Experiment 15 EXPERIMENT 15: Lewis Structures and Molecular Shapes Purpose: To gain experience drawing molecular structures and evaluating 3-D shape.

Experiment 15 EXPERIMENT 15: Lewis Structures and Molecular Shapes

Purpose: To gain experience drawing molecular structures and evaluating 3-D shape.

Objectives:

1) To practice drawing Lewis Structures for various covalently bonded molecules and polyatomic ions.
2) To use model kits to construct these molecules/ions in order to explore their structure and shapes.
3) To practice predicting molecular shapes (using VSEPR theory) and molecular polarity.

BACKGROUND INFORMATION

Non-metal atoms bond covalently, resulting in the formation of either neutral molecules or polyatomic ions. A covalent bond is formed when non-metal atoms share their valence electrons, which they do in order to achieve filled valence orbitals like their nearest noble gas neighbor. This means that most bonded non-metal atoms will acquire a total of eight valence electrons via the sharing process – often referred to as the octet rule. A notable exception is hydrogen, which only needs to acquire two electrons to be like its nearest noble gas neighbor, helium.

Lewis Structures

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 (non-bonded) electrons.

Examples:

The rules for drawing Lewis structures can be found in the Procedure Section of this handout.

Molecular Shapes

The shape of a molecule depends on the distribution of atoms in space about the central atom, and their bond angles. Bond pair electrons and lone pair electrons repel one another, thus they will be arranged around a central atom as far apart as possible in order to minimize repulsions. This is known as:

V
alence
S
hell E
lectron P
air R
epulsion theory, or VSEPR theory (or electron geometry).

The following VSEPR table supplies the names, sketches and descriptions of the most common types of molecular shapes that you will encounter. Note that several other molecular geometries do exist, however, they are beyond the scope of this course.

Electronegativity and Bond Polarity

Some atoms in molecules have the ability to pull shared electrons closer to themselves than other atoms, an ability referred to as electronegativity. Electronegativity (c) increases going across a period and decreases going down a group.

If two bonded atoms have different electronegativities, then the bond pair electrons will be unequally shared. The atom with the greater electronegativity will pull the bond electrons closer towards itself, causing it to obtain a very slight negative charge (d-). The atom with the lower electronegativity will have bond electrons pulled further away from it, causing it to obtain a very slight positive charge (d+). The result is a polar covalent bond. However, if two bonded atoms have the same electronegativity, then the bond pair electrons will be equally shared. The result is a non-polar covalent bond.

Example:

is better represented asbecause the bond electrons are pulled closer to Cl due to its greater electronegativity. Thus, HCl contains a polar covalent bond.

Molecular Polarity

Molecular polarity results when the entire molecule (not just a bond in the molecule) ends up with an unequal distribution of electrons. In general, a molecule will be polar if it contains polar bonds that are distributed in a non-symmetrical arrangement around the central atom. A polar molecule is said to have a net dipole moment. A non-symmetrical arrangement typically results when there are lone pairs on the central atom, or, when different outer atoms surround the central atom.

Not surprisingly, a molecule will be non-polar if it contains all non-polar bonds. It will also be non-polar if it contains polar bonds distributed in a symmetrical arrangement around the central atom. The symmetry causes the individual bond polarities to cancel out, resulting in a net non-polar molecule. A symmetrical arrangement typically results when there are no lone pairs on the central atom, and if all the outer atoms are identical.

Examples:

Non-Polar

Polar

Polar

Procedure

Drawing Lewis Structures

1. Place the atoms relative to each other. The atom with the lower group number usually goes in the center (except for hydrogen).
2. Determine the total number of valence electrons available.
3. Draw a single bond from each surrounding atom to the central atom, and subtract two valence electrons for each bond.
4. Distribute the remaining electrons in pairs (outer atoms 1st) so that each atom obtains eight electrons (or two for H).
5. If after step 4, the central atom still does not have an octet, change the lone pair from one of the surrounding atoms into a bonding pair to the central atom to make a multiple bond.

Constructing Models, Determining Molecular Shapes and Molecular Polarity

1. Use your molecular model kit to construct a three-dimensional model of each of these molecules and polyatomic ions. Sketch a reasonably detailed picture of this model on your Report Form.

Steps for Constructing Molecules with the Model Kit

1. Each colored ball in your kit corresponds to different atom or different group of atoms. Refer to the inside of the container lid for the correct color key.
2. Use the short sticks for single bonds.
3. Use
two long flexible sticks for a double bond and
three long flexible sticks for a triple bond.

2. Compare each model constructed to the molecular shapes described in the Theory section. Then identify and record its correct shape name and its bond angles. If the molecule has no definite central atom, then only record the bond angles.
3. Evaluate the polarity of the bonds in each molecule as well as its overall symmetry in order to determine whether it is polar or non-polar.

Lewis Structures and Molecular Shapes

1. H2S

Total # of Valence Electrons: 8

3-D Model Sketch

Bond Angles

Lewis Structure

104.5 degrees

Polar Bonds: Yes No

Molecular Shape: Bent

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

2. COCl2

Total # of Valence Electrons: 24

3-D Model Sketch

Bond Angles

Lewis Structure

120 degrees

Polar Bonds: Yes No

Molecular Shape: trigonal planar

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

3. SiBr4

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

4. HCN

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

5. NO2-1

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

6. H3O+1 (use the yellow ball for oxygen)

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

7. CHI3

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

8. OF2

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

9. CO3-2

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

10. CS2

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

11. NF3

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

12. NOCl

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

13. Instructor’s choice

Total # of Valence Electrons:

3-D Model Sketch

Bond Angles

Lewis Structure

Polar Bonds: Yes No

Molecular Shape:

Resonance: Yes No

Molecular Polarity: Polar Non-Polar

F
C
F
F
F
C
H
H
H
F
N
F
F
F
H
C
l
H
d
d
+
_
C
l