Molecular Shapes for Steric Numbers 5 & 6 (10.1.3) Flashcards
• Lone pairs are assigned to sites that minimize their interactions with one another
and other bonding electrons.
• Lone pairs are assigned to sites that minimize their interactions with one another
and other bonding electrons.
• Knowing the Lewis dot structure of a molecule and the steric number (SN) of
the central atom allows prediction of the shape and reactivity of that molecule.
• Knowing the Lewis dot structure of a molecule and the steric number (SN) of
the central atom allows prediction of the shape and reactivity of that molecule.
Lone pairs are assigned to sites that minimize their
interactions with one another and other bonding
electrons.
Rules for deciding where to place lone pairs:
1. Eliminate and structure that has a lone
pair-lone pair interaction at 90°.
2. Of the remaining structures, choose the
one with the fewest lone pair-bonding
pair interactions at 90°.
Lone pairs are assigned to sites that minimize their
interactions with one another and other bonding
electrons.
Rules for deciding where to place lone pairs:
1. Eliminate and structure that has a lone
pair-lone pair interaction at 90°.
2. Of the remaining structures, choose the
one with the fewest lone pair-bonding
pair interactions at 90°.
Molecules with a steric number of 5 are built on the
trigonal bipyramidal electronic geometry, with
bond angles of about 120°, 90°, and 180°.
Molecules with SN 5 and no lone pairs have a
trigonal bipyramidal molecular geometry (shape).
Molecules with SN 5 and one lone pair have a
seesaw molecular geometry.
Molecules with SN 5 and two lone pairs have
T-shaped molecular geometry.
Molecules with SN 5 and three lone pairs have a
linear molecular geometry.
Molecules with a steric number of 5 are built on the
trigonal bipyramidal electronic geometry, with
bond angles of about 120°, 90°, and 180°.
Molecules with SN 5 and no lone pairs have a
trigonal bipyramidal molecular geometry (shape).
Molecules with SN 5 and one lone pair have a
seesaw molecular geometry.
Molecules with SN 5 and two lone pairs have
T-shaped molecular geometry.
Molecules with SN 5 and three lone pairs have a
linear molecular geometry.
Molecules with a steric number of 6 are built on the
octahedral electronic geometry, with bond angles of
about 90°.
Molecules with SN 6 and no lone pairs have an
octahedral molecular geometry.
Molecules with SN 6 and one lone pair have a
square pyramidal molecular geometry.
Molecules with SN 6 and two lone pairs have a
square planar molecular geometry.
Molecules with a steric number of 6 are built on the
octahedral electronic geometry, with bond angles of
about 90°.
Molecules with SN 6 and no lone pairs have an
octahedral molecular geometry.
Molecules with SN 6 and one lone pair have a
square pyramidal molecular geometry.
Molecules with SN 6 and two lone pairs have a
square planar molecular geometry.
Which of the following statements about molecules with SN = 6 is not true.
Molecules with SN = 6 and one lone pair of electrons have a trigonal bipyramidal geometry. (D)
Molecules with SN = 6 and one lone pair of electrons have a square pyramidal geometry. The trigonal bipyramidal geometry belongs exclusively to the SN = 5 molecules.
The picture shows three possible locations for the lone pairs in a chlorine trifluoride molecule. Which is the favored configuration and why?
Left. Because the right has two lone pairs at 90° and the middle has the most electron pair-bond angles of 90°. (A)
This is the correct configuration as derived from the appropriate application of the two rules for determining favored configurations.
Which of the following statements about steric number 5 is false?
SN = 5 molecules with two lone electron pairs must have those pairs oriented at 90° to each other. (C)
This statement is incorrect. It should say that SN = 5 molecules with lone electron pairs must not have those pairs oriented at 90° to each other. This is the first rule for deciding the geometry of degenerate SN = 5 molecules.
What is the shape of the carbon tetrachloride (CCl4 ) molecule?
Tetrahedral (D)
Carbon tetrachloride has a steric number of four, therefore is has a tetrahedral electronic geometry. There are no lone pairs on the central atom, so the shape (or molecular geometry) is also tetrahedral.
Which pair of molecules has the same steric number, but different shapes?
NO3− and O3 (A)
Nitrate ion (NO3− ) and ozone (O3 ) both have steric number of three, but ozone has a lone pair on the central atom and nitrate ion does not. Nitrate ion has a molecular shape of trigonal planar and ozone has a bent shape.
Sulfur tetrafluoride (SF4 ) and sulfur hexafluoride (SF6 ) have seesaw and octahedral geometries respectively, what are their steric numbers?
5,6 (D)
Sulfur tetrafluoride has one lone electron pair in addition to the four bonds located around the central sulfur atom giving it steric number 5. The seesaw-shape is one of the degenerate shapes belonging to SN = 5. Sulfur hexafluoride has six fluoride atoms around the central sulfur atom giving it steric number 6. The steric number follows directly from the shape.
What is the bond angle of the F–B–F bond in a boron trifluoride (BF3 ) molecule?
120 degrees (B)
Which of the following has a square planar molecular geometry?
IF4- (C)
The iodine tetrafluoride ion (IF4− ) has a steric number of six. There are two lone pairs of electrons on the iodine atom, so iodine tetrafluoride ion has an electronic geometry of octahedral and a molecular geometry of square planar.
What is the geometry of the molecule chlorine trifluoride, which has steric number 5?
T-shaped. (B)
Which of the following has only one lone pair of electrons on the central atom?
IF5 (B)
Iodine pentafluoride (IF5 ) has a steric number of six and an electronic geometry of octahedral. It has a shape (molecular geometry) of square pyramidal, and thus has one lone pair of electrons on the central atom.
