5 Exp. 5: Molecular Geometry and Hybridization

Pre-Lab:                                                   Name: ________________________

Print this lab (double sided). Read the lab. Complete the following table by hand. Quiz on the tabled structures via BB before lab.

Objectives

The objectives of this exercise are:

• To analyze bond angles of a variety of molecules and ions using molecular model kits.
• To draw Lewis structures (both projection and perspective drawings) for        each of these molecules and ions.

• To determine the hybridization of the central atoms, the number and types of bonds, the geometries, and the polarities of the molecules and ions.

Safety/Housekeeping:

Wear splash goggles when anyone in the lab uses solvents or chemicals

• Wear splash goggles when anyone in the lab uses solvents or chemicals
• Dispose of solvents in the labeled beaker in hood 1
• Dispose of naphthalene in the labeled beaker in hood 1. This may require an acetone rinse. Do not rinse into the sink. It is volatile, and water insoluble.
• Return your disassembled model kit to the back shelf.

Background

Lewis Structures

It would be hard to overstate Gilbert Lewis’ contributions to modern chemistry. His prolific innovations span work in acid base theory, thermodynamics, covalent bond theory and the behavior of electrons in the covalent bond. Lewis Structures bear his name. Lewis also coined the word ‘photon’ as ‘light quanta’ in a 1926 letter to Nature. Lewis mentored countless PhD students including twenty Nobel laureates, many of which collaborated with Lewis. The ‘greatest scientist to never win the Nobel prize ‘, Lewis was nominated for the Nobel award 41 times, but evidence exists that his nominations were blocked for political reasons stemming from early criticisms of his graduate mentor while working in Germany (Kean, 2011). Sadly, Lewis was found dead in his Berkley laboratory after working with hydrogen cyanide. The circumstances behind his death are unclear.

Today you will be creating structures named after Lewis, as well as VSERP theory to predict bond angle and hybridization. A Lewis Structure is a representation of covalent molecules (or polyatomic ions) where all the valence electrons are shown distributed about the bonded atoms as either shared electron pairs (bond pairs) or unshared electron pairs (lone pairs). A shared pair of electrons is represented as a short line (a single bond). Sometimes atoms can share two pairs of electrons, represented by two short lines (a double bond). Atoms can even share three pairs of electrons, represented by three short lines (a triple bond). Pairs of dots are used to represent lone pair electrons. Examples are shown for the molecules SF2 and CH2O below.

Please review (in your text or notes) the rules for drawing Lewis structures before performing this exercise. This includes rules for structures which obey the octet rule as well as those which involve expanded or reduced octets and how to calculate and optimize formal charge.

Resonance Structures

Resonance refers to bonding in molecules or ions that cannot be correctly represented by a single Lewis structure. These structures are often equivalent, meaning that they contain the same number of bonds at different locations. The molecule SO2 (shown above) has two such resonance forms. Resonance structures can also be non-equivalent, in which case they will have different numbers and/or locations of bonds. Note that any valid resonance structure of a molecule can be used to determine its shape and polarity.

VSEPR Theory

The VSEPR (Valence Shell Electron Pair Repulsion) model is used to predict the geometry of molecules based on the number of effective electron pairs around a central atom. The main postulate for the VSEPR theory is that the geometrical structure around a given atom is principally determined by minimizing the repulsion between effective electron pairs. Both the molecular geometry and the polarity of individual bonds then determine whether the molecule is polar or not.

Before determining the shape of a molecule, the Lewis structure must be properly drawn. The shape of a molecule is then determined by the number of areas of electron density (or, number of effective electron pairs) around a central atom. Areas of electrons density include:

• Lone pairs of electrons: these electrons tend to take more space than the bonded pairs in space leading into somewhat distorted structures.
• Bonds (single, double, and triple bonds count as one (1) area of electron density or one effective electron pair).

Before performing this exercise, please review (in your text or notes) the various geometries and bond angles that can be produced by different numbers of effective electron pairs around the central atom.

