Predicting Molecular Characteristics Using VSEPR Theory (10.1.4) Flashcards
• The shape of a molecule (molecular geometry) can be determined using Lewis
dot structures and valence shell electron pair repulsion theory (VSEPR).
• The shape of a molecule (molecular geometry) can be determined using Lewis
dot structures and valence shell electron pair repulsion theory (VSEPR).
• The net dipole moment (polarity) of a molecule can be determined by summing
the bond dipole moment vectors.
• The net dipole moment (polarity) of a molecule can be determined by summing
the bond dipole moment vectors.
The shape of a molecule (molecular geometry) can
be determined using Lewis dot structures and
valence shell electron pair repulsion theory
(VSEPR).
To determine the shape of the ClF2
+ cation, the
central atom must first be assigned. This
assignment can be made by drawing Lewis dot
structures for each arrangement and assigning
formal charges. Because chlorine is less
electronegative than fluorine, it can better tolerate
a +1 formal charge. Therefore, chlorine is more
likely to be the central atom.
Next, the steric number (SN) must be assigned.
Chlorine has two lone pairs and two bonds, so the
steric number of chlorine is four. Since SN = 4, the
shape of ClF2
+
is based on a tetrahedral electronic
geometry (109.5˚ bond angles). Because
chlorine has two lone pairs, ClF2
+
is bent.
The shape of a molecule (molecular geometry) can
be determined using Lewis dot structures and
valence shell electron pair repulsion theory
(VSEPR).
To determine the shape of the ClF2
+ cation, the
central atom must first be assigned. This
assignment can be made by drawing Lewis dot
structures for each arrangement and assigning
formal charges. Because chlorine is less
electronegative than fluorine, it can better tolerate
a +1 formal charge. Therefore, chlorine is more
likely to be the central atom.
Next, the steric number (SN) must be assigned.
Chlorine has two lone pairs and two bonds, so the
steric number of chlorine is four. Since SN = 4, the
shape of ClF2
+
is based on a tetrahedral electronic
geometry (109.5˚ bond angles). Because
chlorine has two lone pairs, ClF2
+
is bent.
The net dipole moment (polarity) of a molecule can
be determined by summing the bond dipole vectors.
If the bond dipole vectors sum to zero, the molecule
is nonpolar. For example, the bond dipole vectors
of boron trifluoride (BF3) cancel each other out, so
BF3 is nonpolar.
If the bond dipole vectors do not cancel one
another, the molecule is a polar molecule. For
example, the bond dipole vectors in phosphorus
trichloride (PCl3) do not sum to zero, so PCl3 has a
net dipole moment and is a polar molecule.
If the surrounding atoms in a molecule differ from
each other in electronegativity, the molecule will
have a net dipole moment even if the bond dipole
moment vectors are in opposite directions. For
example, fluorine is more electronegative than
chlorine, so phosphorus monochloride tetrafluoride
(PClF4) is a polar molecule.
The net dipole moment (polarity) of a molecule can
be determined by summing the bond dipole vectors.
If the bond dipole vectors sum to zero, the molecule
is nonpolar. For example, the bond dipole vectors
of boron trifluoride (BF3) cancel each other out, so
BF3 is nonpolar.
If the bond dipole vectors do not cancel one
another, the molecule is a polar molecule. For
example, the bond dipole vectors in phosphorus
trichloride (PCl3) do not sum to zero, so PCl3 has a
net dipole moment and is a polar molecule.
If the surrounding atoms in a molecule differ from
each other in electronegativity, the molecule will
have a net dipole moment even if the bond dipole
moment vectors are in opposite directions. For
example, fluorine is more electronegative than
chlorine, so phosphorus monochloride tetrafluoride
(PClF4) is a polar molecule.
Which statement best describes why we do not need the electronegativity values for carbon and oxygen to explain why the carbon dioxide molecule is nonpolar?
The dipole moments of the polar O–C bonds cancel each other, so you do not need to compare electronegativity values. (B)
The electronegativity values do affect the bond dipole value and direction in each bond. But the net bond dipole moment is zero because the two dipole moments cancel each other.
Which of the following best describes why phosphorous trichloride has a net dipole moment downward?
There is nothing on the “top” of the molecule to effectively cancel the downward dipole effect of the three Cl–P bonds. (A)
This is the correct choice. Since there is no upward polarity, the result is a downward polarity.
ClF2+ has two possible arrangements, shown in the diagram. The actual preferred arrangement is structure 2. Which statement about the chlorine difluoride ion, ClF2+, is false?
There are a total of 20 electrons in this ion. In structure 2, the Cl atom is in the middle because it has a lower formal charge than the F atom would have if it were in the middle. (D)
This is not true. There are 20 electrons, but if the F atom were in the middle, it also would have a formal charge of 1+. This is shown in Structure 1.
Let’s examine the oxalic acid molecule. It is an organic molecule that is the constituent part of rhubarb that gives it its sour taste.
Which statement is true for either of the carbon atoms in the molecule?
Carbon has an SN = 3. The predicted geometry around it is trigonal planar. The bond angles are exactly 120° because there is no contribution from lone electron pairs. (D)
This is correct and a very thorough explanation of what occurs around either C atom.
Which of the following statement about the four molecules and their bond dipoles shown in the diagram is false?
There is only one molecule that has a net dipole moment. The net polar direction in this molecule is upward. (B)
This is the only choice that is false. The only molecule that has a net dipole moment is the sulfur dioxide molecule. The other three molecules have bond dipoles that cancel each other. In sulfur dioxide, there is no upward polarity to counteract the downward dimension of the net dipole moment resulting from the two dipole vectors.
The PClF4 is a trigonal bipyramidal molecule with chlorine in one of the axial sites. Which statement about the PClF4 molecule is not true?
There is no dipole moment because the trigonal plane atoms cancel each other’s polarities and the two axial bonds cancel each other’s polarities. (A)
This statment is not true. The vector for the P–Cl bond is much shorter than the vector for any P–F bond. The electronegativity of F is greater than the electronegativity of Cl. Since they are both “pulling” on the same P atom, there is a net dipole moment toward the axial fluorine atom.
Which statement best explains why the molecule in the diagram is polar?
The molecule is linear but is polar because the electronegativity of oxygen is greater than the electronegativity of sulfur. (D)
The molecule is linear. But it is polar. This is because the electronegativity of oxygen is greater than the electronegativity of sulfur.
Which of the following statements does not correctly describe the ICl4− anion?
ICl4− has a square planar electronic geometry. (B)
This statement is not true. The molecular geometry of the ion is square planar. The electronic geometry is octahedral.
The Lewis dot structure for oxalic acid is shown in the drawing. Which statement about the bond angles for oxalic acid is not true?
The H–O–C bond angle is 120°. (C)
This statement is not true. The predicted geometry is tetrahedral, since the central O atom has SN = 4. Each angle in this geometry would be 109.5°.
Which of the following statements about molecular polarity is not correct?
There is no net dipole moment for the water molecule because the two bond dipoles that result from the H–O bonds effectively cancel each other. (C)
Remember that water has two lone electron pairs. This causes the molecule to be bent. Therefore, the bond polarities do not cancel and the molecule is polar.
