Question

(a) Predict the electron-domain geometry around the central $\mathrm{S}$ atom in ${\mathrm{SF}}_2,{\mathrm{SF}}_4$, and ${\mathrm{SF}}_6$. ( $\mathbf{b}$ ) 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 ${\mathrm{IO}}_6{}^{5-}$.

Short answer

The electron-domain geometries of the molecules with Sulfur (S) atom are: - ${\mathrm{SF}}_2$: Tetrahedral (two bond pairs and two lone pairs) - ${\mathrm{SF}}_4$: Trigonal Bipyramidal (four bond pairs and one lone pair) - ${\mathrm{SF}}_6$: Octahedral (six bond pairs and no lone pairs) The molecular geometry of ${\mathrm{IO}}_6^{5-}$ is Pentagonal Pyramidal, with its electron-domain geometry being pentagonal bipyramidal (six bond pairs and one lone pair).

Step-by-step solution

Determine the electron domain geometries of the molecules with Sulfur (S) atom

To find the electron domain geometries of ${\mathrm{SF}}_2$, ${\mathrm{SF}}_4$, and ${\mathrm{SF}}_6$, we need to consider the number of bonding pairs and lone pairs around the central sulfur atom. Sulfur (S) is in group 16 and has six valence electrons. In each of the compounds, bond pairs will form as Sulfur bonds with Fluorine (F). - In ${\mathrm{SF}}_2$, there are two bond pairs (S-F bonds) and two lone pairs (non-bonding electrons) on the Sulfur atom. - In ${\mathrm{SF}}_4$, there are four bond pairs (S-F bonds) and one lone pair on the Sulfur atom. - In ${\mathrm{SF}}_6$, there are six bond pairs (S-F bonds) and no lone pairs on the Sulfur atom. Now we can predict the electron-domain geometry for each compound using VSEPR theory: - In ${\mathrm{SF}}_2$, the electron-domain geometry is tetrahedral, as there are four electron domains (two bond pairs and two lone pairs). - In ${\mathrm{SF}}_4$, the electron-domain geometry is trigonal bipyramidal, as there are five electron domains (four bond pairs and one lone pair). - In ${\mathrm{SF}}_6$, the electron-domain geometry is octahedral, as there are six electron domains (six bond pairs).

Predict the molecular geometry of ${\mathrm{IO}}_6^{5-}$

Given that ${\mathrm{IO}}_4^{-}$ has a tetrahedral structure, we can infer that the central Iodine (I) atom forms double bonds with three Oxygen (O) atoms and a single bond with a negatively charged Oxygen atom. In ${\mathrm{IO}}_6^{5-}$, we need to predict the molecular geometry of the molecule considering additional Oxygen atoms. First, let's determine the electron-domain geometry: Iodine (I) is in group 17 and has seven valence electrons. In ${\mathrm{IO}}_6^{5-}$, it forms six I-O single bonds, and the molecule has a 5- charge. The central iodine atom needs to gain an additional five electrons to have a 5- charge, so it will have six bond pairs (I-O bonds) and one lone pair. The electron-domain geometry of ${\mathrm{IO}}_6^{5-}$ will be pentagonal bipyramidal, as there are seven electron domains (six bond pairs and one lone pair). Now, we can predict the molecular geometry: Since there is one lone pair in the electron-domain geometry, the molecular geometry of ${\mathrm{IO}}_6^{5-}$ will be slightly distorted from a perfect pentagonal bipyramidal structure. The lone pair will repel the I-O bond pairs, giving the molecule a "Pentagonal Pyramidal" shape.

Key concepts

Electron-Domain Geometry

The concept of electron-domain geometry is crucial in understanding the shape of molecules. It examines how electron pairs (bonding and non-bonding) arrange themselves around a central atom to minimize repulsion. Using VSEPR (Valence Shell Electron Pair Repulsion) theory, we can predict these arrangements.

Consider a sulfur atom (S) which is central in compounds like ${\text{SF}}_2$, ${\text{SF}}_4$, and ${\text{SF}}_6$. Sulfur belongs to group 16, having six valence electrons. Here's how the electron-domain geometry forms in each:

• ${\text{SF}}_2$: Contains two bond pairs and two lone pairs, leading to a tetrahedral electron-domain geometry.
• ${\text{SF}}_4$: Contains four bond pairs and one lone pair, resulting in a trigonal bipyramidal electron-domain geometry.
• ${\text{SF}}_6$: With six bond pairs and no lone pairs, it forms an octahedral electron-domain geometry.

Understanding these geometries helps us predict molecular shapes in a systematic way.

Molecular Geometry Prediction

Molecular geometry prediction involves determining the spatial arrangement of atoms in a molecule. This is slightly different from electron-domain geometry, which considers all electron pairs.

For example, in ${\text{SF}}_4$, the electron-domain geometry is trigonal bipyramidal, but its molecular geometry is "see-saw" due to one lone pair. Lone pairs push bonding pairs closer together, causing deviations.

• For ${\text{SF}}_2$, the molecular shape is "bent" given the tetrahedral electron-domain geometry but with two lone pairs.
• In ${\text{SF}}_4$, despite the trigonal bipyramidal electron-domain structure, the lone pair adjusts it to a "see-saw" shape.
• ${\text{SF}}_6$'s lack of lone pairs results in a molecular geometry that remains "octahedral," matching its electron-domain geometry.

This prediction guides understanding of individual molecular characteristics and interactions.

SF2, SF4, SF6 Structures

These sulfur-fluorine compounds illustrate how different electron and molecular geometries manifest in chemical structures.

Each compound showcases unique features based on the number of bonding and non-bonding pairs:

• ${\text{SF}}_2$: The presence of two lone pairs gives a bent shape. It resembles water, emphasizing contour differences due to lone electron pairs.
• ${\text{SF}}_4$: Its see-saw shape stems from one lone pair in a trigonal bipyramidal arrangement. The non-equivalent electron positions show how lone pairs skew geometry.
• ${\text{SF}}_6$: Lacking lone pairs leads to a perfect octahedral shape, indicating symmetrical distribution of six fluorine atoms around the sulfur.

These structures demonstrate the impact of lone pairs and bond pairs on shaping molecules, dictating interactions and properties crucial for their application.