Question

Discuss the basic features of the VSEPR model. Explain why the magnitude of repulsion decreases in this order: lone pair-lone pair \(>\) lone pair-bonding pair \(>\) bonding pair-bonding pair.

Short answer

The Valence Shell Electron Pair Repulsion (VSEPR) model predicts the geometric arrangement of atoms in molecules based on the repulsion between electron pairs in an atom's valence shell. Its key features include the determination of molecular geometry by the number of electron pairs, and the minimization of repulsion by spatial arrangement of these pairs. The order of lone pair-lone pair (lp-lp) > lone pair-bonding pair (lp-bp) > bonding pair-bonding pair (bp-bp) repulsion is due to the spatial distribution of the electron pairs, with lone pairs being more diffuse and hence exerting a greater repulsive effect.

Step-by-step solution

Understanding the VSEPR Model

The VSEPR model is a hypothesis that provides a quantitative representation of the spatial arrangements of atoms in molecules. According to this model, electron pairs in the valence shell of an atom repulse each other and try to get as far apart as possible to minimise the repulsion.

Identifying the basic features of VSEPR Model

The main concepts or features of the VSEPR model are: (i) The number of electron pairs around the central atom decides the geometry of the molecule. (ii) The electron pairs (both bonding and non-bonding) take up positions in space to minimize repulsion. (iii) Lone pairs of electrons are spatially more diffuse and thus repel electron pairs (bonding or non-bonding) more than bonding pairs do.

Explaining the Order of Repulsion

The repulsion order of lone pair-lone pair (lp-lp) greater than lone pair-bonding pair (lp-bp) greater than bonding pair-bonding pair (bp-bp) stems from the fact that lone pairs are solely associated with one nucleus, allowing them to spread out more in space, leading to a larger electron cloud. As a result, their repulsion effect is stronger compared to bonding pairs which are shared between two nuclei and hence are more confined in space.

Key concepts

Electron Pair Repulsion

The concept of electron pair repulsion is central to understanding molecular shapes within the VSEPR model. Atoms form bonds by sharing electrons, and these electron pairs arrange themselves into specific shapes around a central atom.

The VSEPR model stands for Valence Shell Electron Pair Repulsion theory. It explains how the geometry of a molecule is determined by repulsions between electron pairs in the valence shell of the central atom. These electron pairs try to be as far apart as possible to minimize repulsion.

• Electron pairs include both bonding pairs (shared with other atoms) and non-bonding pairs (lone pairs that are not shared but belong to one atom).
• Both types of electron pairs affect the shape of the molecule by repelling other electron pairs.

In simple terms, imagine you and your friends trying to sit as comfortably far apart on a picnic blanket. Just like that, electron pairs "push" each other into a formation to avoid as much interaction as possible.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This geometry significantly affects the molecule's properties, such as reactivity and polarity.

The VSEPR model predicts molecular geometry based on the idea of minimizing the repulsion between electron pairs. Each molecule adopts a shape that allows electron pairs to stay as far apart as possible.

• Lone pairs take more space than bonding pairs as they are not shared and only interact with the central atom.
• This geometry can be linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral, among others.

For example, in a water molecule (H₂O), the presence of two lone pairs on the oxygen results in a bent shape, which significantly affects the molecule's behavior.

Lone Pair Repulsion Order

The order of repulsion based on the type of electron pair interaction is crucial for predicting the shape of molecules. In the VSEPR model, the order of repulsion is:

• Lone pair-lone pair (lp-lp) > lone pair-bonding pair (lp-bp) > bonding pair-bonding pair (bp-bp)

This happens because lone pairs are localized on one nucleus and can spread out more than bonding pairs, which are shared between two atoms.

Lone pair-lone pair repulsions are the strongest due to two expansive regions pushing against each other without the involvement of any bonding. When a lone pair interacts with a bonding pair, the repulsion is slightly decreased because the bonded pair is partly spread across another atom.

Finally, bonding pair-bonding pair repulsion is the weakest, as these electrons are shared and therefore more tightly confined to the space between two atoms. Understanding this order helps in predicting the bond angles and thus the shape of the molecule.