Molecular Polarity

A polar bond is one in which the electron cloud is closer to the nucleus of one atom (the more electronegative one) than the other (the less electronegative one). Knowledge of both the bond polarities and the shape are required in the determination of the molecule’s overall polarity (dipole moment). A polar molecule is one that shows an imbalance in its electron distribution. When placed in an electric field, these molecules tend to align themselves with the electric field.
Some molecules have polar bonds but no dipole moment. This happens when the bonds in a molecule are arranged in a way in which polarities cancel each other. One way this occurs is when molecules have all identical bonds and there is no lone pair on the central atom (for example, CO2). Molecules that do not fit both of these criteria may be polar or not depending on how atoms are bonded, and the electron pairs arranged around the central atom (for example, XeCl2F2 shown below).

Take care to distinguish ionic compounds from polar covalent compounds. None of the Lewis structures that you created today depicts an ionic compound. Ionic compounds require a-lot of energy to free ions from their crystalline structure and have both high melting and boiling points. NaCl, for example, melts at just over 800 ºC. Melting of polar covalent structure such as sucrose, on the other hand, is more nuanced. Sugar is found in observable crystals as well. Initial heating will separate the glucose and fructose molecules, an apparent melting called an inversion. Further heating will remove water, leaving nothing but carbon. If this is controlled, caramelization occurs. If the process continues, only carbon remains, and you are left with burned sugar. In contrast to melting an ionic compound, inverting, or even burning sugar occurs at temperatures easily reached in a home kitchen. The melting point of sucrose, for example, is listed at 186 ºCelsius.

Hybridized Central Atom Bonding

Bond hybridization is a complex theory that can be summarized for the central atom by counting the number of electron domains about the central atom. An electron domain is defined as an area of electron density including a lone pair of electrons or a single, double, or triple bond. The following table will help you assign central atom bond hybridization to your molecules.

Procedure

Complete the following for each of the molecules and ions on your Report form:

1. Draw Lewis structures, including all resonance structures if applicable (1).

2. Build models and then draw perspective structures (2) that accurately represent bond angles and molecular shapes.

The molecular model kits contain different colored balls and different size stick connectors. Three-dimensional models will be constructed from these balls and sticks.

The stick connectors represent bonds. Use the short rigid sticks for single bonds. The long flexible sticks must be used to create double (2 sticks) and triple (3 sticks) bonds.

The different colored balls represent different atoms. There is a key on the inside of the lid of the model kit which indicates which colors correspond to which atoms. There are, however, some exceptions to this. For example, the kit indicates that the green balls with just one hole are to be used for the halogens. But if a halogen (such as Cl) appears in a molecule as a central atom with an expanded octet, you would need to use a ball that has 5 or 6 holes to build the model (brown or silver ball). Another example is oxygen. The kit indicates that the red balls with two holes should be used for oxygen. However, if an oxygen atom in a compound requires more than two bonds, the red balls cannot be used. In this case you would substitute a blue ball for oxygen.

The 3-D models will serve as a visual guide to help you with your perspective structures. Use the following guidelines to draw them correctly:

• For bonds lying in the plane of the paper, use a regular solid line (—-).
• For bonds that project down into the paper away from you, use a hatched wedge-shaped

• For bonds that project up out of the paper towards you, use a solid wedge-shaped line

(         )

An example showing both the Lewis structure and perspective representation of CH4 is provided below.

3. Determine the number of atoms bonded to the central atom (or, number of σ-bonds) (3).
4. Determine the number of lone electron pairs on the central atom (4).
5. Predict the electronic geometry using all areas of electron density (or, effective electron pairs) and the ideal bond angles associated with this geometry (5).
6. Predict the actual geometry of the molecule or ion (6).
7. Determine the hybridization of the central atom (7).

8. Determine the polarity of the molecule (8). Use an arrow to show the direction of electron density for polar molecules on the perspective drawing.

Please be sure to keep the model kits discrete and complete for the next group.

