Problem 27 - Chemistry Principles and Practice

Question

Give the bonded-atom lonè-pair arrangement and the molecular shape of the following species. (a) \(\mathrm{SeO}_{2}\) (b) \(\mathrm{N}_{2} \mathrm{O}(\mathrm{N}\) is the central atom \()\) (c) \(\mathrm{H}_{3} \mathrm{O}^{+}\) (d) \(\mathrm{IF}_{5}\) (e) \(\mathrm{SCl}_{4}\)

Step-by-Step Solution

Answer

(a) Bent; (b) Linear; (c) Trigonal pyramidal; (d) Square pyramidal; (e) Seesaw.

Step 1: Determine the VSEPR Shape

For each molecule or ion, determine the number of regions of electron density (bonds and lone pairs) around the central atom using VSEPR theory.

Step 2: Analyze Bonded Atoms and Lone Pairs

Consider bonded-atom lone-pair arrangement (electron geometry) by evaluating the different bonded atoms and lone electron pairs around the central atom.

Step 3: Identify Molecular Shape

Determine the molecular shape based on the arrangements found in Step 2, considering how lone pairs affect the overall molecular geometry.

Step 4: Application to Species (a): SeO2

SeO2 has a total of three regions of electron density around Se (two bonds and one lone pair). The electron geometry is trigonal planar, and the molecular shape is bent or angular.

Step 5: Application to Species (b): N2O

For N2O with N as the central atom, there is a triple bond to one N and a single bond to the other, resulting in two regions of electron density and a linear shape.

Step 6: Application to Species (c): H3O+

H3O+ has four regions of electron density (three bonds and one lone pair). The electron geometry is tetrahedral, leading to a trigonal pyramidal molecular shape.

Step 7: Application to Species (d): IF5

IF5 has a total of six regions of electron density (five bonds and one lone pair). The electron geometry is octahedral, resulting in a square pyramidal molecular shape.

Step 8: Application to Species (e): SCl4

SCl4 has five regions of electron density (four bonds and one lone pair). The electron geometry is trigonal bipyramidal, leading to a seesaw-shaped molecular geometry.

Key Concepts

Molecular Geometry

Understanding molecular geometry is essential in predicting the behavior, reactivity, and properties of molecules. VSEPR (Valence Shell Electron Pair Repulsion) theory is vital for determining the shape of a molecule. It involves analyzing the arrangement of atoms around a central atom, as determined by the number of bonded atoms and lone pairs of electrons.
The shape of a molecule affects physical and chemical properties such as polarity, boiling points, and reactivity. For example:

• In H_3O^+, even though it has four areas of electron density, the presence of a lone pair results in a trigonal pyramidal molecular shape instead of a perfect tetrahedron.
• In IF_5, the lone pair causes the structure to be square pyramidal, not merely octahedral.

Remember, molecular shapes are named based on the positions of atoms, not electron pairs.

Electron Density

Electron density refers to the distribution of electron clouds in a molecule, which directly influences its shape and geometry. According to VSEPR theory, electron pairs, both bonded and lone pairs, distribute themselves in space to minimize repulsion between them by staying as far apart as possible. These repulsions between different electron pairs define the shape of the molecule.
Typically, electron density is counted as:

• Single, double, and triple bonds are considered regions of electron density.
• Lone pairs are considered individual regions of electron density.

For example, in the case of SeO_2, the central Se atom is surrounded by two bonding regions and one lone pair, leading to an electron geometry that is trigonal planar. However, the molecular shape is bent due to the lone pair. In SCl_4, the central S atom is surrounded by four bonding regions and one lone pair, resulting in a seesaw-shaped geometry due to these electron densities.

Bonded-Atom Lone-Pair Arrangement

The arrangement of bonded atoms and lone pairs around a central atom typically determines the molecular shape and electron geometry. This aspect is important because lone pairs occupy more space than bonded atoms, influencing the shape markedly.
For instance, consider the examples:

• With N_2O, the linear shape is due to the two regions of electron density, one being a triple bond and the other a single bond, without lone pairs on the central N atom.
• Conversely, in IF_5, the presence of a lone pair despite the octahedral electron geometry causes the actual molecular shape to be square pyramidal.

When identifying bonded-atom lone-pair arrangements, remember that these arrangements lead to various geometries because lone pairs exert greater repulsion than bonding pairs.