Question

The \(\mathrm{SF}_{5}^{-}\) ion is formed when \(\mathrm{SF}_{4}(g)\) reacts with fluoride salts containing large cations, such as \(\mathrm{CsF}(s)\). Draw the Lewis structures for \(\mathrm{SF}_{4}\) and \(\mathrm{SF}_{5}^{-}\), and predict the molecular structure of each.

Short answer

The molecular structures for \(\mathrm{SF}_4\) and \(\mathrm{SF}_5^- \) can be predicted using Lewis structures and VSEPR theory. The Lewis structures are \(\mathrm{S}(\mathrm{F})_4\) for \(\mathrm{SF}_4\) and \(\mathrm{S}(\mathrm{F})_5^-\) for \(\mathrm{SF}_5^- \). Based on VSEPR theory, the molecular structure for \(\mathrm{SF}_4\) is seesaw (distorted tetrahedral) and for \(\mathrm{SF}_5^- \) is square pyramidal.

Step-by-step solution

Determine the total number of valence electrons per molecule

Count the number of valence electrons for each atom in the given molecules. For \(\mathrm{SF}_4\), we have one sulfur atom with 6 valence electrons and four fluorine atoms with 7 valence electrons each. For \(\mathrm{SF}_5^-\), we have one sulfur atom with 6 valence electrons, five fluorine atoms with 7 valence electrons each, and one extra electron from the negative charge. Total valence electrons for \(\mathrm{SF}_4 = 1 \times 6 + 4 \times 7 = 34\) Total valence electrons for \(\mathrm{SF}_5^- = 1 \times 6 + 5 \times 7 + 1 = 42\)

Draw the Lewis structures

Distribute the electrons so that each atom achieves an octet (or expanded octet). In both cases, sulfur will be the central atom. In \(\mathrm{SF}_4\), we can distribute the electrons as single bonds between S and each of the four F atoms, and then add lone pairs on each F atom to achieve an octet. In \(\mathrm{SF}_5^- \), we can distribute the electrons as single bonds between S and each of the five F atoms, and then add lone pairs on each F atom to achieve an octet. Lewis structures: \(\mathrm{SF}_4 : \mathrm{S}(\mathrm{F})_4\) \(\mathrm{SF}_5^- : \mathrm{S}(\mathrm{F})_5^-\) Note that in \(\mathrm{SF}_4\), Sulfur has an expanded octet with 10 electrons, and in \(\mathrm{SF}_5^- \), Sulfur has an expanded octet with 12 electrons.

Predict molecular structure using VSEPR theory

VSEPR theory (Valence Shell Electron Pair Repulsion) will help us predict the molecular structure. The electron pairs will arrange themselves as far apart as possible to minimize repulsion forces. For \(\mathrm{SF}_4\), there are 4 bonded electron pairs and 1 lone pair around the sulfur atom. The molecular geometry can be described as "seesaw" or "distorted tetrahedral" since the lone pair causes a distortion. For \(\mathrm{SF}_5^- \), there are 5 bonded electron pairs and 1 lone pair around the sulfur atom. The molecular geometry can be described as "square pyramidal" since the bonded electrons form a pyramid with a square base. In summary, the molecular structures for \(\mathrm{SF}_4\) and \(\mathrm{SF}_5^- \) are seesaw (distorted tetrahedral) and square pyramidal, respectively.