Discover that we have used only 8 out of 10 valence electrons. Therefore, add a third bond between Carbon and Nitrogen such that we account for all 10 valence electrons. The final Lewis structure for HCN should have a single bond between H and C atoms, and a triple bond between C and N atoms (1 sigma and 2 pi bonds), with no lone pairs on Carbon and one lone pair on Nitrogen.
2. Second, identify the hybridization of the central Carbon atom:
- Carbon has 4 electron domains (1 bond to Hydrogen, 1 sigma bond to Nitrogen, and 2 pi bonds to Nitrogen). This corresponds to sp hybridization (3 electron domains would correspond to sp2 hybridization and 4 electron domains would correspond to sp3 hybridization).
3. Draw the valence bond theory diagram:
- Position the sp-hybridized Carbon in the center of the molecule, with the sp orbitals pointing towards both Hydrogen and Nitrogen, since they form sigma bonds with Carbon.
- For the Hydrogen atom to the left, draw its 1s orbital overlapping with the sp orbital of Carbon, forming the H-C sigma bond.
- On the Nitrogen side, draw the sp-hybridized Nitrogen with one sp orbital pointing towards Carbon for the formation of the sigma bond. Then, draw the two remaining 2p orbitals on Nitrogen perpendicular to each other and perpendicular to the sigma bond.
- Indicate the overlap between the 2p orbitals of Nitrogen and two 2p orbitals of Carbon perpendicular to the C-N sigma bond axis, forming two pi bonds.
- Place a lone pair on the Nitrogen atom in the remaining sp orbital.
In summary, to draw the valence bond theory diagram for HCN:
- Start with the Lewis structure, which has a single bond between H and C, and a triple bond (1 sigma and 2 pi bonds) between C and N, with one lone pair on N.
- Identify the hybridization of the carbon atom as sp.
- Draw the sp hybrid orbitals of Carbon forming sigma bonds with Hydrogen and Nitrogen.
- Overlap the 1s orbital of Hydrogen and the sp orbital of Nitrogen with Carbon's sp orbitals.
- Draw the C-N pi bonds formed by the overlapping of 2p orbitals on both Carbon and Nitrogen.
- Finally, add a lone pair to the remaining sp orbital on Nitrogen.