Problem 96 - Chemistry The Central Science In Si Units

Question

(a) Predict the electron-domain geometry around the central S atom in \(\mathrm{SF}_{2}, \mathrm{SF}_{4}\), and \(\mathrm{SF}_{6}\). (b) The anion \(\mathrm{IO}_{4}^{-}\) has a tetrahedral structure: three oxygen atoms form double bonds with the central iodine atom and one oxygen atom which carries a negative charge forms a single bond. Predict the molecular geometry of \(\mathrm{IO}_{6}{\underline{\phantom{xx}}}^{5-}\).

Step-by-Step Solution

Answer

SF2: Bent, SF4: Trigonal bipyramidal, SF6: Octahedral. IO6^{5-} is pentagonal bipyramidal.

Step 1: Identify Lewis Structures for SF Molecules

For \(\text{SF}_2\): Sulfur is the central atom with six valence electrons, and two fluorine atoms each forming a single bond. Total electron domains: 4. \(\text{SF}_4\): Sulfur is the central atom with four fluorine atoms each forming a single bond, totaling 4 plus 2 lone pair electrons. Total electron domains: 5.\(\text{SF}_6\): Sulfur forms six single bonds with six fluorine atoms. Total electron domains: 6.

Step 2: Predict Electron-Domain Geometry for SF Molecules

For \(\text{SF}_2\): 4 electron domains indicate a tetrahedral electron-domain geometry. However, the molecular geometry is bent due to two lone pairs.For \(\text{SF}_4\): 5 electron domains suggest a trigonal bipyramidal electron-domain geometry.For \(\text{SF}_6\): 6 electron domains result in an octahedral electron-domain geometry.

Step 3: Revisit Electron-Domain Geometry for IO Anions

For \(\text{IO}_4^{-}\): Confirmed tetrahedral structure as stated in the problem.For \(\text{IO}_6^{5-}\): Iodine with 12 electrons total from six oxygen atoms means there are six bonds making a total of five single bonds and one double bond with oxygen. Two lone pairs needed to complete iodine's octet (or more accurately, extension past the octet). Total electron domains: 7.

Step 4: Predict Molecular Geometry of IO Anion

For \(\text{IO}_6^{5-}\): With 7 electron domains, the electron-domain geometry suggests a pentagonal bipyramidal arrangement due to the 5 single bonds and one double bond with oxygen.

Key Concepts

Electron-Domain Geometry

Electron-domain geometry is a vital concept used to predict the spatial arrangement of electron pairs around a central atom. This arrangement includes both bonding and non-bonding (lone) pairs of electrons, which repel each other and spread out, striving to minimize repulsion. Understanding electron-domain geometry is essential as it lays the groundwork for predicting the molecule's eventual shape. For instance, a molecule with four electron domains typically has a tetrahedral electron-domain geometry. However, the presence of lone pairs can alter the molecular shape from what might initially be expected. The key idea is that the overall structure aims to keep electron pairs as far apart as possible.

Lewis Structures

Lewis structures are visual representations that help illustrate the arrangement of electrons in a molecule. They show how atoms share electrons and fill their valence shells, providing insight into the molecule's bonding and lone pairs. These structures are critical for predicting electron-domain geometry, as they highlight the number of electron pairs surrounding the central atom. For example, the Lewis structure of \( ext{SF}_2\) reveals two bonding pairs and two lone pairs around sulfur, leading to a tetrahedral electron-domain geometry but with a bent molecular geometry due to the lone pairs. Similarly, \( ext{SF}_4\) has a total of five electron domains, leading to a trigonal bipyramidal electron-domain geometry due to its four bonds and one lone pair.

Sulfur Compounds

Sulfur compounds often feature strong and stable bonds due to sulfur's ability to expand its octet and accommodate more than eight electrons in its valence shell. Sulfur, unlike oxygen, can engage in various bonding environments, including the ability to form up to six bonds as seen in \(\text{SF}_6\). In sulfur hexafluoride, sulfur forms single bonds with six fluorine atoms, resulting in an octahedral electron-domain geometry. Each sulfur-fluorine bond shares an electron pair, and there are no lone pairs on sulfur in this molecule. This is a classical example of an expanded octet, showcasing how sulfur can adapt to different bonding requirements compared to elements in the same group.

Iodine Anions

Iodine anions are interesting in their ability to exist in various oxidation states and form complexes with multiple ligands. When forming \(\text{IO}_4^-\), iodine is centrally positioned with a tetrahedral structure - featuring three double-bonded oxygens and one single-bonded oxygen carrying a negative charge. In \(\text{IO}_6^{5-}\), iodine bonds with six oxygen atoms, accumulating five single bonds and one double bond, resulting in seven electron domains. This configuration predicts a pentagonal bipyramidal electron-domain geometry due to the arrangement of bonds around iodine. The presence of five single bonds and one double bond maximizes iodine's bonding capabilities, demonstrating its flexibility in forming complex anions.