Question

Using the VSEPR theory, predict the molecular structure of each of the following molecules. a. \(\mathrm{NCl}_{3}\) b. \(\mathrm{H}_{2} \mathrm{Se}\) c. \(\mathrm{SiCl}_{4}\)

Short answer

The molecular structures of the given molecules using the VSEPR theory are as follows: a. NCl3 has a trigonal pyramidal molecular structure. b. H2Se has a bent (angular) molecular structure. c. SiCl4 has a tetrahedral molecular structure.

Step-by-step solution

a. Molecular structure of NCl3

Step 1: Determine the central atom In \(\mathrm{NCl}_{3}\), the central atom is Nitrogen (N). Step 2: Write the Lewis structure The Lewis structure is: N is surrounded by three Cl atoms, with a single bond between N and each of the Cl atoms and a lone pair on the N atom. Step 3: Predict the molecular structure using the VSEPR theory Since N has 3 bonding electron pairs (from N-Cl single bonds) and one lone pair, there are 4 electron pairs in total. According to the VSEPR theory, the molecular structure is trigonal pyramidal.

b. Molecular structure of H2Se

Step 1: Determine the central atom In \(\mathrm{H}_{2} \mathrm{Se}\), the central atom is Selenium (Se). Step 2: Write the Lewis structure The Lewis structure is: Se is surrounded by two H atoms, with a single bond between Se and each of the H atoms and two lone pairs on the Se atom. Step 3: Predict the molecular structure using the VSEPR theory Since Se has 2 bonding electron pairs (from Se-H single bonds) and two lone pairs, there are 4 electron pairs in total. According to the VSEPR theory, the molecular structure is bent (angular).

c. Molecular structure of SiCl4

Step 1: Determine the central atom In \(\mathrm{SiCl}_{4}\), the central atom is Silicon (Si). Step 2: Write the Lewis structure The Lewis structure is: Si is surrounded by four Cl atoms, with a single bond between Si and each of the Cl atoms. Step 3: Predict the molecular structure using the VSEPR theory Since Si has 4 bonding electron pairs (from Si-Cl single bonds) and no lone pairs, there are 4 electron pairs in total. According to the VSEPR theory, the molecular structure is tetrahedral.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This arrangement is crucial for understanding the molecule's properties, including reactivity, polarity, and even color. The Valence Shell Electron Pair Repulsion (VSEPR) theory is a widely accepted model used to predict molecular geometry. The theory is based on the idea that electron pairs surrounding a central atom will arrange themselves as far apart as possible to minimize repulsion.
For example, in a molecule like SiCl extsubscript{4}, the four electron pairs around the central atom (silicon) arrange themselves symmetrically, resulting in a tetrahedral structure. Understanding molecular geometry helps chemists determine how molecules interact with one another, which is important in fields ranging from pharmacology to environmental science.

Lewis Structure

A Lewis structure is a simple way to represent the valence electrons of atoms in a molecule. These structures help predict the distribution of electrons and the kinds of bonds that might form between atoms. They consist of elemental symbols surrounded by dots that represent outer shell electrons. Single, double, or triple lines between atoms indicate covalent bonds formed by shared electron pairs.
For instance, the Lewis structure of Cl extsubscript{3} (nitrogen trichloride) shows nitrogen at the center with three lines (representing single bonds) connecting to three chlorine atoms, and a pair of dots on nitrogen representing a lone pair. Lewis structures are a foundational step in determining the shape and polarity of molecules using VSEPR theory.

Electron Pairs

Electron pairs are groups of two electrons occupying an orbital within an atom. In the context of molecular geometry, electron pairs include bonding pairs, which are shared between atoms, and lone pairs, which are unshared and reside on a single atom. The arrangement of these electron pairs around a central atom determines the molecule's shape according to the VSEPR theory.
Bonding electron pairs form the framework of the molecular structure by creating covalent bonds. For example, in SiCl extsubscript{4}, there are four bonding pairs. Lone pairs, such as those on the Se atom in H extsubscript{2}Se, exert extra repulsion compared to bonding pairs, often resulting in bent or non-linear geometries like the bent structure of water.

Trigonal Pyramidal

The trigonal pyramidal shape is a molecular geometry that results from a central atom surrounded by three bonding pairs and one lone pair. This geometry is common in molecules where the central atom is from Group 15 of the periodic table, such as nitrogen in NCl extsubscript{3}.
In a trigonal pyramidal structure, the lone pair exerts a stronger repulsive force than bonding pairs, causing the bonded atoms to push downward and form a pyramid-like shape. This arrangement is not symmetrical, resulting in a polar molecule if the bonds are polar.

• Occurs typically with four electron pairs, one being a lone pair.
• Has a characteristic bond angle slightly less than 109.5 degrees due to the presence of the lone pair.

Bent Structure

Bent or angular molecular geometry occurs in molecules with two bonded pairs and at least one lone pair on the central atom. This shape is seen in molecules like H extsubscript{2}Se, where selenium is the central atom with two hydrogen atoms bonded to it.
The presence of lone pairs affects the bond angle, pushing the bonded atoms closer together, resulting in an angle less than the typical tetrahedral angle of 109.5 degrees. The lone pairs' increased electron repulsion distorts the bond angles, creating the bent structure preferred in such conditions. This geometry is crucial for polar molecules, contributing to their dipole moments.

• Commonly results in polar molecules due to asymmetric shape.
• Lone pairs influence bond angles, generally leading to angles less than 120 degrees.

Tetrahedral Structure

A tetrahedral structure is a common molecular geometry where a central atom is symmetrically surrounded by four bonded atoms, forming a shape like a four-cornered pyramid. This structure is typical in molecules with no lone pairs on the central atom, such as SiCl extsubscript{4}.
In a perfect tetrahedral shape, all bond angles are equal at approximately 109.5 degrees, maintaining symmetry and often resulting in non-polar molecules if the surrounding atoms are identical. The tetrahedral structure alleviates repulsive forces by maximizing the distance between electron pairs, which is an inherent goal of the VSEPR theory.

• Applies to molecules with four bonding pairs.
• Results in equal bond angles, contributing to its symmetry.