Problem 68 - General Chemistry: Principles and Modern Applications

Question

Use the VSEPR theory to predict the shape of (a) the molecule OSF \(_{2} ;\) (b) the molecule \(\mathrm{O}_{2} \mathrm{SF}_{2} ;\) (c) the ion \(\mathrm{SF}_{5}^{-} ;\) (d) the ion \(\mathrm{ClO}_{4}^{-} ;\) (e) the ion \(\mathrm{ClO}_{3}^{-}\).

Step-by-Step Solution

Answer

The molecule OSF2 has a T-shaped, O2SF2 and ClO4- are tetrahedral, SF5- is square pyramidal, and ClO3- is a Trigonal Pyramid.

Step 1: Identify the central atom

For OSF2, O2SF2, SF5-, and ClO4-, the central atom is S. For ClO3-, it's Cl.

Step 2: Drawing the Lewis structures

Draw structures showcasing all involved bonds and lone electron pairs. This helps to visualize the layout of the molecules/ions. You get for OSF2: S is bounded to O and two F with two lone pairs on S. O2SF2 has S bounded to two O and two F, with no lone pairs. SF5- has S surrounded by five one-bonded F atoms with one lone pair. ClO4- has Cl surrounded by four one-bonded O atoms with no lone pairs. ClO3- has one double-bonded O and two one-bonded O surrounding Cl with one lone pair.

Step 3: Applying the AXE notation

This notation helps to predict the geometry of a molecule. The 'A' stands for the central atom, 'X' represents the number of atoms attached to A and 'E' is the number of lone electron pairs. For OSF2, A=S, X=3, E=2 (AX3E2). For O2SF2, A=S, X=4, E=0 (AX4E0). For SF5-, A=S, X=5, E=1 (AX5E1). For ClO4-, A=Cl, X=4, E=0 (AX4E0). For ClO3-, A=Cl, X=3, E=1 (AX3E1).

Step 4: Predicting the shape

Now you can use the AXE notation to predict the shape of the molecules and ions. For OSF2 (AX3E2), it is a T-Shaped. For O2SF2 (AX4E0), it's Tetrahedral. For SF5- (AX5E1), it's a square pyramidal. For ClO4- (AX4E0), it's also Tetrahedral. For ClO3- (AX3E1), it's a Trigonal Pyramid.

Key Concepts

Molecular Geometry

Molecular geometry is about understanding the specific three-dimensional arrangement of atoms within a molecule. Using the VSEPR (Valence Shell Electron Pair Repulsion) theory, we can predict the shape of molecules by considering the repulsions between electron pairs.

Different arrangements affect how molecules physically interact and react with others. Generally, different shapes like linear, bent, tetrahedral, or trigonal pyramidal describe how atoms surround a central atom, impacting molecular details like polarity and reactivity.

• For instance, in a bent shape like water (H₂O), the angle between the hydrogen atoms is determined by the electron pair repulsions around the oxygen atom.
• Simplifying to basic geometries helps chemists predict chemical behaviors and molecular interactions in reactions.

Lewis Structures

Lewis structures are simplified diagrams used to represent the bonding between atoms within a molecule. They also show lone pairs of electrons that do not participate directly in bonding.

Creating a Lewis structure involves:
- Counting the total number of valence electrons available in the molecule or ion.
- Arranging atoms to show which are bonded together, usually putting the more electronegative atoms in terminal positions.
- Adding electron dots to symbolize shared and lone pairs, aimed at fulfilling each atom’s octet (or duet for hydrogen).

• Lewis structures for the molecules in the exercise, such as OSF₂ and SF₅^-, help display the possible arrangements of atoms and the role of lone pairs affecting molecule shape.
• Understanding how to draw Lewis structures is crucial because it forms the foundation for using the VSEPR theory to predict molecular geometries.

AXE Notation

AXE notation is a specific semantic shortcut used by chemists to apply the VSEPR theory efficiently. It names molecules based on the number of bonded atoms and lone pairs around a central atom.

The notation consists of:

• 'A' representing the central atom.
• 'X' for the number of bonded atoms.
• 'E' for the number of lone electron pairs on the central atom.

For example:

• In OSF₂, AX₃E₂ indicates a T-shaped molecule due to three bonds and two lone pairs around the central sulfur atom.
• This approach strives to simplify molecular geometry predictions, making it more accessible for both students and professionals to determine molecular shapes quickly and accurately.

Central Atom Identification

Identifying the central atom in a molecule is the initial step in predicting molecular structure using VSEPR theory. The central atom is typically the least electronegative, unlike hydrogen which is rarely central.

To identify it:

• Examine the composition of the molecule to see which atom can make the most connections, i.e., the most versatile in bonding.
• In exercises here, like OSF₂ and SF₅^-, sulfur (S), which can accommodate multiple bonds due to its abundance of available valence electrons, acts as the central atom.

Recognizing the central atom is vital as it serves as the anchor for understanding the entire molecular geometry through methods like Lewis structures and axial/equatorial positioning within the 3D structure of the molecule, aiding the visualization of molecular shapes considerably.