Ch.22 - Chemistry of the Nonmetals
Chapter 22, Problem 46
Complete the exercises below. The SF₅⁻ ion is formed when SF₄ (g) reacts with fluoride salts containing large cations, such as CsF(s). Draw the Lewis structures for SF₄ and SF₅⁻, and predict the molecular structure of each.
Verified step by step guidance1
Step 1: Identify the total number of valence electrons for each molecule. For SF₄, sulfur (S) has 6 valence electrons and each fluorine (F) has 7 valence electrons. Therefore, SF₄ has a total of 6 + 4(7) = 34 valence electrons. For SF₅⁻, add one more electron for the negative charge, resulting in 34 + 1 = 35 valence electrons.
Step 2: Draw the Lewis structure for SF₄. Place sulfur in the center and arrange the four fluorine atoms around it. Connect each fluorine to sulfur with a single bond. Distribute the remaining electrons to satisfy the octet rule for each fluorine atom. Sulfur can have an expanded octet, so any remaining electrons can be placed on sulfur.
Step 3: Draw the Lewis structure for SF₅⁻. Place sulfur in the center and arrange the five fluorine atoms around it. Connect each fluorine to sulfur with a single bond. Distribute the remaining electrons to satisfy the octet rule for each fluorine atom. The extra electron from the negative charge should be placed on sulfur, allowing it to have an expanded octet.
Step 4: Predict the molecular geometry of SF₄ using VSEPR theory. SF₄ has 4 bonding pairs and 1 lone pair on the central sulfur atom. This results in a seesaw shape due to the lone pair occupying an equatorial position in a trigonal bipyramidal arrangement.
Step 5: Predict the molecular geometry of SF₅⁻ using VSEPR theory. SF₅⁻ has 5 bonding pairs and no lone pairs on the central sulfur atom. This results in a trigonal bipyramidal shape, as all positions are occupied by bonding pairs.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lewis Structures
Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They help visualize the arrangement of valence electrons and the connectivity of atoms, which is crucial for predicting molecular geometry and reactivity. In the case of SF₄ and SF₅⁻, drawing their Lewis structures will reveal the number of bonds and lone pairs around the sulfur atom.
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Molecular Geometry
Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is determined by the number of bonding pairs and lone pairs of electrons around the central atom, following the VSEPR (Valence Shell Electron Pair Repulsion) theory. Understanding the molecular geometry of SF₄ and SF₅⁻ is essential for predicting their physical and chemical properties, as well as their reactivity.
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Formal Charge
Formal charge is a concept used to determine the charge distribution within a molecule, calculated based on the number of valence electrons, the number of non-bonding electrons, and the number of bonds. It helps assess the stability of a Lewis structure by indicating how electrons are shared among atoms. For SF₄ and SF₅⁻, calculating formal charges can help identify the most stable resonance structures and predict the favored molecular configuration.
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