Ch.8 - Covalent Compounds: Bonding Theories and Molecular Structure

Problem 39

Chapter 8, Problem 39

What shape do you expect for molecules that meet the following descriptions? (a) A central atom with two lone pairs and three bonds to other atoms (b) A central atom with two lone pairs and two bonds to other atoms (c) A central atom with two lone pairs and four bonds to other atoms

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Identify the electron pair geometry around the central atom using the VSEPR theory, which considers both bonding pairs and lone pairs of electrons. The total number of electron pairs (both bonding and lone pairs) determines the electron pair geometry.

For part (a), with two lone pairs and three bonding pairs (total of five pairs), the electron pair geometry is trigonal bipyramidal. The presence of lone pairs modifies the molecular shape from the basic geometry.

For part (b), with two lone pairs and two bonding pairs (total of four pairs), the electron pair geometry is tetrahedral. Again, the presence of lone pairs affects the final molecular shape.

For part (c), with two lone pairs and four bonding pairs (total of six pairs), the electron pair geometry is octahedral. The molecular shape is influenced by the arrangement of the lone pairs within this geometry.

Determine the molecular shape by considering the positions of the lone pairs within the electron pair geometry. Lone pairs tend to occupy more space than bonding pairs, leading to adjustments in bond angles and the overall shape of the molecule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is a model used to predict the geometry of individual molecules based on the repulsion between electron pairs surrounding a central atom. According to this theory, electron pairs, whether bonding or lone pairs, will arrange themselves to minimize repulsion, leading to specific molecular shapes.

Lone Pairs vs. Bonding Pairs

Lone pairs are pairs of valence electrons that are not involved in bonding, while bonding pairs are shared between atoms to form covalent bonds. The presence of lone pairs affects the molecular geometry by altering bond angles and overall shape, as they occupy more space than bonding pairs, leading to distortions in the expected geometry.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs around the central atom, which influences the angles between bonds and the overall shape, such as linear, trigonal planar, tetrahedral, or bent configurations.

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