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Chapter 9: Problem 23

What is the difference between the electron-domain geometry and the molecular geometry of a molecule? Use the water molecule as an example in your discussion. Why do we need to make this distinction?

Short Answer

Expert verified
The electron-domain geometry of a molecule describes the spatial arrangement of all electron pairs around the central atom, including bonding pairs and lone pairs. In contrast, molecular geometry focuses only on the arrangement of the atoms in the molecule, excluding lone pairs. Using the water molecule (H2O) as an example, its electron-domain geometry is tetrahedral due to the central oxygen atom having four electron domains (two bonding pairs and two lone pairs) while its molecular geometry is bent, considering only the positions of the oxygen and hydrogen atoms. This distinction is essential because electron-domain geometry provides insight into the overall arrangement of electron pairs, whereas molecular geometry determines the shape of the molecule, influencing properties such as polarity, reactivity, and intermolecular forces.

Step by step solution

01

Define Electron-Domain Geometry

Electron-domain geometry (also known as the electron-pair geometry) is the spatial arrangement of all electron pairs around the central atom, including both bonding pairs (shared between atoms) and lone pairs (not shared and belonging to a single atom).
02

Define Molecular Geometry

Molecular geometry (or molecular shape) is the spatial arrangement of only the atoms in a molecule, not considering the lone pairs. It is the shape formed by the positions of the atoms in relation to each other.
03

Using the Water Molecule as an Example

The water molecule (H2O) has one central oxygen atom surrounded by two hydrogen atoms, bonded together by two single covalent bonds. The central oxygen atom also has two lone pairs. To visualize this, we can use the VSEPR (Valence Shell Electron Pair Repulsion) model.
04

Electron-Domain Geometry of Water

In the water molecule, the central oxygen atom has four electron domains (two bonding pairs and two lone pairs). According to the VSEPR model, these electron domains repel each other and adopt a tetrahedral arrangement to minimize the repulsion. Therefore, the electron-domain geometry of water is tetrahedral.
05

Molecular Geometry of Water

Molecular geometry focuses only on the positions of the atoms in a molecule. In the case of water, only the positions of the oxygen and hydrogen atoms are considered, while the lone pairs on the oxygen atom are not included. When we exclude the lone pairs, we obtain a bent molecular geometry for the water molecule.
06

Importance of the Distinction

It is essential to make the distinction between electron-domain geometry and molecular geometry because they provide different information about a molecule. Electron-domain geometry gives us insight into the overall arrangement of all electron pairs around the central atom, while molecular geometry represents the shape of the molecule, which is crucial for understanding its properties, such as polarity, reactivity, and intermolecular forces. By comparing these two geometries, we can also deduce the presence and effects of lone pairs, which play a significant role in determining the characteristics of a molecule.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electron-Domain Geometry
The concept of electron-domain geometry is vital to understanding the arrangement of electron pairs in a molecule. In simple terms, it refers to how both bonding pairs (the ones shared between atoms) and lone pairs (the non-shared electrons) arrange themselves around a central atom. This arrangement aims to minimize repulsion as suggested by the VSEPR (Valence Shell Electron Pair Repulsion) Theory.

To put it into context, let's examine the water molecule, H鈧侽. Here, the oxygen atom is the central atom, and it has a total of four electron domains: two bonding pairs and two lone pairs. According to VSEPR Theory, these domains will spread out as much as possible, leading to a tetrahedral electron-domain geometry. The shape ensures that the electron pairs (bonding and lone pairs alike) are as far apart from each other as possible, minimizing repulsion and stabilizing the molecule.

This arrangement is essential as it influences not just the electron interactions, but also the ultimate shape of the molecule once the lone pairs are considered.
Molecular Geometry
Understanding molecular geometry helps in visualizing the actual shape formed by the atoms in a molecule. Unlike electron-domain geometry, which considers all electron pairs, molecular geometry only looks at how the atoms themselves are arranged in space.

For the water molecule, while the electron-domain geometry is tetrahedral, its molecular geometry is quite different. This is because molecular geometry does not include the lone pairs in its form. Once you only consider the positions of the hydrogen and oxygen atoms, ignoring the lone pairs on oxygen, the shape appears as 'bent' or 'angular.'

