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VSEPR Theory
Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure around a central atom from an examination of…
The valence shell electron-pair repulsion, or VSEPR, theory assumes that electron groups involved in single bonds, multiple bonds, or lone pairs repel each other and try to stay at the maximum possible distance from each other.
The molecular geometry is dictated by the arrangement of various electron groups around the central atom.
There are five basic molecular shapes: linear for two bonding electron groups, trigonal planar for three, tetrahedral for four, trigonal bipyramidal for five, and octahedral for six.
While predicting the molecular geometry, remember that lone pair–lone pair repulsions are greater than lone pair–bonding pair and bonding pair–bonding pair repulsions.
Consider the Lewis structures of methane, ammonia, and water. In each, the central atom is surrounded by four electron groups.
In methane, the four bonding electron pairs are arranged tetrahedrally with an ideal H–C–H angle of 109.5°.
In ammonia, the nitrogen atom has three bonding pairs and one lone pair.
The lone pair of electrons occupies a larger space than the bonding pairs. This is because a lone pair is bound to only one nucleus, whereas a bonding electron group is shared by two nuclei.
The H–N–H bond angles are smaller than the expected tetrahedral angle of 109.5°, as observed in methane. This compression of the bond angle is attributed to the repulsive force exerted by a lone pair on the adjacent bonding electron groups.
The arrangement of electron pairs is called electron-pair geometry. The molecular geometry describes the arrangement of the atoms, and differs from the electron-pair geometry. The electron-pair geometry for ammonia is tetrahedral, whereas the molecular shape is trigonal pyramidal.
A water molecule also has four electron groups around the central atom. The electron pair geometry is also tetrahedral with two bonding electron groups and two lone pairs.
The greater repulsion exerted by two lone pairs further compresses the H–O–H bond angle in water molecules. It is much smaller than the ideal tetrahedral bond angle, and the molecular geometry is bent.
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Q1: What is the basic principle behind VSEPR theory?
VSEPR theory assumes that electron groups—including single bonds, multiple bonds, and lone pairs—repel each other and arrange themselves to maximize distance and minimize repulsions. The molecular geometry around a central atom is dictated by how these electron groups position themselves to stay as far apart as possible.
Q2: What are the five basic molecular shapes predicted by VSEPR theory?
VSEPR theory predicts five basic shapes: linear for two electron groups, trigonal planar for three, tetrahedral for four, trigonal bipyramidal for five, and octahedral for six electron groups. Each shape represents the optimal arrangement that minimizes electron pair repulsion around the central atom.
Q3: How do lone pairs affect bond angles in molecules?
Lone pairs occupy more space than bonding pairs because they are bound to only one nucleus, whereas bonding pairs are shared by two nuclei. This causes lone pair–lone pair repulsions to be greater than bonding pair repulsions, compressing bond angles. In ammonia and water, lone pairs reduce H–N–H and H–O–H angles below the ideal tetrahedral angle of 109.5°.
Q4: What is the difference between electron-pair geometry and molecular geometry?
Electron-pair geometry describes the arrangement of all electron groups—both bonding pairs and lone pairs—around the central atom. Molecular geometry describes only the positions of atoms, not electrons. These differ when lone pairs are present. For example, ammonia has tetrahedral electron-pair geometry but trigonal pyramidal molecular geometry.
Q5: Why does water have a bent molecular structure?
Water has four electron groups around the central oxygen atom: two bonding pairs and two lone pairs. The electron-pair geometry is tetrahedral, but the two lone pairs exert greater repulsive forces than bonding pairs, compressing the H–O–H bond angle significantly below 109.5°, resulting in a bent molecular geometry.
Q6: How do lone pairs position themselves in trigonal bipyramidal geometry?
In trigonal bipyramidal electron-pair geometry, the three equatorial positions have more available space due to 120° bond angles compared to the two axial positions. Lone pairs preferentially occupy the equatorial positions because the larger space reduces repulsion. When two lone pairs and four bonding pairs are arranged octahedrally, the lone pairs position 180° apart, creating a square planar molecular structure.
Q7: Why is the H–C–H bond angle in methane different from ammonia?
Methane has four bonding electron pairs and no lone pairs around carbon, so all electron groups repel equally, creating an ideal tetrahedral geometry with H–C–H angles of 109.5°. Ammonia has three bonding pairs and one lone pair; the lone pair's stronger repulsion compresses the H–N–H angles to less than 109.5°, demonstrating how lone pairs distort ideal geometries.