Lewis Structures and
Molecular Geometry
Constructed Response
Lewis Structures and Molecular Geometry
Teacher Guide
Students will need about 30 minutes to complete these constructed response tasks.
Objectives assessed:
• Predict the type of bond between atoms of a molecule from the electronegativities of
those atoms.
• Use Lewis structures to represent covalent molecules and polyatomic ions.
• Use Lewis structures and VSEPR to predict the polarity, geometry, and bond angles of
covalent molecules.
1. Electronegativities of elements, electron configurations, Lewis valence electron dot
structures, and VSEPR theory can be used to predict the polarity of bonds and to predict
the polarity and shape of molecules.
• Consider the compound phosphorus trichloride (PCl3). The electronegativities of
phosphorus and chlorine are 2.1 and 3.0, respectively. Use these values to
determine whether the bonds in PCl3 are nonpolar, polar covalent, or ionic. Show
your work and explain your answer.
• Express the arrangement of electrons in atoms by creating an electron
configuration for each type of atom in PCl3.
• Express the arrangement of electrons in atoms by creating the Lewis valence
electron dot structure of PCl3. Show all valence electrons.
• Use VSEPR theory to predict the angles between the P-Cl bonds in PCl3. Explain
your answers.
• Describe the shape of the PCl3 molecule and explain your answer.
• Predict whether the PCl3 molecule will be polar or nonpolar, and explain why.
Correct response(s):
Sample Correct Response: P-Cl bonds are polar covalent because 3.0 – 2.1 = 0.9, which
is greater than 0.4 and less than 1.7. The electron configuration for each atom is:
The Lewis valence electron dot structure is:
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Lewis Structures and Molecular Geometry
PCl3 bonds are 109.5° apart because bonding and non-bonding electrons all repel each
other toward the corners of a tetrahedron. The PCl3 molecule is trigonal pyramidal because
only the three P-Cl bonds, not the unbonded electron pair, determine the shape of the
molecule. PCl3 is polar because the more electronegative chlorine atoms are shifted to one
side of the center of the molecule.
Response scoring tool:
Score
Content
5
Response does all of the following:
• demonstrates that P-Cl bonds are polar covalent
• creates correct electron configurations for P and Cl
• creates correct Lewis valence electron dot structure for PCl3
• predicts bond angle between P-Cl bonds
• predicts the molecular shape of PCl3
• cites electronegativity and molecular geometry to explain why the
molecule will be polar
4
Response applies electronegativity difference, Lewis valence electron
dot structures, and/or VSEPR theory to correctly complete four of the five
tasks.
3
Response applies electronegativity difference, Lewis valence electron
dot structures, and/or VSEPR theory to correctly complete three of the
five tasks.
2
Response applies electronegativity difference, Lewis valence electron
dot structures, and/or VSEPR theory to correctly complete two of the five
tasks.
1
Response applies electronegativity difference, Lewis valence electron
dot structures, and/or VSEPR theory to correctly complete one of the five
tasks.
0
No response or response not appropriate to the question.
Objectives assessed:
• Explain the intermolecular forces between polar molecules and their relative strengths.
• Use Lewis structures and VSEPR to explain the unique properties of water.
2. Three main types of intermolecular forces exist between molecules. Intermolecular
forces, Lewis structures, and VSEPR theory explain the unique properties of water.
• Identify three types of intermolecular forces and explain the cause of each.
• Rank the three forces by strength, from weakest to strongest.
• Identify the intermolecular force that is responsible for the high surface tension of
water (H2O). Create Lewis structures for two water molecules to show the
intermolecular force between them. Show this force with a dashed line.
• Relate the structure of the water molecule to the following two properties: boiling
point; and the ability of water to act as a solvent for ionic compounds.
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Lewis Structures and Molecular Geometry
•
Explain why the properties of water are important for living organisms. Give at
least three reasons.
Correct response(s):
Sample Correct Response: Dipole-dipole forces are caused by attractions between the
negative ends of polar molecules and the positive ends of other polar molecules. Hydrogen
bonding is caused by the attraction of hydrogen atoms in a molecule to highly
electronegative atoms in other molecules. London dispersion forces are caused by attraction
between positive and negative ends of temporary, instantaneous dipoles caused by the
constant motion of electrons.
London dispersion forces are weaker than dipole-dipole forces, which are weaker than
hydrogen bonds. Water has high surface tension because of the hydrogen bonding that
occurs between two water molecules.
Because water is a polar molecule and forms relatively strong hydrogen bonds, its boiling
point is higher than it would otherwise be, since the water molecules are more likely to
stick together than to separate and move apart into gaseous form.
In addition, because water is a polar molecule, it acts as an excellent solvent for ionic
compounds. The negative dipole on a water molecule helps to stabilize cations, while the
positive dipole helps to stabilize anions.
The properties of water are important for living organisms because many organisms are
primarily composed of water. Water fills the cells in living organisms, and cellular reactions
take place in water. In addition, water is an essential habitat for many living organisms.
Water drives weather systems, which affect living organisms on a daily basis.
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Lewis Structures and Molecular Geometry
Response scoring tool:
Score
Content
3
Response does all of the following:
• identifies and describes the cause of three intermolecular forces
• ranks the three forces by strength
• identifies hydrogen bonding as the force responsible for surface
tension, and correctly uses Lewis structures to show hydrogen
bonding
• relates the polarity of the water molecule to its boiling point and
ability as a solvent for ionic compounds
• gives three reasons why water is important for living organisms
2
Correctly answers two out of the following three: identifies and describes
the cause of three intermolecular forces and ranks the three forces OR
identifies hydrogen bonding as the force responsible for surface tension
and correctly uses Lewis structures to show hydrogen bonding OR
relates the structure of the water molecule to its properties and explains
why water is important to living organisms
1
Identifies and describes the cause of three intermolecular forces, and
makes an attempt to explain some of the properties or importance of
water.
0
No response or response not appropriate to the question
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