Problem 15 - Chemistry The Molecular Science

Question

In each of these ions, three oxygen atoms are attached to a central atom. Draw the Lewis structure for each ion, and then describe the electron-region geometry and the molecular geometry. Comment on similarities and differences in the series. (a) \(\mathrm{BO}_{3}^{3-}\) (b) \(\mathrm{CO}_{3}^{2-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{ClO}_{3}^{-}\)

Step-by-Step Solution

Answer

\(\mathrm{BO}_3^{3-}\) and \(\mathrm{CO}_3^{2-}\) are trigonal planar, \(\mathrm{SO}_3^{2-}\) and \(\mathrm{ClO}_3^{-}\) are trigonal pyramidal.

Step 1: Draw Lewis Structures

To draw the Lewis structure for each ion, start by counting the valence electrons. For \(\mathrm{BO}_3^{3-}\) (borate ion), count 3 from boron and 6 from each oxygen atom, plus 3 additional electrons for the 3- charge, giving a total of 24 valence electrons. Similarly, for \(\mathrm{CO}_3^{2-}\) (carbonate ion), 4 from carbon, 6 from each oxygen, plus 2 additional for the charge, totaling 24. For \(\mathrm{SO}_3^{2-}\) (sulfite ion), 6 from sulfur, 6 from each oxygen, plus 2 additional, totaling 26. Finally, for \(\mathrm{ClO}_3^{-}\) (chlorate ion), 7 from chlorine, 6 from each oxygen, plus 1 additional, totaling 26. Draw the skeleton with the central atom connected to the oxygen atoms, and distribute remaining electrons to satisfy the octet rule, starting with the outer atoms.

Step 2: Determine Electron-Region Geometry

The electron-region geometry considers all electron groups (bonds and lone pairs) around the central atom. For \(\mathrm{BO}_3^{3-}\) and \(\mathrm{CO}_3^{2-}\), there are three bonding groups and no lone pairs, resulting in a trigonal planar electron-region geometry. \(\mathrm{SO}_3^{2-}\) has three bonding pairs plus one lone pair, leading to tetrahedral electron-region geometry. \(\mathrm{ClO}_3^{-}\) has three bonding pairs plus one lone pair, also resulting in a tetrahedral geometry.

Step 3: Determine Molecular Geometry

Molecular geometry describes the arrangement of the atoms. For \(\mathrm{BO}_3^{3-}\) and \(\mathrm{CO}_3^{2-}\), with no lone pairs on the central atom, the molecular geometry is also trigonal planar. For \(\mathrm{SO}_3^{2-}\) and \(\mathrm{ClO}_3^{-}\), the presence of a lone pair changes the molecular shape to trigonal pyramidal.

Step 4: Analyze Similarities and Differences

\(\mathrm{BO}_3^{3-}\) and \(\mathrm{CO}_3^{2-}\) are similar with trigonal planar geometry. \(\mathrm{SO}_3^{2-}\) and \(\mathrm{ClO}_3^{-}\) differ due to the lone pair on the central atom, resulting in trigonal pyramidal geometry. Overall, while all ions contain oxygen atoms attached to a central atom, the presence of lone pairs affects geometries.

Key Concepts

Electron-Region Geometry

In chemistry, the electron-region geometry refers to the spatial arrangement of electron groups around the central atom in a molecule or ion. These electron groups can be bonding pairs or lone pairs. Understanding the electron-region geometry helps you predict the overall shape and properties of the molecule or ion.

For example, in the ions

• \(\mathrm{BO}_3^{3-}\) (borate)
• \(\mathrm{CO}_3^{2-}\) (carbonate)
• \(\mathrm{SO}_3^{2-}\) (sulfite)
• \(\mathrm{ClO}_3^{-}\) (chlorate)

we consider both the bonding and lone pairs of electrons. In the case of \(\mathrm{BO}_3^{3-}\) and \(\mathrm{CO}_3^{2-}\), there are three groups of electrons all being bonding pairs, leading to a trigonal planar electron-region geometry. Meanwhile, the ions \(\mathrm{SO}_3^{2-}\) and \(\mathrm{ClO}_3^{-}\) feature one lone pair together with the bonding pairs, resulting in a tetrahedral electron-region geometry.

Recognizing electron-region geometry is crucial for understanding how a molecule or ion will interact with others and helps to predict its physical and chemical characteristics.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule, which is influenced by the electron-region geometry but focuses only on the atoms, not the lone pairs.

In the borate \(\mathrm{BO}_3^{3-}\) and carbonate \(\mathrm{CO}_3^{2-}\), all electron groups are bonded to atoms, leading to a trigonal planar molecular geometry. It means the atoms are arranged in a flat triangular shape around the central atom.

However, for sulfite \(\mathrm{SO}_3^{2-}\) and chlorate \(\mathrm{ClO}_3^{-}\), the presence of a lone pair changes the arrangement. The lone pair occupies space and pushes the atoms to form a trigonal pyramidal shape. This shape resembles a three-sided pyramid with the central atom at the peak.

Molecular geometry is important as it affects the behavior and reactivity of the molecule. It also impacts physical properties such as polarity and the ability to engage in interactions like hydrogen bonding.

Trigonal Planar

Trigonal planar is a type of geometry seen in molecules where three regions of electron density (bonds) form around a central atom, creating a flat, triangular shape.

In the ions \(\mathrm{BO}_3^{3-}\) and \(\mathrm{CO}_3^{2-}\), the central atom connects with three oxygen atoms, all lying on the same plane. There are no lone pairs to distort the shape, hence maintaining the planar arrangement.

Characteristics of trigonal planar geometry include:

• All bond angles are approximately 120°.
• The molecule is symmetrical, which often leads to non-polar molecules.

Maintaining a planar structure helps in understanding how these ions might stack, interact, and fit in larger structures. In cases where the electron groups are identical, the molecule's geometry often leads to a cancellation of dipoles, resulting in non-polarity.

Trigonal Pyramidal

Trigonal pyramidal is a geometry that occurs when a molecule has three bonds and one lone pair around the central atom. The presence of the lone pair pushes the bonds away, distorting the otherwise planar structure into a three-sided pyramid.

Ions such as \(\mathrm{SO}_3^{2-}\) and \(\mathrm{ClO}_3^{-}\) exhibit trigonal pyramidal geometry. The lone pair remains on the central atom, affecting the spatial distribution of the bound oxygen atoms.

Some features of trigonal pyramidal geometry include:

• Bond angles are slightly less than 109.5° due to lone pair repulsion.
• The presence of a lone pair results in a less symmetrical structure, leading to polarity.

Understanding trigonal pyramidal shapes is important for predicting properties such as solubility, reactivity, and mechanisms of interaction with other molecules.