Problem 29 - Chemistry in Focus

Question

Draw a Lewis structure and use VSEPR to determine the geometry of each of the following molecules. If the molecule has more than one central atom, indicate the geometry about each of these and draw the threedimensional structure. a. ClNO (nitrogen is the central atom) b. \(\mathrm{H}_{3} \mathrm{CCH}_{3}\) (two carbon atoms in the middle, each with three hydrogen atoms attached) c. \(\mathrm{N}_{2} \mathrm{~F}_{2}\) (nitrogen atoms in the center and fluorine atoms on the ends) d. \(\mathrm{N}_{2} \mathrm{H}_{4}\) (nitrogen atoms in the center and two hydrogen atoms attached to each nitrogen)

Step-by-Step Solution

Answer

ClNO has a bent geometry around the nitrogen atom. \(\mathrm{H}_{3}CCH_{3}\) has a tetrahedral geometry around each carbon atom. \(\mathrm{N}_{2} \mathrm{~F}_{2}\) has a linear geometry, and \(\mathrm{N}_{2} \mathrm{H}_{4}\) has a trigonal pyramidal geometry around each nitrogen atom.

Step 1: Draw Lewis Structures

First, draw the Lewis structures for each molecule showing all the valence electrons around each atom. To determine the number of valence electrons, look at the group number of each element in the periodic table.

Step 2: Determine Molecular Geometries using VSEPR

Determine the molecular geometry for each central atom in the molecule using VSEPR (Valence Shell Electron Pair Repulsion) theory. For each central atom, count the number of bond pairs and lone pairs and use this to determine the shape according to VSEPR shapes.

Step 3: Describe Geometries for ClNO

For ClNO, Nitrogen is the central atom with one lone electron pair, two bonding domains that is, a single bond with chlorine and a double bond with oxygen, leading to a bent shape.

Step 4: Describe Geometries for \(\mathrm{H}_{3}CCH_{3}\)

For \(\mathrm{H}_{3}CCH_{3}\), there are two central carbon atoms, each of which has four single bonds with no lone pairs, resulting in a tetrahedral geometry around each carbon atom.

Step 5: Describe Geometries for \(\mathrm{N}_{2} \mathrm{~F}_{2}\)

For \(\mathrm{N}_{2}\mathrm{~F}_{2}\), both nitrogen atoms are at the center with a triple bond between them and a single bond with fluorine atoms on each end, which gives a linear geometry.

Step 6: Describe Geometries for \(\mathrm{N}_{2} \mathrm{H}_{4}\)

For \(\mathrm{N}_{2} \mathrm{H}_{4}\), each nitrogen atom is bonded to the other by a single bond, and each has one lone pair and three single bonds with hydrogen atoms, resulting in a trigonal pyramidal geometry.

Step 7: Draw 3D Structures

Finally, draw the three-dimensional structures indicating the arrangement of atoms in space for each molecule.

Key Concepts

Lewis structures

Understanding Lewis structures is crucial when studying chemistry as they provide visual representations of the valence electrons in molecules and ions. The Lewis structure is a two-dimensional diagram that uses dots around chemical symbols to illustrate where the valence electrons are located.

When drawing a Lewis structure, one must take several steps to ensure accuracy:

• Count all valence electrons from each atom in the molecule.
• Place electrons around atoms to fill their octet or duet (in the case of hydrogen) as per the octet rule.
• Connect atoms with single, double, or triple bonds to represent shared electron pairs.
• Put any remaining electrons around atoms that can hold more than eight electrons (expanded octets) or as lone pairs for atoms needing fewer electrons to achieve stability.

Following this sequence helps in predicting molecule structures and their reactivity.

Molecular shapes

The molecular shape of a compound is a three-dimensional arrangement of atoms within a molecule. Molecular shapes are influenced by the electron pairs surrounding the central atom(s), whether they are bonded to other atoms or are non-bonding pairs of electrons (known as lone pairs).

The shape of a molecule can be linear, bent, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral among others. These shapes are not arbitrarily decided but are derived from analyzing Lewis structures and leveraging theories like VSEPR to understand electron pair repulsions.

Valence shell electron pair repulsion theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is employed to predict the geometry of a molecule based on the idea that electron pairs are negatively charged and, therefore, repel each other. This repulsion causes the molecule to take on a shape where the valence electron pairs are as far apart from each other as possible, minimizing the repulsive forces.

The VSEPR theory is a valuable tool to use after drawing Lewis structures, as it helps to visualize molecules in three dimensions. The number of bonding domains and lone pairs dictate the resulting geometry, from which the molecular shape is derived.

Bonding domains

Bonding domains refer to the regions around a central atom where electrons are shared between atoms to form chemical bonds.

In the context of VSEPR, each single, double, or triple bond counts as one bonding domain regardless of the number of shared electron pairs. Having multiple bonding domains may lead to a variety of molecular shapes, and their specific arrangement is used to predict the molecule's structure. For example, four single bonds (four bonding domains) around a carbon atom result in a tetrahedral shape.

Lone electron pairs

Lone electron pairs play a significant role in determining molecular geometry. They are valence electrons not involved in bonding and thus occupy their own space around the central atom. Compared to bonding pairs, lone pairs take up more space because their electron density is solely focused around one atom.

When using VSEPR theory to determine a molecule's structure, lone pairs must be considered as they alter bond angles and ultimately influence the molecular shape. They're often the reason a molecule like water, with two hydrogen atoms and two lone pairs, has a bent shape instead of being linear.