VSEPR Theory and the Basic Shapes

Valence shell electron-pair repulsion or VSEPR theory serves as a tool to predict molecular structure.

It assumes that the negatively charged electron groups, which may be electrons involved in a single bond, multiple bonds, or lone pairs, repel one another and try to stay at the maximum possible distance from each other to minimize repulsions.

Imagine a set of balloons tied together. Each balloon orients itself away from the other as much as possible.

Molecular geometry is dictated by the arrangement of various electron groups around the central atom.

Beryllium fluoride has two electron groups around the central atom. According to VSEPR, the minimum repulsion between these electron groups is achieved through a maximum separation. Thus, the bond angle is 180°, and the molecular shape is linear.

Boron trifluoride has three electron groups around the central boron atom. The repulsion between these groups can be minimized by assuming a 120° bond angle. VSEPR theory predicts that the molecule exhibits trigonal planar geometry.

In the case of methane, there are four electron groups surrounding the central carbon atom. They are farthest when the bond angle is 109.5°, and the molecule assumes a three-dimensional tetrahedral geometry.

If five balloons are tied together, maximum separation is achieved when the three balloons are in one plane, and the remaining two are placed on either side of the plane.

Phosphorus pentachloride has five electron groups around the central atom. The three equatorial chlorine atoms are separated by the bond angle of a 120° and assume a trigonal planar arrangement.

There is one chlorine atom each above and below the plane. The angle between the equatorial and axial chlorines is 90°. The molecule has trigonal bipyramidal geometry.

In sulfur hexafluoride, there are six electron groups around the sulfur atom. The four groups occupy a single plane. The two other groups lie on either side of this plane. The geometry of the molecule is octahedral. All the bonds are equivalent and bond angles are 90°.

These examples reveal that two to six bonding electron groups around the central atom lead to five basic molecular shapes: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.