Problem 11 - Chemistry: An Atoms-Focused Approach

Question

Why is the shape of a molecule determined by repulsions between electron pairs and not by repulsions between nuclei?

Step-by-Step Solution

Answer

Answer: The shape of a molecule is determined by repulsions between electron pairs because of the Valence Shell Electron Pair Repulsion (VSEPR) theory, which states that electron pair repulsive forces are the primary factor influencing molecular geometry. While nuclear repulsions also occur, they are not the dominant factor because attractive forces between positive nuclei and negative electron pairs counteract these repulsions and maintain the molecular structure.

Step 1: 1. Understanding Electron Pair Repulsion

Electron pairs, both bonding and non-bonding, are negatively charged. As a result, they repel each other due to the electrostatic forces between them. In a molecule, the electron pairs are arranged in a way that minimizes these repulsions, leading to the most stable molecular geometry. This principle is known as Valence Shell Electron Pair Repulsion (VSEPR) theory.

Step 2: 2. Role of Electron Pairs in Molecular Geometry

According to VSEPR theory, the repulsion between electron pairs is the main factor that determines the molecular geometry. When electron pairs are arranged around a central atom, their repulsion forces the bonding and non-bonding electrons to occupy positions as far apart as possible from one another. This results in specific geometric arrangements, such as linear, trigonal planar, tetrahedral, octahedral, and others, depending on the number of electron pairs.

Step 3: 3. Nuclear Repulsions in Molecules

Nuclei of atoms also repel each other due to their positive charges. However, this repulsion is not the dominant factor in determining molecular shape. This is because the effect of nuclear repulsion is mitigated by the attractive force between the positively charged nuclei and the negatively charged electron pairs within the molecule. These attractive forces help to maintain the stability of the molecular structure and prevent it from breaking apart due to nuclear repulsions.

Step 4: 4. Conclusion

The shape of a molecule is determined by repulsions between electron pairs and not by repulsions between nuclei because electron pair repulsive forces are the primary factor influencing molecular geometry, according to VSEPR theory. While nuclear repulsions do play a role in the overall stability of a molecule, they are not the dominant factor in determining the shape of a molecule. Instead, the attractive forces between positive nuclei and negative electron pairs help to maintain the molecular structure and counteract the repulsion between nuclei.