Question

Discuss VSEPR theory and the trend of repulsive interactions in lone pair-lone pair and lone-pairbond pair.

Short answer

The VSEPR theory predicts the molecular geometry based on the repulsion between electron pairs in the valence shell of an atom. The repulsion follows the trend: lone pair-lone pair > lone pair-bond pair > bond pair - bond pair. This is because lone pairs are located closer to the nucleus and spread out more. This repulsion further affects the shape of the molecule, such as causing a water molecule to have a 'bent' shape.

Step-by-step solution

Understand the VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model in chemistry used to predict the geometry of molecules based on the number of valence electron pairs surrounding their central atoms. Under this theory, electron pairs are treated as charges and will orient themselves so they are as far apart as possible surviving each other's repulsion.

Discuss Repulsion Force in Lone Pair-Bond Pair and Lone Pair-Lone Pair

According to VSEPR theory, the pair of electrons in the valence shell experiences repulsive forces from other pairs. This repulsion force follows a hierarchy: Lone pair (LP) - Lone pair repulsion > Lone pair - Bond pair (BP) repulsion > Bond pair - Bond pair repulsion. Lone pairs repel more than bonding pairs because they are closer to the nucleus and spread out more; therefore, they occupy a larger space.

Describe the Implication of Repulsion Trend

The trend in repulsion impacts the molecular geometry. For example, a water molecule (H2O) has two bond pairs and two lone pairs around the central oxygen atom. The lone pairs, due to their greater repulsion, push down the bond pairs, resulting in a 'bent' or 'V-shape' molecule despite the four electron pairs theoretically arranging themselves tetrahedrally.

Key concepts

Lone pair-lone pair repulsion

In VSEPR theory, understanding repulsion forces begins with lone pair-lone pair interactions. Lone pairs are unshared electrons that remain after bonding, clustered close to the central atom. Since they aren’t involved in bonding, lone pairs have a higher density of negative charge around them. This results in greater repulsion compared to bonded pairs.

The strong repulsion effect is due to lone pairs occupying more space than bonding pairs. Their proximity to the central atom leads to a significant push away from each other, maximizing their distance. This repulsion is crucial because:

• It dictates that lone pairs repel each other more strongly than they do bonded pairs, leading them to try and maintain maximum distance from each other.
• This strong repulsion can alter molecular shapes, deviating from ideal geometries usually seen in simple molecules.
• The extent of lone pair-lone pair repulsion often guides the prediction of molecular shapes in complex geometry scenarios.

Lone pair-bond pair repulsion

Lone pair-bond pair repulsion is the next step down in terms of severity in the hierarchy of electron pair repulsions described by VSEPR. While not as intense as lone pair-lone pair repulsion, this interaction is still stronger than bond pair-bond pair repulsion.

This repulsion occurs because lone pairs are closer to the nucleus and spread out more, trying to avoid the bond pairs. In essence, they still exert a significant outward push. This repulsion is important for several reasons:

• It affects bond angles by pushing bond pairs closer together, impacting the overall structure of the molecule.
• In molecules like ammonia (NH₃), the presence of a lone pair on nitrogen causes a smaller H-N-H bond angle compared to methane (CH₄), where all valence electrons are in bonding pairs.
• This lone pair-bond pair repulsion largely influences distorted geometries in molecules, such as in trigonal pyramidal shapes.

Molecular geometry prediction

A fundamental application of VSEPR theory is the prediction of molecular geometry. By analyzing the repulsions between different types of electron pairs, scientists can predict the shape of a molecule.

Predictions start by counting the number of electron pairs around the central atom, including both bonding pairs and lone pairs.

• Linear Geometry: Occurs with two areas of electron density, pointing 180° apart.
• Trigonal Planar: Three areas of density, spaced at 120° angles.
• Tetrahedral: Four areas, with 109.5° angles between them.

The presence of lone pairs modifies these ideal angles, leading to bent, trigonal pyramidal, and other molecular shapes. For instance, in water (H₂O), the lone pairs force the molecule into a bent shape instead of the ideal tetrahedral arrangement.

Understanding this process allows chemists to make informed predictions about molecular behavior and reactivity based on geometry.