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BONDING AND MOLECULAR GEOMETRIES post lab questions

BONDING AND MOLECULAR GEOMETRIES REMOTE INSTRUCTION ACTIVITY

In today’s laboratory activity, you will be using an online simulation to investigate the structure of several molecules. The shapes of molecules are predicted by VSEPR (valence shell electron pair repulsion) theory, which states that electron groups are positioned as far from one another as possible in a molecule. The electron groups considered by the VSEPR theory can be either in the form of lone pairs of electrons, or shared electrons in a chemical bond. The following shapes are predicted by VSEPR:

Two electron groups

Two bonding pairs: linear

Three electron groups

Three bonding pairs: trigonal planar

Two bonding pairs, one lone pair: bent

Four electron groups

Four bonding pairs: tetrahedral

Three bonding pairs, one lone pair: trigonal pyramidal

Two bonding pairs, two lone pairs: bent

Five electron groups

Five bonding pairs: trigonal bipyramidal

Four bonding pairs, one lone pair: see-saw

Three bonding pairs, two lone pairs: t-shape

Two bonding pairs, three lone pairs: linear

Six electron groups

Six bonding pairs: octahedral

Five bonding pairs, one lone pair: square pyramidal

Four bonding pairs, two lone pairs: square planar

Valence bond theory states that when electron groups are positioned around an atom, they are placed in hybridized atomic orbitals. That is, rather than filling in s, p, d, or f orbitals, they occupy orbitals that are combinations of those simpler atomic orbitals. The number of atomic orbitals used in hybridization is equal to the number of electron groups around the atom. For example, in ammonia, the central nitrogen atom is bonded to three hydrogen atoms and has a single lone pair of electrons. According to VSEPR theory, the shape of ammonia is trigonal pyramidal, and according to valence bond theory, the nitrogen is sp³ hybridized since it uses its 2s and all three 2p orbitals in bonding.

The PhET Molecule Shapes simulation provides three-dimensional models of several small molecules as well as a set of generic models that illustrate the possible geometries of the theory. Each model can be moved and manipulated. In Part I of the worksheet, you will use the simulation together with your understanding of Lewis structures to complete the following entries. In Part II of the experiment, you will use Lewis theory to evaluate the shapes of larger molecules.

Part I: Small molecules

Use the PhET Molecule Shapes simulation to answer the following questions. You can find it at https://phet.colorado.edu/sims/html/molecule-shapes/latest/molecule-shapes_en.html.

NOTE: Not all of the following molecules are included in the PhET Molecule Shapes simulation. If the molecule is not in the simulation, use your knowledge of Lewis, VSEPR, and hybridization theories to help you answer the questions.

Find two molecules from the worksheet above that are bent, but with different hybridizations:

____________________________________

Do the two bent molecules have the same bond angle? __________________

Why? ______________________________________________________________________________

___________________________________________________________________________________

Compare H₂O, NH₃, and CH₂Cl₂. Which has the smallest bond angle? __________________________

Why? ______________________________________________________________________________

___________________________________________________________________________________

Part II: Larger molecules

Answer the following questions about the geometry of larger molecules. You do not need the PhET simulations.

Alanine is one of the simplest and most common amino acids. The structure of alanine is:

structural formula                                                ball-and-stick model

1. Label each internal atom in the structure above with its hybridization and
2. How many sigma bonds and how many pi bonds are present in alanine?

______________s, ______________ p

Cyclohexane, C₆H₁₂, and benzene, C₆H₆, are both molecules containing six carbon atoms arranged in a ring. Draw the Lewis structures of the two molecules:

3. What are the hybridizations of the carbon atoms in each of the molecules?

Cyclohexane:      ______________          Benzene:              ______________

4. What are the C-C-C bond angles in each of the molecules?

Cyclohexane:      ______________          Benzene:              ______________

5. Which of these molecules is likely to be entirely in one plane? ______________
6. How many sigma bonds and how many pi bonds are in each molecule?

Benzene:             ______________s, ______________ p

Cyclohexane:     ______________s, ______________ p