What is the basic principle behind VSEPR theory?

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.

How does VSEPR theory explain linear molecular geometry?

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.

What determines the tetrahedral shape in molecules like methane?

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.

How are five electron groups arranged in trigonal bipyramidal geometry?

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.

What is the molecular geometry of sulfur hexafluoride?

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.

Why does trigonal planar geometry occur in boron trifluoride?

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.

What are the five basic molecular shapes predicted by VSEPR theory?

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.

10.1

VSEPR Theory and the Basic Shapes

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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.