Problem 57 Determine the electron geometry,... [FREE SOLUTION] (2024)

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Chapter 6: Problem 57

Determine the electron geometry, molecular geometry, and idealized bond angles for each molecule. In which cases do you expect deviations from the idealized bond angle? \begin{equation}\mathrm{ a. } {PF}_{3} \quad \text { b. }\mathrm{SBr}_{2}\end{equation} \begin{equation}\mathrm{ c. }{CHCl}_{3} \quad\text { d. } \mathrm{CS}_{2}\end{equation}

Short Answer

Expert verified

PF3 has a tetrahedral electron geometry and trigonal pyramidal molecular geometry, with bond angles slightly less than 109.5 degrees. SBr2 has a tetrahedral electron geometry and bent molecular geometry, with bond angles less than 109.5 degrees. CHCl3 has a tetrahedral electron and molecular geometry, with bond angles close to 109.5 degrees. CS2 has a linear electron and molecular geometry, with bond angles of 180 degrees.

Step by step solution


Determine Electron Geometry of PF3

Consider the valence shell electron pair repulsion (VSEPR) theory. Phosphorus (P) has 5 valence electrons and forms 3 bonds with fluorine (F) atoms, using 3 electrons. The remaining 2 electrons form a lone pair. The total number of electron pairs around phosphorus is 4. The electron geometry for 4 electron pairs is a tetrahedral.


Determine Molecular Geometry of PF3

Since there is one lone pair on the central phosphorus atom, the molecular geometry is determined by the positions of the 3 bonded pairs of electrons which push apart to a trigonal pyramidal shape.


Idealized Bond Angles and Deviations for PF3

The idealized bond angles for a tetrahedral electron geometry are approximately 109.5 degrees. However, the lone pair on phosphorus will repel the bonded electron pairs more strongly, causing a slight reduction in the bond angle. Therefore, we can expect a deviation from the idealized bond angle which will be less than 109.5 degrees.


Determine Electron Geometry of SBr2

Sulfur (S) has 6 valence electrons and forms 2 bonds with bromine (Br) atoms, using 4 electrons. The remaining 2 lone pairs of electrons are on the sulfur atom. There are 4 areas of electron density around the sulfur which results in a tetrahedral electron geometry.


Determine Molecular Geometry of SBr2

With two bonding pairs and two lone pairs, the molecular geometry of SBr2 is bent.


Idealized Bond Angles and Deviations for SBr2

The ideal angle in a tetrahedral geometry is about 109.5 degrees. However, with two lone pairs, the bond angle in SBr2 is expected to decrease due to increased repulsion, resulting in a bond angle smaller than 109.5 degrees.


Determine Electron Geometry of CHCl3

Carbon (C) forms 4 single bonds with one hydrogen (H) and three chlorine (Cl) atoms, using all its 4 valence electrons. There are 4 areas of electron density which corresponds to a tetrahedral electron geometry.


Determine Molecular Geometry of CHCl3

Since there are no lone pairs on the central carbon atom, and all 4 areas of electron density are from bonds, the molecular geometry of CHCl3 is also tetrahedral.


Idealized Bond Angles for CHCl3

With a tetrahedral molecular geometry, the idealized bond angles for CHCl3 are 109.5 degrees. Because there are no lone pairs to distort the geometry, we expect the bond angles to be very close to the idealized value.


Determine Electron Geometry of CS2

Carbon (C) in CS2 forms 2 double bonds with two sulfur (S) atoms, using all its 4 valence electrons. There are 2 areas of electron density which results in a linear electron geometry.


Determine Molecular Geometry of CS2

With two areas of electron density from double bonds, and no lone pairs on the central atom, the molecular geometry of CS2 is linear.


Idealized Bond Angles for CS2

The idealized bond angle for a linear geometry is 180 degrees. Since there are no lone pairs to cause deviations, we expect CS2 to have bond angles of exactly 180 degrees.

Key Concepts

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

VSEPR theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the repulsion between the sets of valence electrons around a central atom.

VSEPR theory postulates that electron pairs located in the valence shell of an atom will arrange themselves to minimize repulsion between these pairs. This creates a molecule whose shape is determinable based on the number of bonding electron pairs (shared by atoms) and non-bonding electron pairs (lone pairs) on the central atom.

Understanding VSEPR theory is fundamental for predicting molecule shapes. As we saw with PF3, although the electron geometry is tetrahedral, the presence of a lone pair alters the molecular geometry to trigonal pyramidal, which in turn affects bond angles.

Electron Geometry

Electron geometry describes the spatial arrangement of all pairs of electrons around the central atom, including both the bond pairs and lone pairs.

Even though we cannot see electrons, we can predict their arrangement: two electron pairs shape up linearly, three form a trigonal planar, four create a tetrahedral, five a trigonal bipyramidal, and six an octahedral. These geometries consider each pair of electrons—bonding or non-bonding—to be positioned at the corners of a shape, with the central atom at its center.

Bond Angles

Bond angles are the angles between adjacent lines representing bonds. They are determined by the electron geometry of the molecule. Clear examples are given in the step by step solutions where we learn that for tetrahedral electron geometries the idealized bond angles are approximately 109.5 degrees.

In molecules with no lone pairs, such as CHCl3, the observed bond angles closely match the idealized bond angles. However, the presence of lone pairs can cause deviations from the idealized angles due to the repulsion they exert, which is seen in molecules like PF3 and SBr2.

Lone Pairs

Lone pairs refer to the pairs of valence electrons on an atom that are not shared with another atom and do not participate in bonding. Lone pairs are important as they occupy more space than bonding pairs, causing the geometry to adjust to accommodate them, which impacts the bond angles.

For example, in SBr2, the two lone pairs present on the sulfur atom push down the bonding pairs to create a bent molecular geometry instead of a tetrahedral, which is what the electron geometry would suggest if we only considered areas of electron density.

Electron Pair Repulsion

Electron pair repulsion is a cornerstone concept of VSEPR theory, which dictates that electron pairs surrounding a central atom tend to repel each other due to their negative charge. This repulsion is the driving force behind the arrangement of electron pairs that leads to the molecular geometry of the molecule.

The repulsion between lone pairs is greater than that between a lone pair and a bond pair, which is greater than the repulsion between bond pairs. This hierarchy of repulsions is crucial in understanding why certain deviations from ideal bond angles occur in molecular structures.

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Problem 57 Determine the electron geometry,... [FREE SOLUTION] (3)

Most popular questions from this chapter

Draw the Lewis structure (including resonance structures) for the acetate ion \(\left(\mathrm{CH}_{3} \mathrm{COO}^{-}\right) .\) For each resonancestructure, assign formal charges to all atoms that have formal charge.A molecule with the formula AB3 has a trigonal pyramidal geometry. How many electron groups are on the central atom (A)?Write Lewis structures for each molecule or ion. Use expanded octets as necessary. \begin{equation}\mathrm{ a. }{ClF}_{5} \quad \text { b. }\mathrm{AsF}_{6}-\quad \text { c. } \mathrm{Cl}_{3} \mathrm{PO} \quad \text {d. IF }_{5}\end{equation}Write the Lewis structure for each molecule. a. \(\mathrm{NF}_{3} \quad\) b. HBr \(\quad\) c. \(\mathrm{SBr}_{2} \quad\) d.\(\mathrm{CCl}_{4}\)
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Problem 57 Determine the electron geometry,... [FREE SOLUTION] (2024)
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