Question

The reaction between \(\mathrm{SbF}_{3}\) and \(\mathrm{CsF}\) produces, among other products, the anion \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\). This anion has no \(\mathrm{F}-\mathrm{F}\) bonds and no \(\mathrm{Sb}-\mathrm{Sb}\) bonds. (a) Propose a Lewis structure for the ion. (b) Assign a hybridization scheme to the Sb atoms.

Short answer

The \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\) anion's Lewis structure can be drawn with two Sb atoms, each bonded to three F atoms and one shared F atom that bridges the two Sb atoms, resulting in sp3d hybridization for each Sb atom. No lone pairs are present on either Sb atom.

Step-by-step solution

Determining the Total Number of Valence Electrons

To draw the Lewis structure for the \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\) anion, first determine the total number of valence electrons. Antimony (Sb) has 5 valence electrons and fluorine (F) has 7. For two Sb atoms, there are 10 valence electrons, and for seven F atoms, there are 49 valence electrons. Including the extra electron due to the negative charge gives a total of 60 valence electrons.

Drawing the Skeleton Structure

Arrange the Sb and F atoms to satisfy the conditions of no Sb-Sb or F-F bonds. Place an Sb atom in the center with the seven fluorine atoms surrounding it. To satisfy the conditions, form a bridge with two fluorine atoms bonded to both Sb atoms. This will use up four of the seven fluorine atoms.

Completing the Octets

Add lone pairs to the surrounding F atoms to complete their octets. Each F atom needs 8 electrons to complete its octet, which is achieved by forming single bonds with Sb and adding lone pairs. Since there are two bridge F atoms, they're each bonded to two Sb atoms.

Placing Remaining Electrons

After all F atoms have complete octets, distribute the remaining valence electrons to satisfy the octet rule for the Sb atoms. The bridge fluorine atoms share electrons with both Sb atoms, and the remaining three fluorine atoms form single bonds with one Sb atom.

Verifying Electron Count

Ensure that the total count of electrons matches the number of valence electrons calculated in Step 1. Every bond counts as two electrons, and each lone pair counts as two electrons.

Assigning Hybridization to Sb Atoms

Hybridization is assigned based on the number of electron pairs (bonding and non-bonding) surrounding the Sb atoms. As each Sb atom is surrounded by five electron pairs, the hybridization for each Sb atom will be sp3d.

Key concepts

Valence Electrons

Understanding valence electrons is essential in predicting how an atom will react chemically. Valence electrons are the electrons located in the outermost shell of an atom, and they play a pivotal role in forming chemical bonds. To draw a Lewis structure, like the one for the \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\) anion mentioned in our exercise, determining the number of valence electrons is the first step.

In the solution, we counted the valence electrons from antimony (Sb) and fluorine (F). Remember, each element has a characteristic number of valence electrons; for instance, Sb has 5 and F has 7. This is easily verifiable on the periodic table where elements in the same group (vertical column) have the same number of valence electrons. Knowing the total count of valence electrons, which for \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\) totals 60, including the extra one from the negative charge, is crucial for properly constructing the Lewis structure and ensuring stability within the molecule.

Octet Rule

The octet rule is a fundamental concept in chemistry that posits atoms tend to form bonds until they are surrounded by eight electrons, reaching a stable electron configuration akin to noble gases. This rule is useful for understanding why atoms share electrons in covalent bonds or transfer electrons in ionic bonds.

In our \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\) anion example, applying the octet rule informed us where to place the electrons. The fluorine atoms, each with seven valence electrons, will share one more to complete their octets, hence they form single bonds with antimony atoms. Importantly, there are exceptions to the octet rule; certain molecules and ions do not follow it strictly, often due to the presence of d-orbitals in the valence shell, as is the case with many transition metals and heavier p-block elements.

Hybridization

Hybridization is a model that describes the electronic structure of an atom within a molecule. It's derived from the concept that atomic orbitals within an atom mix to form new hybrid orbitals which can form chemical bonds. The type of hybridization depends on the number of electron pairs around the central atom.

When it comes to the Sb atoms in the \(\left[\mathrm{Sb}_{2} \mathrm{~F}_{7}\right]^{-}\) anion, the solution determined the hybridization to be sp3d. This is because each Sb atom is surrounded by five pairs of electrons - one for each fluorine it's bonded to and the lone pair. The shapes of these hybrid orbitals dictate the geometry of the molecule or ion, ensuring that electron pairs are as far apart as possible, in accordance to VSEPR theory, which minimizes electron pair repulsion and stabilizes the structure.