Question

Under what conditions is the electron-group geometry for a molecule the same as the molecular shape?

Short answer

The electron-group geometry and molecular shape for a molecule are the same under the condition that there are no lone pair electrons on the central atom. This occurs when all electron groups around the central atom are bonding electron groups, resulting in no influence from lone pair repulsion on the arrangement of the terminal atoms.

Step-by-step solution

Define electron-group geometry and molecular shape

Electron-group geometry refers to the spatial arrangement of electron groups (bonding and lone pair electrons) around the central atom in a molecule. On the other hand, molecular shape describes the arrangement of only the terminal atoms (ignoring lone pairs) around the central atom.

Role of lone pair electrons in molecular geometry

Lone pair electrons influence the molecular geometry by repelling the bonding electron groups. Since lone pair electrons occupy more space than bonding electrons, the repulsion between a lone pair and bonding pair is stronger than the repulsion between two bonding pairs. This causes the bonding pairs to be pushed away from the lone pairs, altering the molecular shape.

Identifying when electron-group geometry is the same as molecular shape

The electron-group geometry and molecular shape will be the same when there are no lone pair electrons on the central atom. This is because the absence of lone pair electrons means that the spatial arrangement of the electron groups, as well as the terminal atoms, is not affected by repulsion from lone pairs. In summary, for the electron-group geometry to be the same as the molecular shape in a molecule, there should be no lone pair electrons on the central atom. Similarly, the molecule will exhibit the same electron-group geometry and molecular shape when all the electron groups around the central atom are bonding electron groups.

Key concepts

Electron-Group Geometry

Understanding the arrangement of electrons around a central atom is key to predicting the structure and reactivity of a molecule. Electron-group geometry, also known as electron-domain geometry, is the three-dimensional spatial orientation of all the electron groups around the central atom, which includes both the bonding pairs and lone pairs of electrons. This geometry is determined using the Valence Shell Electron Pair Repulsion (VSEPR) model, which assumes that electron groups repel one another and will therefore adopt an arrangement that minimizes this repulsion, resulting in specific geometrical shapes.

For example, when there are four electron groups, the geometry is tetrahedral, while two electron groups form a linear geometry. The possible electron-group geometries are linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, each accommodating different numbers of electron groups. An essential concept to grasp is that the electron-group geometry is the broader framework within which the actual shape of the molecule is determined.

Lone Pair Electrons

Lone pair electrons are nonbonding electrons that are located on the central atom and do not participate in bonding with other atoms. Despite being silent spectators in bond formation, they have a substantial impact on the molecule's overall shape due to their negative charge, which exerts repulsive forces on bonding electron pairs.

Lone pairs occupy more space than bonding pairs because their electron density is exclusively around the central atom, whereas bonding pairs are shared between two atoms. This increased electron density of lone pairs leads to stronger repulsive interactions compared to bonding pairs, thus altering the orientation of atoms and resulting in a molecular geometry that is different from the basic electron-group geometry. For instance, a tetrahedral electron-group geometry with one lone pair would lead to a trigonal pyramidal molecular shape. Understanding the presence and influence of lone pairs is crucial for predicting the more refined and actual shape of a molecule.

Molecular Geometry

Molecular geometry, often referred to as molecular shape, is specifically the arrangement of atoms in a molecule, disregarding the lone pairs. It's this shape that dictates many of the molecule's physical and chemical properties, including reactivity, polarity, and phase of matter under normal conditions. According to the VSEPR theory, the molecular geometry considers only the positions of the terminal atoms, not the lone pairs. Factors such as bond angles and lengths can be directly inferred from the molecular geometry.

The molecular geometry can be classified into several basic shapes like linear, bent, trigonal planar, tetrahedral, and so on. It is the observable shape of the molecule, which can be distinctly different from the electron-group geometry in the presence of lone pairs. For a molecular shape to match the electron-group geometry, the molecule must have all electron groups involved in bonding without any lone pairs on the central atom. The symmetry and simplicity of such a molecule can lead to nonpolar characteristics if all the bonding atoms are identical.