If you have four electron pairs around a central atom, under...

Name: Problem 146, Chapter 10: General Chemistry: Principles and Modern Applications
Uploaded: 2020-08-21T10:30:41-08:00
Duration: 1 min 26 s
Channel: Lottie Adams
Description: If you have four electron pairs around a central atom, under what circumstances can you have a pyramidal molecule? Similarly, how can you have a bent molecule? What are the expected bond angles in each case?

If you have four electron pairs around a central atom, under what circumstances can you have a pyramidal molecule? Similarly, how can you have a bent molecule? What are the expected bond angles in each case?

Key Concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the repulsions between electron pairs in the valence shell of the central atom. The theory posits that electron pairs, whether bonding or non-bonding, will arrange themselves as far apart as possible to minimize repulsive forces, which in turn determines the overall shape of the molecule.

Electron Pair Geometry

Electron pair geometry considers all regions of electron density (including bonding pairs and lone pairs) around the central atom. For a central atom with four electron pairs, the ideal arrangement is tetrahedral with approximate bond angles of 109.5°. However, the presence of lone pairs can distort these angles, leading to different molecular geometries.

Lone Pair Effects

Lone pairs of electrons are localized closer to the central atom and occupy more space than bonding pairs. Their repulsive effect is stronger than that of bonding pairs, causing the bond angles between bonding pairs to be reduced compared to the idealized geometry. This effect explains why geometries with lone pairs, such as trigonal pyramidal or bent shapes, have bond angles smaller than 109.5°.

Trigonal Pyramidal Geometry

When a central atom with four electron pairs has one lone pair and three bonding pairs, the molecular shape is described as trigonal pyramidal. The lone pair occupies one of the tetrahedral positions, which pushes the bonding pairs closer together, resulting in bond angles that are slightly smaller than 109.5° (typically around 107° as seen in ammonia). This geometry can be described as a pyramidal shape due to its three-sided base.

Bent Molecular Geometry

A bent molecular geometry occurs when a central atom with four electron pairs has two lone pairs and two bonding pairs. The two lone pairs occupy more space and exert greater repulsive forces on the bonding pairs, which results in a geometry where the bonding pairs are forced closer together, yielding a bond angle that is considerably reduced from 109.5° (typically around 104.5° in water). This arrangement is known as bent or V-shaped.

Transcript

Please give Ace some feedback