10.1.3 Molecular Shapes for Steric Numbers 5 & 6

Question 1

Molecular Shapes for Steric Numbers 5 & 6

Answer 1

• 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.

Question 2

note

Answer 2

• 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 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.

Question 3

Which pair of molecules has the same steric number, but different shapes?

Answer 3

NO3− and O3

Question 4

Sulfur tetrafluoride (SF4 ) and sulfur hexafluoride (SF6 ) have seesaw and octahedral geometries respectively, what are their steric numbers?

Answer 4

5, 6

Question 5

Which of the following statements about steric number 5 is false?

Answer 5

SN = 5 molecules with two lone electron pairs must have those pairs oriented at 90° to each other.

Question 6

What is the shape of the carbon tetrachloride (CCl4 ) molecule?

Answer 6

Tetrahedral

Question 7

What is the geometry of the molecule chlorine trifluoride, which has steric number 5?

Answer 7

T-shaped

Question 8

What is the bond angle of the F–B–F bond in a boron trifluoride (BF3 ) molecule?

Answer 8

120°

Question 9

Which of the following has a square planar molecular geometry?

Answer 9

IF4−

Question 10

Which of the following statements about molecules with SN = 6 is not true.

Answer 10

Molecules with SN = 6 and one lone pair of electrons have a trigonal bipyramidal geometry.

Question 11

Which of the following has only one lone pair of electrons on the central atom?

Answer 11

IF5

Question 12

The picture shows three possible locations for the lone pairs in a chlorine trifluoride molecule. Which is the favored configuration and why?

Answer 12

Left. Because the right has two lone pairs at 90° and the middle has the most electron pair-bond angles of 90°.