Question

Name the proper three-dimensional molecular shapes for each of the following molecules or ions, showing lone pairs as needed: \((\mathbf{a}) \mathrm{ClO}_{2}^{-}(\mathbf{b}) \mathrm{SO}_{4}^{2-}(\mathbf{c}) \mathrm{NF}_{3}(\mathbf{d}) \mathrm{CCl}_{2} \mathrm{Br}_{2}(\mathbf{e}) \mathrm{SF}_{4}^{2+}\)

Short answer

a) ClO2- has a bent molecular shape. b) SO4 2- has a tetrahedral molecular shape. c) NF3 has a trigonal pyramidal molecular shape. d) CCl2Br2 has a tetrahedral molecular shape. e) SF4 2+ has a seesaw (or distorted tetrahedron) molecular shape.

Step-by-step solution

a) ClO2-

Firstly, we must determine the central atom, which is Chlorine (Cl). Next, we'll calculate the total number of valence electrons: 7 (from Cl) + 2(6) (from two Oxygen atoms) + 1 (from the extra electron due to negative charge) = 20 valence electrons. The electron domain geometry can be found by determining the number of electron domains around the central atom. Here, we have three domains (two O atoms and one lone pair). The domains are arranged in a trigonal planar geometry. Since there is one lone pair and two bonded atoms, the molecular shape is bent.

b) SO4 2-

The central atom is Sulfur (S). We calculate the total valence electrons as follows: 6 (from S) + 4(6) (from four Oxygen atoms) - 2 (due to the 2- charge) = 32 valence electrons. In this case, we have four electron domains (four O atoms) surrounding the central sulfur atom, which means the Sulfur atom has a tetrahedral electron domain geometry. Since all the domains are occupied by oxygen atoms, the molecular shape is also tetrahedral.

c) NF3

The central atom is Nitrogen (N). The total valence electrons can be calculated as 5 (from N) + 3(7) (from Fluorine atoms) = 26 valence electrons. Nitrogen has four electron domains (three F atoms and one lone pair) with tetrahedral electron domain geometry. Since there is one lone pair and three bonded atoms, the molecular shape is trigonal pyramidal.

d) CCl2Br2

Here, the central atom is Carbon (C) with four valence electrons. The total valence electrons are 4 (from C) + 2(7) (from Chlorine atoms) + 2(7) (from Bromine atoms) = 32 valence electrons. There are four electron domains around the carbon atom (two Cl and two Br), and the electron domain geometry is tetrahedral. Since all domains are occupied by other atoms, the molecular shape is also tetrahedral.

e) SF4 2+

The central atom is Sulfur (S). We calculate the total valence electrons: 6 (from S) + 4(7) (from four Fluorine atoms) - 2 (due to the 2+ charge) = 30 valence electrons. With five electron domains (four F atoms and one lone pair) around the central sulfur atom, the electron domain geometry is trigonal bipyramidal. There is one lone pair and four bonded atoms; hence, the molecular shape is called a "seesaw" or "distorted tetrahedron."

Key concepts

Electron Domain Geometry

Electron domain geometry is a concept used in molecular chemistry to describe the arrangement of electron groups around a central atom. These electron groups or domains include both bonding electrons (the electrons involved in forming chemical bonds) and lone pairs (non-bonding electron pairs). Understanding the electron domain geometry is crucial because it influences the shape of the molecule and determines physical and chemical properties.

• For example, if a molecule has four electron domains around a central atom, the electron domains will arrange themselves in a tetrahedral geometry to minimize repulsion.
• Similarly, when the central atom has three electron domains, as seen in the first example with \(\text{ClO}_2^-\), the geometry tends to be trigonal planar.

This understanding provides a scaffold for predicting molecular shapes.

Valence Electrons Calculation

Calculating valence electrons is a fundamental step when determining molecular shapes, as it tells how many electrons are available for bonding in a molecule. Valence electrons are the outermost electrons and they play a key role in forming bonds.
To count valence electrons:

• Sum the valence electrons from each atom in the molecule.
• Add or subtract electrons to account for any charges indicated by ions.

Consider the molecule \(\text{SO}_4^{2-}\), where sulfur and oxygen collectively contribute and account for the charge by subtracting two electrons due to the negative charge. This results in a total of 32 valence electrons.

Molecular Shapes

Molecule shape is determined by the electron domain geometry, and it's crucial for understanding how molecules interact with each other. The shape influences everything from reactivity to color to biological activity.
Key molecular shapes based on electron domains include:

• Bent shape, like in \(\text{ClO}_2^-\), occurs when there are lone pairs in a trigonal planar geometry, producing an angular shape.
• Tetrahedral shape, found in \(\text{SO}_4^{2-}\) and \(\text{CCl}_2\text{Br}_2\), happens when four bonding pairs surround a central atom.
• Trigonal pyramidal, seen with \(\text{NF}_3\), arises due to one lone pair in a tetrahedral arrangement which alters the bond angles.
• Seesaw or distorted tetrahedron, as in \(\text{SF}_4^{2+}\), is due to lone pairs causing asymmetry in a trigonal bipyramidal geometry.

Lone Pairs

Lone pairs are pairs of valence electrons that are not shared with another atom, and hence, not involved in bonding. Even though they don't contribute to bonding, they still play a critical role in determining the shape and properties of a molecule.
Lone pairs exert repulsive forces that can influence bond angles and molecular shape:

• In \(\text{NF}_3\), there is a lone pair that pushes down the three bonding pairs, resulting in a trigonal pyramidal shape.
• The presence of a lone pair in \(\text{SF}_4^{2+}\) breaks the symmetry of a perfect trigonal bipyramidal shape, causing a seesaw configuration instead.

Understanding lone pair interactions is essential for accurately predicting molecular shapes because they can significantly distort the geometry from what is expected when only considering bonding pairs.