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Chapter 3: Problem 27

Predict the shapes and estimate the bond angles of (a) the thiosulfate ion, \(\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}\); (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{Be}\); (c) \(\mathrm{BH}_{2}{ }^{-}\); (d) \(\mathrm{SnCl}_{2}\).

Short Answer

Expert verified
The shapes and estimated bond angles: (a) Thiosulfate ion (\(S_2O_3^{2-}\)): trigonal pyramidal, < 109.5°; (b) (\(CH_3\right)_2 \rm Be\right): linear, 180°; (c) \rm BH_2^{-}: bent, < 120°; (d) \rm SnCl_2: bent, < 120°.

Step by step solution

01

- Determine Geometry of Thiosulfate Ion

First, draw the Lewis structure for the thiosulfate ion, \(\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}\). The central atom sulfur has a steric number of 4 (two oxygens are bonded with double bonds and one sulfur is bonded with a single bond and there is also one lone pair), predicting a trigonal pyramidal shape, distorted due to the presence of lone pair of electrons on sulfur. The estimated bond angle is less than \(109.5^\circ\) due to the lone pair-bond pair repulsion.
02

- Determine Geometry of (CH3)2Be

The compound \(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{Be}\) has a linear structure as there are two groups attached to the beryllium atom and there are no lone pairs. The bond angle is \(180^\circ\).
03

- Determine Geometry of BH2-

For \(\mathrm{BH}_{2}{ }^{-}\), boron is the central atom with 3 electrons from the two hydrogen atoms and 2 from the negative charge, totaling to 5 electrons which forms 3 groups (2 hydrogen atoms and 1 lone pair). The molecular geometry is predicted to be bent or v-shaped, and the bond angle is estimated to be slightly less than \(120^\circ\) due to the repulsion from the lone pair.
04

- Determine Geometry of SnCl2

In \(\mathrm{SnCl}_{2}\), tin is the central atom and it is bonded to two chlorine atoms with no lone pairs on the tin atom. The shape is angular or bent with an estimated bond angle of less than \(120^\circ\) since it is based on a trigonal planar arrangement where a pair of non-bonding electrons would have occupied one corner.

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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 shape of individual molecules based on the idea that electron pairs in the valence shell repel each other and therefore tend to be as far apart as possible.

This theory helps explain the structure of molecules by considering the number of bonds and lone pairs associated with a specific atom. The shape of a molecule directly affects its physical and chemical properties, making VSEPR theory a crucial concept in the study of chemistry.
Lewis structure
Lewis structures are visual representations of an atom’s valence electrons and any potential bonding between atoms within a molecule. Depicting electrons as dots, Lewis structures facilitate an understanding of the types of bonds—whether single, double, or triple—between atoms as well as the presence of lone, or non-bonded, electron pairs.

To draw such a structure, it's essential to know the number of valence electrons for each atom. This graphical approach is powerful in predicting molecule behavior during bonding and reactions, making it a fundamental tool in chemistry.
Bond angles
Bond angles are the angles between adjacent bonds around a central atom in a molecule, which are determined by the molecule's geometry. The VSEPR theory provides a guide to predict these angles, largely based on the arrangement that minimizes repulsion between electron pairs.

For example, in a molecule with a tetrahedral geometry, the bond angles would be approximately 109.5°. Deviations from these standard angles occur due to the presence of lone pairs or differences in electron pair repulsions, and are key to understanding molecular shape and function.
Electronic geometry
Electronic geometry describes the spatial arrangement of all electron pairs (bonding and lone pairs) around a central atom. It determines the idealized shape of the molecule if all electron pairs are considered equivalent.

Common geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. These geometric frameworks serve as a basis to deduce molecular geometry, which is the actual arrangement of atoms influenced by variations caused by lone pairs.
Lone pair repulsion
Lone pairs are pairs of valence electrons not involved in bonding. According to the VSEPR theory, lone pairs exert a greater repulsive force on neighboring electron pairs compared to bonded pairs. This is because lone pairs are located closer to the nucleus and occupy more space.

The presence of lone pairs can decrease bond angles in a molecule, altering its expected geometry. This effect is evident in the comparison between the ideal bond angles of a purely geometric arrangement and the observed bond angles in real molecules.

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Most popular questions from this chapter

Draw Lewis structures for each of the following species and predict the hybridization at each carbon atom: (a) \(\mathrm{H}_{2} \mathrm{CCH}^{+}\); (b) \(\mathrm{H}_{2} \mathrm{CCH}_{3}{ }^{+}\); (c) \(\mathrm{H}_{3} \mathrm{CCH}_{2}{ }^{-}\).

Both \(\mathrm{NH}_{2}^{-}\)and \(\mathrm{NH}_{2}{ }^{+}\)are angular species, but the bond angle in \(\mathrm{NH}_{2}^{-}\)is less than that in \(\mathrm{NH}_{2}{ }^{+}\). (a) What is the reason for this difference in bond angles? (b) Take the \(x\)-axis as lying perpendicular to the plane of the molecule. Does the \(\mathrm{N} 2 \mathrm{p}_{x}\) orbital participate in the hybridization for either species? Briefly explain your answer.

\mathrm{Xenon}\( forms \)\mathrm{XeO}_{3}, \mathrm{XeO}_{4}\(, and \)\mathrm{XeO}_{6}^{4-}\(, all of which are powerful oxidizing agents. Give their Lewis structures, their bond angles, and the hybridization of the xenon atom. Which would be expected to have the longest \)\mathrm{Xe}-\mathrm{O}$ distances?

Write the Lewis structure and VSEPR formula and identify the molecular shape and bond angles of the interhalogen ion \(\mathrm{IF}_{6}^{+}\).

Write the Lewis structure, VSEPR formula, molecular shape, and bond angles for each of the following species: (a) \(\mathrm{I}_{3}{ }^{-}\); (b) \(\mathrm{POCl}_{3}\); (c) \(\mathrm{IO}_{3}{ }^{-}\); (d) \(\mathrm{N}_{2} \mathrm{O}\).
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