AP C HEMISTRY E XAM Q UESTION
P ROFILER
VSEPR Models

AP E XAMS FROM 1999 - 2011

There were eleven questions that directly involved VSEPR, Lewis structures, and polarity.
Many of these questions also incorporated other topics.

There was an average of 3.7 subparts related to the above topics per question.

There was a total of 41 subparts that included
VSPER.

B REAKDOWN OF Q UESTIONS RELATED TO
VSEPR
Exam Year Other Topics Included
2002B
2004
2005
2006
2006B
2007
2007B
2008
2009B
2010
2011
Bonding
Gas Law
--
Oxidation numbers
--
Factors affecting reactions, including kinetic and thermodynamic factors
Acid-base buffer
Ionization energy
Thermodynamics
Intermolecular Forces
Inter- versus intra-molecular forces, thermodynamics
Number of Subparts
Related to VSEPR
5
2
7
7
5
4
1
5
1
4
2

E NTRY P OINTS ON F REE R ESPONSE QUESTIONS

The questions are designed to have an “entry point” for the student.

An entry point is a relatively easy question
(usually worth one point).

It is designed to give the students a feeling of “I can do this problem”

Build confidence in the student so they attempt to do the problem.

E NTRY P OINT FOR Q UESTIONS WITH VSEPR

In each of the questions that involved VSEPR, drawing a Lewis dot diagram and/or identifying the shape of a species is the entry point to a question.

This means: This is a topic you should feel comfortable with and be able to do with ease.

Year
Possible
Points for
VSEPR subparts
Performance
2011 2
The mean score was 3.04 out of 8 possible points, with most of the points earned by completing the Lewis diagram and identifying the arrangement of atoms or citing the electron-pair repulsion.
2010 6
The mean score was 4.09 out of 8 possible points. The distribution was bimodal with one maximum between 5 and 6 and the other at zero. Most students earned points on the Lewis diagrams.

Year
Possible Points for VSEPR subparts
Performance
2008
2005
5
9
The mean score was 3.92 out of 9 possible points, with scores of 4 and 5 both being modal. This question had a wide bell-shaped distribution of scores.
Most students drew correct Lewis diagrams.
Performance varied when students attempted to use these diagrams to reason out the shape, the central atom hybridization, and then molecular polarity.
Almost all students attempted this required question. The mean score was 4.97 out of a possible 9 points.

C OMMON M ISTAKES M ADE B Y S TUDENTS

Students did not draw diagrams in the boxes provided, and sometime they drew multiple diagrams without indicating which was their final answer.

Forgetting to answer questions completely.

Miscounting valence electrons

They showed all electrons, not just valence electrons

C OMMON M ISTAKES M ADE B Y S TUDENTS

Students sometimes completed an octet for hydrogen.

Placing unbonded electron pairs in the wrong position

Students gave an overemphasis on the octet rule and “happy” electrons.

Occasionally structures included lone electrons

C OMMON M ISTAKES M ADE B Y S TUDENTS

Unable to identify a triple bond as being stronger than a single bond

Misunderstanding the term “resonance.”

Many students did not understand the term “hybridization of the S atom.” Some confused this with atomic structure or a statement of the geometry

C OMMON M ISTAKES M ADE B Y S TUDENTS

Incomplete justification given for a molecular geometry. A statement that lone electron pairs are present is a fact and does not provide an explanation for the geometry of the molecule. Describing the two-dimensional image you have drawn on the paper,
as above or below one another is not sufficient justification.

A NALYSIS OF
F REE R ESPONSE Q UESTION
2006

This question was worth
15% of the Free
Response section, or
7.5% of the entire exam.
A choice of two questions was given on this exam. The other question tested the student on periodic trends and ions.
There is no longer any choice on the exam – you MUST do ALL problems.

Recall that halogens only form single bonds – not part of CNOPS!
Notice that both stems in this question are worded in the same manner.
Similarly, parts i, ii, and iii ask for the same things in both questions, but
(a) examines a cation and (b) an anion.

You must justify your answer in order to earn credit for this portion.

