Kaplan MCAT General Chemistry Review

Because Lewis structures do not in any way suggest or reflect the actual geometric arrangement of atoms in a compound, we need another system that provides that information. One such system is known as valence shell electron pair repulsion theory (VSEPR theory). This theory is actually dependent upon Lewis structure, so they go hand in hand.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory uses Lewis structure to predict the molecular geometry of covalently bonded molecules. It states that the three-dimensional arrangement of atoms surrounding a central atom is determined by the repulsions between the bonding and the nonbonding electron pairs in the valence shell of the central atom. These electron pairs arrange themselves as far apart as possible, thereby minimizing the repulsive forces (see Table 3.2). The following steps are used to predict the geometrical structure of a molecule using the VSEPR theory:

• Draw the Lewis structure of the molecule.

• Count the total number of bonding and nonbonding electron pairs in the valence shell of the central atom.

• Arrange the electron pairs around the central atom so that they are as far apart from each other as possible. For example, the compound AX₂ has the Lewis structure X : A : X. The A atom has two bonding electron pairs in its valence shell. To position these electron pairs as far apart as possible, their geometric structure should be linear:

X-A-X

MCAT Expertise

Knowing the tetrahedral shape will be particularly useful because it is often present in carbon, nitrogen, and oxygen.

Table 3.2

Example: Predict the geometry of NH₃.

Solution:

1. The Lewis structure of NH₃ is

2. The central atom, N, has three bonding electron pairs and one nonbonding electron pair, for a total of four electron pairs.

3. The four electron pairs will be farthest apart when they occupy the corners of a tetrahedron. As one of the four electron pairs is a lone pair, the observed geometry is trigonal pyramidal, shown in Figure 3.5.

Figure 3.5

In describing the shape of a molecule, only the arrangement of atoms (not electrons) is considered. Even though the electron pairs are arranged tetrahedrally, the shape of NH₃ is pyramidal. It is not trigonal planar because the lone pair repels the three bonding electron pairs, causing them to move as far away as possible.

Example: Predict the geometry of CO₂.

Solution: The Lewis structure of CO₂ is

The double bond behaves just like a single bond for purposes of predicting molecular shape. This compound has two groups of electrons around the carbon. According to the VSEPR theory, the two sets of electrons will orient themselves 180° apart, on opposite sides of the carbon atom, minimizing electron repulsion. Therefore, the molecular structure of CO₂ is linear: .

Key Concept

The shapes from Table 3.2 refer to electronic geometry, which is different from molecular geometry. In Figure 3.5, we see an ammonia molecule that has a tetrahedral electronic structure but is considered to have a molecular structure that is trigonal pyramidal.

One subtlety that students sometimes miss is the difference between electronic geometry and molecular geometry. Electronic geometry describes the spatial arrangement of all pairs of electrons around the central atom, including the bonding and the lone pairs. Molecular geometry describes the spatial arrangement of only the bonding pairs of electrons, in much the same way that the geometry of an object made from our sophisticated construction set would be determined by the position of the wooden spools attached to the dowels. For example, consider the fact that CH₄ (methane), NH₃ (ammonia), and H₂O all have the same electronic geometry: In each compound, four pairs of electrons surround the central atom. This is tetrahedral electronic geometry. However, because each has a different number of lone pairs, each has a different molecular geometry. Methane has tetrahedral geometry, ammonia has trigonal pyramidal, and water has angular or bent geometry. The distinction is important, and the MCAT will primarily focus on molecular geometry, but there is one important implication of electronic geometry: the determination of the ideal bond angle. Tetrahedral electronic geometry, for example, is associated with an ideal bond angle of 109.5°. Thus, molecular geometries that deviate from tetrahedral electronic geometry, such as those of ammonia and water, have bond angles that are deviations from 109.5°. You may have been tempted to say, for example, that water’s bond angle is a deviation from linear geometry with its ideal bond angle of 180°, but this is not the case. The actual bond angle in water is around 104.5°, a deviation from 109.5°.

Polarity of Molecules