10.1.6 Molecular Shapes: The AXE Method, Part 2

Question 1

Molecular Shapes: The AXE Method, Part 2

Answer 1

• Every molecule has an AXE designation that provides information about the number of atoms and lone pairs of electrons surrounding the central atom.
• The AXE designation can be used to determine the geometry and bond angle(s) of a molecule.
• When multiple molecular structures are possible, the structure with the lowest energy is the correct structure.

Question 2

note 1

Answer 2

• The AXE designation is a method for organizing the
  geometries of molecules. By determining the AXE
  designation of a molecule, the geometry and bond angles can also be determined.
• Molecules having a central atom surrounded by six atoms and no lone pairs of electrons have an AX 6 designations. Molecules having an AX 6 designation have an octahedral geometry and bond angles of 90o. An example of a molecule having an AX 6 designation is sulfur hexafluoride.
• Molecules having a central atom surrounded by five atoms and one lone pair of electrons have an AX 5 E designations. Molecules having an AX 5 E designation have an square pyramidal geometry and bond angles of 90o. An example of a molecule having an AX 5 E designation is bromine pentafluoride.
• Molecules having a central atom surrounded by four atoms and two lone pairs of electrons have an AX 4 E 2 designation. Because there are two possible structures for this AXE designation, the rules for determining the lowest energy structure must be applied. Once the structure having a lone pair-lone pair interaction has been eliminated, the structure having the trans oriented lone pairs is the only option for AX 4 E 2 molecules. Molecules having an AX 4 E 2 designation have an square planar geometry and bond angles of 90o. An
  example of a molecule having an AX 4 E 2 designation is xenon tetrafluoride.

Question 3

note 2

Answer 3

• Molecules having a central atom surrounded by five atoms and no lone pairs of electrons have an AX 5 designations. Molecules having an AX 5 designation have an trigonal bipyramidal geometry and bond angles of 90o and 120o. An example of a molecule having an AX 5 designation is phosphorus pentachloride.
• Molecules having a central atom surrounded by four atoms and one lone pair of electrons have an AX 4 E designation. Because there are two possible structures for this AXE designation, the rules for determining the lowest energy structure must be applied. The structural choice with the fewest lone pair-bonding interactions at 90o means that molecules having an AX 4 E designation have an see-saw geometry. These molecules have bond angles of 90o and 120o. An example of a molecule having an AX 4 E designation is sulfur tetrafluoride.
• Molecules having a central atom surrounded by three atoms and two lone pairs of electrons have an AX 3 E 2 designation. Because there are three possible structures for this AXE designation, the rules for determining the lowest energy structure must be applied. One structure can be eliminated because of a lone pair-lone pair interaction at 90o. The structural choice with the fewest lone pair-bonding interactions at 90o means that molecules having an AX 3 E 2 designation have an T-shaped geometry and bond angles of 90o. An example of a molecule having an AX 3 E 2 designation is chlorine trifluoride.
• Molecules having a central atom surrounded by three atoms and two lone pairs of electrons have an AX 2 E 3 designation. Because there are three possible structures for this AXE designation, the rules for determining the lowest energy structure must be applied. After eliminating all of the structures that have lone pair-lone pair interactions at 90o. there is only one remaining strutural possibility. Molecules having an AX 2 E 3 designation have an linear geometry and bond angles of 180o. An example of a molecule having an AX 2 E 3 designation is the triiodide ion.

Question 4

Which of the following geometries is associated with an AX4E designation?

Answer 4

see-saw

Question 5

What is the AXE designation for the triiodide ion?

Answer 5

AX2E3

Question 6

Which of the following molecules have a T-shaped geometry?

Answer 6

Iodine trichloride

Question 7

What pair of compounds listed both have an AX2E3 designated geometry?

Answer 7

Xenon difluoride and triiodide ion

Question 8

Which of the following compounds is incorrectly matched with its AXE designation?

Answer 8

All of these are correctly matched

Question 9

Which of the following pairs of AXE designations have different shapes?

Answer 9

AX4 and AX4E

Question 10

What is the correct AXE designation for the XeO3 molecule?

Answer 10

AX3E

Question 11

Sufur hexafluoride has the same AXE designation as which of the following compounds?

Answer 11

None of the above

Question 12

Which compound has an AX4E designation?

Answer 12

SF4

Question 13

Which of the following compounds has a square planar geometry?

Answer 13

xenon tetrafluoride

Question 14

What is the correct AXE designation and geometry of the orthotellurate ion, TeO66−?

Answer 14

AX6, octahedral

Question 15

Which of the following compounds has an AX3E2 designated geometry?

Answer 15

Chlorine trifluoride

Question 16

Which pair of compounds both have an AX5E designation?

Answer 16

bromine pentafluoride and iodine pentafluoride

Question 17

Which of the following pairs of compounds have the same AXE designation?

Answer 17

triiodide ion and xenon difluoride

Question 18

What is the bond angle associated with an AX2E3 designation?

Answer 18

180º

Question 19

What is the correct AXE designation of IF5?

Answer 19

AX5E