Question 9.SE.6: Methanal has the Lewis structure Describe how the bonds in m...
Methanal has the Lewis structure
Describe how the bonds in methanal are formed in terms of overlaps of hybrid and unhybridized orbitals.
Analyze We are asked to describe the bonding in methanal in terms of hybrid orbitals.
Plan Single bonds are σ bonds, and double bonds consist of one σ bond and one π bond. The ways in which these bonds form can be deduced from the molecular geometry, which we predict using the VSEPR model.
Solve The C atom has three electron domains around it, which suggests a trigonal-planar geometry with bond angles of about 120°. This geometry implies sp² hybrid orbitals on C (Table 9.4). These hybrids are used to make the two C—H and one C—O σ bonds to C. There remains an unhybridized 2p orbital on carbon, perpendicular to the plane of the three sp² hybrids.
The O atom also has three electron domains around it, and so we assume it has sp² hybridization as well. One of these hybrid orbitals participates in the C—O σ bond, while the other two hold the two nonbonding electron pairs of the O atom. Like the C atom, therefore, the O atom has a p orbital that is perpendicular to the plane of the molecule. The two p orbitals overlap to form a C—O π bond (Figure 9.24).
| TABLE 9.4 Geometric Arrangements Characteristic of Hybrid Orbital Sets | |||
| Atomic Orbital Set | Hybrid Orbital Set | Geometry | Examples |
| s,p | Two sp |  |
\mathrm{Bef}_{2},\,\mathrm{HgCl}_{2} |
| s,p,p | Three sp² |  |
{\mathrm{BF}}_{3},{\mathrm{SO}}_{3} |
| s,p,p,p | Four sp³ |  |
\mathrm{CH}_{4},\mathrm{NH}_{3},\mathrm{H}_{2}O,\mathrm{NH}_{4}^{+} |
