D11.2 Three-dimensional Bond Geometry

The two examples so far were a linear (one-dimensional) molecule, BeCl2, and a planar (two-dimensional) molecule, BF3. What happens when a molecule is three dimensional?

Exercise: Molecular geometry of CH4

Exercise: Molecular geometry of NH3

Wedge-dash Notation

The Lewis structures in the two exercises above are drawn using wedge and dash notation. Straight lines represent bonds in the plane of the page/screen, solid wedges represent bonds coming toward you out of the plane, and dashed wedges represent bonds going away from you behind the plane.

Examine the following rotatable 3D model of NH3 and rotate it until it looks like the Lewis structure drawn in the answer in the preceding exercise. The NH3 molecule has trigonal pyramidal geometry because the lone pair on nitrogen occupies one corner of the tetrahedron, leaving the three N-H bonds occupying the other three corners; this gives a three-cornered (trigonal) pyramid.

Redraw the Lewis structure you drew for ammonia in the preceding exercise using wedge-dash notation. Then, rotate the 3D model until it matches your drawing. If you can find an orientation that matches, your wedge-dash Lewis structure is probably correct; if you cannot find a match, your Lewis structure is probably incorrect.

For simplicity, a wedge-dash Lewis structure draws as many of a molecule’s bonds in a plane as possible. However, as is the case with CH4 and NH3, most molecules do not have all bonds in the same plane.

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