This geometric shape is crucial for properties like the molecule's polarity. The bent shape leads to an unequal distribution of charge, making water a polar molecule. The understanding of molecular geometry offers insights into how molecules will interact with each other, affecting everything from boiling points to how substances dissolve.
Lone Pairs
Lone pairs are essentially unshared electrons situated around a central atom in a molecule, and they play a pivotal role in shaping geometry. These electron pairs are not involved in bonding but impact both electron-domain and molecular geometries due to their negative charge and larger space needs.

In the case of water, the central oxygen atom contains two lone pairs that significantly affect its geometry. Though they do not directly form "bonds" with hydrogen atoms, they assert repulsion on neighboring electrons. Consequently, lone pairs occupy a larger volume than bonding pairs, leading VSEPR to predict a tetrahedral electron-domain arrangement.

When discernible within molecular geometry, lone pairs are left out, but their influence remains profound. They effectively "push" hydrogen atoms closer together, leading to a distinctive bent shape, rather than linear. Understanding the presence and impact of lone pairs helps identify and predict molecular behavior and reactivity.

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Most popular questions from this chapter

In ozone, \(\mathrm{O}_{3},\) the two oxygen atoms on the ends of the molecule are equivalent to one another. (a) What is the best choice of hybridization scheme for the atoms of ozone? (b) For one of the resonance forms of ozone, which of the orbitals are used to make bonds and which are used to hold nonbonding pairs of electrons? (c) Which of the orbitals can be used to delocalize the \(\pi\) electrons? (d) How many electrons are delocalized in the \(\pi\) system of ozone?

Azo dyes are organic dyes that are used for many applications, such as the coloring of fabrics. Many azo dyes are derivatives of the organic substance azobenzene, \(\mathrm{C}_{12} \mathrm{H}_{10} \mathrm{~N}_{2}\). A closely related substance is hydrazobenzene, \(\mathrm{C}_{12} \mathrm{H}_{12} \mathrm{~N}_{2} .\) The Lewis structures of these two substances are (Recall the shorthand notation used for benzene.) (a) What is the hybridization at the \(\mathrm{N}\) atom in each of the substances? (b) How many unhybridized atomic orbitals are there on the \(\mathrm{N}\) and the \(C\) atoms in each of the substances? (c) Predict the \(\mathrm{N}-\mathrm{N}-\mathrm{C}\) angles in each of the substances. (d) Azobenzene is said to have greater delocalization of its \(\pi\) electrons than hydrazobenzene. Discuss this statement in light of your answers to (a) and (b). (e) All the atoms of azobenzene lie in one plane, whereas those of hydrazobenzene do not. Is this observation consistent with the statement in part (d)? (f) Azobenzene is an intense red-orange color, whereas hydrazobenzene is nearly colorless. Which molecule would be a better one to use in a solar energy conversion device? (See the "Chemistry Put to Work" box for more information about solar cells.)

What hybridization do you expect for the atom indicated in red in each of the following species? (a) \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-} ;\) (b) \(\mathrm{PH}_{4}^{+}\) (c) \(\mathrm{AlF}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}^{+}\)

Propylene, \(\mathrm{C}_{3} \mathrm{H}_{6},\) is a gas that is used to form the important polymer called polypropylene. Its Lewis structure is (a) What is the total number of valence electrons in the propylene molecule? (b) How many valence electrons are used to make \(\sigma\) bonds in the molecule? (c) How many valence electrons are used to make \(\pi\) bonds in the molecule? (d) How many valence electrons remain in nonbonding pairs in the molecule? (e) What is the hybridization at each carbon atom in the molecule?

Give the electron-domain and molecular geometries for the following molecules and ions: (a) \(\mathrm{HCN},\) (b) \(\mathrm{SO}_{3}^{2-},\) (c) \(\mathrm{SF}_{4}\), (d) \(\mathrm{PF}_{6}^{-}\), (e) \(\mathrm{NH}_{3} \mathrm{Cl}^{+}\), (f) \(\mathrm{N}_{3}^{-}\).
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