C HIEF R EADER ’ S C OMMENTS
2006

C HIEF R EADER ’ S C OMMENTS
Key words:
• Lewis Structures
• Hybridization
• Geometry
• Effect of nonbonding pairs of electrons
• Bond angles
• Expanded valence shell hybridization and geometry
• Oxidation number on element in an ionic compound

C HIEF R EADER ’ S C OMMENTS
• This is a problem where a well-prepared student can earn full points.
• Make sure you take full advantage of problems, like this one, that focus on a basic concept and information.

C HIEF R EADER ’ S C OMMENTS

C HIEF R EADER ’ S T IPS F OR S TUDENTS

Learn how to count electrons for charged species.

Are consistent in the representation of electron pairs – use either dotes or dashes so they do not represent more electrons than the number of valance electrons present in the Lewis structures drawn.

Learn hybridization and the corresponding shapes.

C HIEF R EADER ’ S T IPS F OR S TUDENTS

Learn the correct names of the shapes for expanded valence geometries.

Learn the difference between bonding and nonbonding pairs.

Use the correct vocabulary for unshared pairs
(not referring to them as “unpaired electrons,”
“lone electrons,” “extra pairs”)

C HIEF R EADER ’ S T IPS F OR S TUDENTS

Represent the bonding and not the complementary angle for a structure (often there was not a clear idea as to what F—S—F angle represented).

Use models so that they can see the shapes of species (either hands-on models or computer models).

Understand hybridization in general terms (e.g., students should recognize that sp 4 hybrids do not exist).

C HIEF R EADER ’ S T IPS F OR S TUDENTS

Know the effect of nonbonding electrons on the shape of a species.

Know that the geometry of a species represents the shape of the species that one would see if the species could be magnified. The use of threedimensional models in teaching is helpful for students to understand the geometric shapes of species.

S CORING S TANDARD
2006

S CORING S TANDARDS
Notice that it is okay to use dots or dashes, but
BE CONSISTENT!
Be sure to add the charge

S CORING S TANDARDS
The Readers prefer the lines for a simple reason – they are easier to see!

These do not need to be answered in full sentences.
An exact angle does not need to be given.
The question does not ask for one.

If you have drawn the
Lewis structure incorrectly in part (i), you can still get credit for part (iii).
This is also true for part a(iii).

H OW TO S UCCEED WITH T HESE
T YPES OF P ROBLEMS
Tips from the Chief Reader

I N G ENERAL :

Read the question carefully and answer the question asked. Begin by writing the answer and then providing supporting evidence.

When responding to the question of whether a statement is true or false, begin by writing down one of these choices and then provide supporting evidence. Do not begin the answer with the word
“yes.”

Be sure to address each substance within a given question stem. When asked to compare two substances, always talk about both. Simply discussing one and expecting the reader to assume that the other is implied will not receive credit.

I N G ENERAL :

Often information is used for multiple parts of a question; be sure to refer to information that is given.

Avoid vague generalizations when answering questions. Give details as often as possible.

Write legibly and be sure to write all answers in the lined spaces provided instead of squeezing words between question parts.

Be sure to label each response; this is especially true when choosing to answer the questions out of order.

D RAWING L EWIS S TRUCTURES :

Take time to calculate the total number of electrons before beginning.

Students should clearly represent the correct number of electrons in a Lewis diagram with easy-to-discern dots or dashes (to represent electron pairs).

Know that molecules are three-dimensional and that bond angles should reflect this

S HAPES AND A NGLES

Know the difference between electronic and molecular structure.

Appreciate the difference between the doubleheaded resonance arrow, which implies that neither individual Lewis diagram is correct but that the actual structure is between the two presented structures, and the two arrows that are presented together pointed in opposite directions which represent a dynamic process, with bonds rapidly oscillating between two positions.

O THER T IPS :

Limit the use of the “octet rule” and avoid discussing “happy” molecules, ions, and electrons

Practice determining hybridization and number of sigma and pi bonds in molecules

O THER T IPS :

Students need to practice the reasoning sequence employed in understanding molecular structure: complete a Lewis electron-dot diagram, use this to determine the electron-pair orientation and thus the molecular shape, and then use the shape to find the central atom hybridization and molecular polarity. Symmetry plays a major role in determining polarity.

O THER T IPS :
“People like to open books”

Shapes that can be symmetrical and therefore non-polar:



Planar
Linear
Tetrahedral


Octahedral
Bipyramidal.