VSEPR理論と基本形状

VSEPR theory assumes that negatively charged electron groups, including bonding pairs and lone pairs, repel one another and position themselves as far apart as possible to minimize repulsions. This principle, similar to balloons tied together orienting away from each other, allows us to predict molecular structure and bond angles around a central atom.

When a central atom has two electron groups, maximum separation occurs at a 180° bond angle, resulting in linear geometry. Beryllium fluoride exemplifies this shape, where the two fluorine atoms position themselves on opposite sides of the central beryllium atom to minimize electron pair repulsion.

Methane has four electron groups surrounding the central carbon atom. These groups achieve maximum separation when positioned at 109.5° bond angles, creating a three-dimensional tetrahedral geometry. This arrangement minimizes repulsion between the four bonding electron pairs around the carbon center.

In trigonal bipyramidal geometry, three electron groups occupy an equatorial plane separated by 120° angles, while two groups position themselves axially above and below the plane at 90° angles. Phosphorus pentachloride demonstrates this arrangement, with equatorial and axial chlorine atoms achieving optimal separation.

Sulfur hexafluoride has six electron groups around the central sulfur atom, resulting in octahedral geometry. Four groups occupy a single plane while two groups lie on opposite sides of this plane, with all bonds equivalent and bond angles measuring 90° or 180° throughout the structure.

Boron trifluoride has three electron groups around the central boron atom. Repulsion between these groups is minimized when they assume 120° bond angles, positioning the three fluorine atoms in a single plane around the boron center to maximize their separation.

VSEPR theory predicts five basic molecular shapes based on electron group arrangements: linear (two groups), trigonal planar (three groups), tetrahedral (four groups), trigonal bipyramidal (five groups), and octahedral (six groups). Each shape maximizes the distance between electron groups to minimize repulsion.