Part 2: Polarity of Solvents

1. Create the following solvent mixtures in a test tube with one pipette full of each solvent. You may invert a test tube if covered by a gloved thumb, parafilm, or a small stopper.

2. Test the following solids in each solvent by adding a small amount of solid to one pipette full of each solvent. Note: adding too much solid will overwhelm the solvent, giving a false negative for solubility. Unknowns #1 and #2 are selected from the given list of solutes.
3. If you are still unsure of the identities of unknowns #1 and #2, devise another test based on the information you read in the lab introduction. Be sure to use proper safety techniques such as the fume hood and clear your idea with your lab instructor. Be prepared to clean any mess that occurs in your glassware. You may consider testing what does not happen, rather than what does.

Additional Tests:

Results:

Lab Data:

You may hand draw and fill in the appropriate structures and data for submission. See report requirements for details.

Table 2: Electron Domain Geometries

HCN

CH3OH (carbon as center)

CH3OH (oxygen as center)

SeF6

NO2–

AsF5

TeF5–

XeF2

H2CO

SF4

PO43-

XeF4

BrF3

NH3

CH3NH2 (carbon as center)

CH3NH2 (nitrogen as center)

SBr2

Molecule (Choose any molecule that interests you):

Find each molecule that displays resonance; include the structures here:

Post Lab:

1. List the formula for each molecule that has lone pairs on the central atom and is non-polar. Explain why these molecules are non-polar.

2. Ionic compounds have extremely high melting and boiling points. NaCl, for example, melts at just over 800 º Celsius. It takes a-lot of energy to free the ions from a rigid crystalline structure. In light of this fact, why does it appear so effortless for a polar solvent such as water to dissolve or separate an ionic compound?

3. Write a short paragraph contrasting the importance of earning a prize, formal recognition, getting published, etc. with advancing the body of scientific or other knowledge and increasing in personal learning.

4. Connect the shape or geometry of a molecule or ion to its function in paragraph form. Pharmaceuticals, metal ion ligands, hormone receptors, and piezoelectric detectors are some options to begin your search. *In order to help you practice journal searches and citation for the upcoming formal report, your citation must be journal sourced and cited (ACS (American Chemical Society) preferred, APA accepted). The CWI library is a great starting point; https://cwi.edu/current-students/library

Please see the Journalistic Format Reference Document in you BB shell for parameters.

Lab Report:

You will complete your lab report differently than in prior labs.

1. Complete the pre-lab and take the pre-lab quiz in Blackboard before attending lab.
2. Print or reproduce and complete the data tables for work during and after lab. You must build the model associated with each compound. You may hand draw Lewis structures and perspective diagrams, and may label the remaining data requirements by hand, including polarity, # lone pairs, etc., as well as the summary data If the molecule appears to have no central atom, assign carbon as the central atom. One section of data is marked Molecule. Chose a molecule or geometry that interest you. It must be different from your lab partner’s molecules.
3. Include a brief printed document with answers to any post lab questions. Include data and observations for the wet portion of the lab including solubility and other observations organized into a table. You will turn in all your printed materials; your printed lab with drawings and post lab document stapled together at the beginning of your next lab appointment.

Sources:

Adapted from: Chem 11 Experiments, Santa Monica College http://www.smc.edu/AcademicPrograms/PhysicalSciences/Chemistry/Lab-Manual/Pages/Chem-11-Experiments.aspx (accessed Jun 25, 2020).

Lewis, G. The Conservation of Photons. Nature 118, 874–875 (1926). https://doi.org/10.1038/118874a0

Kean, S. (2011). The Disappearing Spoon. Boston, MA: Back Bay Books.

“The Posthumous Nobel Prize in Chemistry. Volume 1. Correcting the Errors and Oversights of the Nobel Prize Committee”. doi:10.1021/bk-2017-1262.ch006

The Science of Melting Sugar. https://www.finedininglovers.com/article/science-melting-sugar (accessed Jun 27, 2020).