Problem 22 - General Chemistry: Principles and Modern Applications

Question

Which of the following species has a linear structure: \(\mathrm{ClF}_{2}^{+}, \mathrm{IBrF}^{-}, \mathrm{OCl}_{2}, \mathrm{ClF}_{3},\) or \(\mathrm{SF}_{4} ?\) (Central atom underlined.) Do any two of these species have the same structure?

Step-by-Step Solution

Answer

Only the IBrF- species has a linear structure and no two species share the same structure.

Step 1: Lewis Structures and Electron Domain Geometry

First, create the Lewis structures for each molecule and determine the electron domain geometry. Use the following information: a) ClF2+ has 7(1)+7(2)+1 = 22 total valence electrons (VE), b) IBrF- has 7(3)-1 = 20 VE, c) OCl2 has 6(1)+7(2) = 20 VE, d) ClF3 has 7(1)+7(3) = 28 VE, e) SF4 has 6(1)+7(4) = 34 VE.

Step 2: Deduce the Geometry

Compare the configuration of bonding and non-bonding electron pairs to known molecular geometries. a) ClF2+ has three electron domains (the Cl atom is surrounded by two single bonds and one lone pair), which would give a trigonal planar electron domain geometry. However, with one lone pair, the molecular geometry becomes bent or V-shaped. b) IBrF- also has three electron domains, but the molecular geometry is linear because the central iodine atom has two lone pairs, leaving the F and Br atoms on opposite ends. c) OCl2 has three electron domains with no lone pair, which is a bent or V-shaped structure. d) ClF3 has five electron domains resulting in a T-shaped molecular geometry. e) SF4 has five electron domains with one lone pair, creating a see-saw structure.

Step 3: Compare the Structures

Now compare the geometries of the molecules. Only IBrF- has a linear structure. None of the species share the same molecular structure.

Key Concepts

Lewis Structures

When figuring out the geometry of molecules, Lewis structures are a great way to start. They show how atoms bond in a molecule, as well as the lone pairs of electrons on atoms. To draw a Lewis structure, follow these simple steps:

• Count the total valence electrons for all atoms in the molecule.


• Place the atoms relative to how they are bonded, usually starting with the central atom.


• Connect the atoms with single bonds and distribute remaining electrons to complete the octets (or duets for hydrogen).


• If there are still electrons left, add them as lone pairs to the central atom, or form double or triple bonds if needed.

Lewis structures are not only a tool to represent the arrangement of electrons, but are crucial in predicting both the electron domain geometry and molecular geometry that ultimately reveal how a molecule will behave chemically.

Electron Domain Geometry

The concept of electron domain geometry builds on Lewis structures by considering regions of electron density around the central atom. We think of each bond—whether single, double, or triple—and each lone pair as an electron domain. When you summarize these domains:

• Two electron domains form a linear shape.


• Three domains form a trigonal planar shape.


• Four domains result in a tetrahedral geometry.


• Five domains lead to a trigonal bipyramidal form.


• Six domains shape into an octahedral geometry.

These geometries provide a foundation from which we can understand the molecular geometry. It's important to remember that the presence of lone pairs can distort these perfect geometries into alternative molecular shapes.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a vital role in chemical bonding and molecular geometry. The number of valence electrons determines how atoms combine to form compounds. Here's how you can find them:

• Using the periodic table, identify an element's group number—this often equates to the number of valence electrons.


• Add up the valence electrons for all atoms involved to determine the total for a molecule.


• Consider any extra electrons involved due to charges, as seen in ions like \(\text{ClF}_2^+\) and \(\text{IBrF}^-\).

Knowing the total valence electrons helps in accurately drawing Lewis structures and predicting molecular properties. Understanding quadrants and exceptions such as the octet rule is key in mastering this step.