Question

Using Lewis structures and VSEPR, give the VSEPR formula for each of the following species and predict its shape: (a) sulfur tetrachloride; (b) iodine trichloride; (c) \(\mathrm{IF}_{4}{ }^{-}\); (d) xenon trioxide.

Short answer

(a) Sulfur Tetrachloride: AX4, Tetrahedral; (b) Iodine Trichloride: AX3E2, T-shaped; (c) \text{IF}_4^-: AX4E, Square Planar; (d) Xenon Trioxide: AX3, Trigonal Planar.

Step-by-step solution

Determine the Lewis Structures

Firstly, draw the Lewis structures for each molecule. Count the number of valence electrons each atom has and distribute them to satisfy the octet rule (or duet rule for hydrogen), while assigning formal charges as necessary.

Identify Electron Pairs Around the Central Atom

Identify the number of bonding pairs and lone pairs around the central atom in each molecule's Lewis structure. The sum of bonding pairs and lone pairs will give the steric number.

Predict Shapes Using VSEPR Theory

Use the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the molecular shape, taking into account the repulsion between the electron pairs.

Sulfur Tetrachloride (a)

Sulfur Tetrachloride (SCl4) has one sulfur central atom with no lone pairs and four bonding pairs of electrons. The VSEPR formula is AX4, indicating a steric number of 4, which corresponds to a tetrahedral shape.

Iodine Trichloride (b)

Iodine Trichloride (ICl3) has one iodine central atom with two lone pairs and three bonding pairs. The VSEPR formula is AX3E2, indicating a steric number of 5, which corresponds to a T-shaped molecular geometry.

IF4- (c)

The ion IF4- has one iodine central atom with one lone pair and four bonding pairs. The VSEPR formula is AX4E, indicating a steric number of 5, which corresponds to a square planar molecular geometry.

Xenon Trioxide (d)

Xenon trioxide (XeO3) has one xenon central atom with no lone pairs and three bonding pairs. The VSEPR formula is AX3, indicating a steric number of 3, which corresponds to a trigonal planar molecular geometry.

Key concepts

Lewis Structures

Understanding Lewis structures is fundamental in predicting the shape and properties of molecules. These structures are diagrams that illustrate the bonding between atoms within a molecule as well as any non-bonding valence electrons.

Let's start by recognizing that every atom wants to achieve a stable electronic configuration. For many elements, this involves obtaining an 'octet,' or eight electrons in their valence shell. Hydrogen, however, aims for two, called a duet. A Lewis structure is drawn by representing each atom with its symbol and surrounding it with dots to signify valence electrons, using lines to depict bonds.

Steps to Draw a Lewis Structure

• Count all the valence electrons from the atoms involved.
• Sketch a skeletal structure of the molecule, connecting the atoms with single bonds.
• Fill the octet (or duet for hydrogen) of the outer atoms.
• If electrons remain, place them on the central atom or make double/triple bonds to satisfy the octet rule.
• Assign formal charges to make sure the best structure is chosen.

Remember, the Lewis structure is a two-dimensional representation that doesn't fully communicate three-dimensional shape, but it's the starting point for predicting molecular geometry using VSEPR theory.

Molecular Geometry

Once the Lewis structure is assembled, VSEPR theory allows us to predict the three-dimensional arrangement of atoms around a central atom, which is known as the molecular geometry.The VSEPR theory (Valence Shell Electron Pair Repulsion) is based on the premise that electron pairs will arrange themselves to be as far apart as possible to minimize repulsion. This arrangement determines the molecular shape. Here are some fundamental shapes from VSEPR:

• Linear: 180° bond angles, found in diatomic molecules like O₂ and simple compounds like CO₂.
• Bent or V-Shaped: Often seen with a central atom bonded to two other atoms and having one or more lone pairs – for an AX2E1 molecule like water.
• Trigonal Planar: 120° bond angles, for molecules with an AX3 formula, such as BH₃.
• Tetrahedral: 109.5° bond angles, for molecules like CH₄ with an AX4 formula.
• Trigonal Bipyramidal and Octahedral: More complex geometries found in molecules with five or six groups of electrons around the central atom, respectively.

Keep in mind that lone pairs take up more space than bonding pairs, leading to distinctions such as the difference between tetrahedral and trigonal pyramidal shapes.

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom and are involved in forming bonds. They are critical to determining how an atom will interact with others to form molecules.

Determining Valence Electrons

For main-group elements, the group number in the periodic table often equals the number of valence electrons. For example, carbon, located in group 14, has four valence electrons. Transition metals are more complex and may vary in the number of valence electrons.Knowing the count of an atom's valence electrons is the first step in creating Lewis structures, as it determines how many electrons are available for bonding. Valence electrons also play a significant role in the reactivity of an element, its electronegativity, and its ability to conduct electricity.The Lewis structures improve greatly by considering formal charges, which can predict the most stable structure by calculating the charge on an atom assuming electrons in bonds are shared equally. While predicting molecular shapes with VSEPR, remember that electrons will arrange themselves to minimize repulsion, which directly relates to how valence electrons are distributed around the central atom.