2.8 Geometry - Molecular Shapes

Geometry

Now that we can draw organic compounds, let’s take a closer look at common geometries we will encounter. The simple theory that helps us understand the shapes of molecules is the valence shell electron pair repulsion theory, VSEPR (pronounced “Vesper”) theory. The idea is that all of the electron groups on an atom (single bonds, double bonds, triple bonds, lone pairs of electrons) are all negatively charged groups. Since like charges repel each other, these groups get as far away from each other as they can. The farthest two groups can get away from each other on an atom is 180°, a linear shape.

The farthest three groups can get away from each other on an atom is 120°, a trigonal planar shape.

The farthest four groups can get away from each other is 109.5°, a tetrahedral shape. We use wedges and dashes to help us draw these shapes. A regular line means that atom lies in the plane of the paper. A wedge indicates an atom is coming towards you, in front of the page. A dashed line indicates an atom is going away from you, behind the page.

Even though these all have the same basic shape, we call them different things. Whenever we name a shape, we treat lone pairs of electrons like they are invisible. You can eve cover them up with your hand and ignore them. We just look at the atoms and name the shape we see. On a side note, notice “trigonal pyramid” is the name of a pyramid with a triangle as its base. This is different than a pyramid with a square pyramid as its base (named obviously “square pyramid”).