Covalent Bonding And Molecular Geometr Essay Research

Covalent Bonding And Molecular Geometr Essay, Research Paper

Covalent Bonding and Molecular Geometry

Objective

The objective of this exercise is to help in understanding the geometric relationships of atoms in simple molecules and the relationship of hybridization to the geometry present.

Discussion

In the last 30 years, data obtained from spectrometric measurements, X﷓ray and electron diffraction studies, and other experiments have yielded precise information about bond distances, angles, and energies. In many cases, the data confirmed conclusions reached earlier. In other cases, valuable new insights were acquired. Structure theory has advanced far beyond the simple electron dot representations and now rests securely on the foundations of quantum and wave mechanics. Although problems involving only simple molecules can now be solved with mathematical rigor, approximations such as the valence bond theory and the molecular orbital theory are very successful in giving results that agree with experimental measurements.

This exercise will use valence bond theory or hybridization to look at the geometry formed from various hybridizations. You will use a framework model kit which gives the correct angles for the each of these hybridizations.

The first bond formed between any two atoms is always a sigma (s)﷓bond (one that is symmetric about the bond axis). Additional bonds between the same two atoms will be pi (p)﷓bonds (perpendicular to the bond axis). It is the sigma﷓bonds and any lone﷓pairs of electrons occupying the sigma hybrid orbitals that determine the geometry of a molecule. Pi﷓bonds are always perpendicular to the sigma﷓bonds and follow the geometry formed by the sigma﷓bonding.

Procedure

Check out a molecular model kit from the stockroom. Read the kit directions to see which framework center is used for each hybridization.

Tetrahedral (sp3 hybridization)

CH4

Construct a model of methane using a tetrahedral center (4 prongs) and four rods of the same color to show how the 4 H’s are attached.

Geometry Lewis dot diagram # of s bonds on C Approximate H-C-H angel Max # atoms (incl. C) in one plane Is there a mirror plane(divides the molecule in equal halves) .

H3C﷓CH3

Construct a model of ethane using a tetrahedral center for each C and the same color rods for all 6 H’s with a C﷓C bond present.

Geometry Lewis dot diagram # of s bonds on each C Approximate H-C-H angle Approximate H-C-H angle

The C-C bond is a single bond and has free rotation about it. Arrange the ethane molecule so that each C﷓H bond on one C atom is exactly parallel to a C﷓H bond on the second C atom. (This is the eclipsed position.) View this arrangement by looking along the C﷓C bond such that the atoms on the front C blank out those on the back C. From this view the model is represented as in I ( a Newman projection.)

Now rotate the front carbon 60 degrees clockwise producing II.

Which of the above arrangements allows more space per atom.

Which arrangement would you expect to be more stable.

CIH2C﷓CH2Cl

Make a model of 1,2﷓dichloroethane by replacing an H atom with a C1 atom on each C in your ethane model. Use a different color rod to show the C1 atoms. The eclipsed Newman projection is given in 111. Rotate the front C by 60 degrees and show the result on IV, rotate another 60 degrees and show the result on V, etc., until you have completed a 360 degree rotation about the front C. Remember that a C1 atom is much larger than an H atom

Which of the structures would you expect to be the most stable. Explain why.

Which of the structures would you expect to be the least stable. Explain why.

NH3,

Use a tetrahedral center for the N atom and three rods of the same color for the 3 H’s to make a model of ammonia.

Geometry Lewis dot diagram # of s bonds on N # on non-bonding pairs on N Approximate H-N-H angle

H2O

Use a tetrahedral center for the O atom and two rods of the same color for the 2 H’s to make a model of water.

Sketch the geometry Draw a dot diagram # of s bonds on O # on non-bonding pairs on O Approximate H-O-H angle

Trigonal Planar (sp2 hybridization)

BF3

Use a trigonal planar center (has 3 or 5 prongs depending on the model kit) and three rods of the same color for the three F’s to make a model of boron trifluoride. (If you have a model kit without a 3 prong center, use the 5 prong center and put the three Fs on the 3 equivalent positions.)

Geometry Lewis dot diagram # of s bonds on B # atoms in one plane Approximate F-B-F angle

NO31-

The BF3 model is the same model needed for the nitrate ion.

Geometry Lewis dot diagram Total # of bonds on N_________# of s bonds on N_________# of p bonds on N_________ If Px & Py are used for SP2 hybridization what is the function of Pz . Location of p bond compared to plane of the molecule__________Approximate O-N-O angle________

H2C=CH2

Use sp2 centers for both C atoms and four rods of the same color for the four H’s to make a model of ethene, which has a C=C bond. This is a planar molecule. If your

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