Question

Use VSEPR theory to predict the shape of each of the following: a. \(\mathrm{NCl}_{3}\) b. \(\mathrm{OBr}_{2}\) c. \(\mathrm{SiF}_{2} \mathrm{Cl}_{2}\) d. \(\mathrm{BeBr}_{2}\)

Short answer

The shapes are as follows: \( \text{NCl}_3 \) - Trigonal Pyramidal, \( \text{OBr}_2 \) - Bent, \( \text{SiF}_2\text{Cl}_2 \) - Tetrahedral, \( \text{BeBr}_2 \) - Linear.

Step-by-step solution

- Determine the Lewis structure for each molecule

Draw the Lewis structures for \(\text{NCl}_3\), \(\text{OBr}_2\), \(\text{SiF}_2\text{Cl}_2\), and \(\text{BeBr}_2\).

- Count the electron pairs around the central atom

For each molecule, count the number of bonding pairs and lone pairs around the central atom.

- Apply VSEPR theory to determine the molecular geometry

Use the VSEPR theory to predict the shape of the molecule based on the number of bonding pairs and lone pairs around the central atom.

Solution for \(\text{NCl}_3\)

The Lewis structure shows \( \text{N} \) with three \( \text{Cl} \) atoms and one lone pair. Using VSEPR theory, \( \text{NCl}_3\) has 4 electron pairs (3 bonding pairs and 1 lone pair). The shape is trigonal pyramidal.

Solution for \(\text{OBr}_2\)

The Lewis structure shows \( \text{O} \) with two \( \text{Br} \) atoms and two lone pairs. Using VSEPR theory, \( \text{OBr}_2\) has 4 electron pairs (2 bonding pairs and 2 lone pairs). The shape is bent.

Solution for \(\text{SiF}_2\text{Cl}_2\)

The Lewis structure shows \( \text{Si} \) with four atoms (2 \( \text{F} \) and 2 \( \text{Cl} \)) and no lone pairs. Using VSEPR theory, \( \text{SiF}_2\text{Cl}_2\) has 4 bonding pairs. The shape is tetrahedral.

Solution for \(\text{BeBr}_2\)

The Lewis structure shows \( \text{Be} \) with two \( \text{Br} \) atoms and no lone pairs. Using VSEPR theory, \( \text{BeBr}_2\) has 2 bonding pairs. The shape is linear.

Key concepts

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. This concept is vital because the shape of a molecule affects its properties and reactivity. VSEPR theory is commonly used to predict molecular geometry. VSEPR stands for Valence Shell Electron Pair Repulsion. This theory states that electron pairs around a central atom will arrange themselves to be as far apart as possible to minimize repulsion. For example, \(\text{NCl}_3\) has a trigonal pyramidal shape due to four electron pairs (three bonding pairs and one lone pair) around the nitrogen atom. Understanding the shape helps in predicting how the molecule will interact with others.

Lewis Structure

The Lewis structure is a diagram that shows the bonding between atoms of a molecule and any lone pairs of electrons that may exist. This structure is key for predicting molecular geometry. To draw a Lewis structure:

• Identify the total number of valence electrons.
• Arrange the atoms with the least electronegative atom in the center.
• Distribute the electrons to form bonds and satisfy the octet rule.

For example, the Lewis structure of \(\text{OBr}_2\) shows oxygen with two bonding pairs of electrons shared with bromine atoms and two lone pairs. This structure helps to figure out the shape and other properties of the molecule.

Electron Pairs

Electron pairs consist of two electrons that occupy the same orbital, and they play a crucial role in determining molecular shape. They can be classified into two types: bonding pairs and lone pairs. Bonding pairs are shared between atoms, forming a covalent bond. Lone pairs are localized around a single atom. The arrangement of these electron pairs around a central atom influences the molecule’s geometry, as seen in VSEPR theory. For instance, in \(\text{SiF}_2\text{Cl}_2\), silicon has four bonding pairs and no lone pairs, giving it a tetrahedral shape.

Bonding Pairs

Bonding pairs are pairs of electrons that are shared between two atoms, creating a covalent bond. The number of bonding pairs affects the molecular geometry. For example, in \(\text{BeBr}_2\), there are two bonding pairs and no lone pairs, resulting in a linear shape. Bonding pairs determine the distance and angles between atoms, crucial for understanding the molecule’s physical and chemical properties. The more bonding pairs around a central atom, the more complex the geometry becomes.

Lone Pairs

Lone pairs are pairs of electrons that are not involved in bonding and are localized on a single atom. They exert repulsive forces on nearby bonding pairs, affecting the molecular geometry. For example, in \(\text{OBr}_2\), oxygen has two bonding pairs and two lone pairs, resulting in a bent shape due to the repulsion between the lone pairs and bonding pairs. Lone pairs can also affect the molecule's polarity and reactivity. Their presence is crucial in predicting the shape and behavior of molecules using VSEPR theory.