10.1.5 Molecular Shapes: The AXE Method, Part 1

Question 1

Molecular Shapes: The AXE Method, Part 1

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.
• By determining the AXE designation of a molecule, the molecular geometry and bond angles can also be determined.

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.
• AX 2 molecules include a central atom surrounded by two other atoms and having no lone pairs of electrons. Molecules with an AX 2 designation have a linear geometry and a bond angle of 180o. Examples of AX 2 molecules are beryllium hydride and carbon dioxide.
• AX 3 molecules include a central atom surrounded by three other atoms and having no lone pairs of electrons. Molecules with an AX 2 designation have a trigonal planar geometry and a bond angle of 120o. Examples of AX 3 molecules are boron trifluoride, nitrate ion, and formaldehyde.
• AX 2 E molecules include a central atom surrounded by two other atoms and having one lone pair of electrons. Molecules with an AX 2 E designation have a bent geometry and a bond angle of less than 120o. Examples of AX 2 E molecules are sulfur dioxide and ozone.
• AX 4 molecules include a central atom surrounded by three other atoms and having no lone pairs of electrons. Molecules with an AX 4 designation have a tetrahedral geometry and a bond angle of 109.5o. Examples of AX

Question 3

note 2

Answer 3

• AX 3 E molecules include a central atom surrounded by three other atoms and having one lone pair of electrons. Molecules with an AX 3 E designation have a tetrahedral geometry and a bond angle of slightly less than 109.5o. Examples of AX 2 E molecules are ammonia and thionyl chloride.
• AX 2 E 2 molecules include a central atom surrounded by three other atoms and having one lone pair of electrons. Molecules with an AX 2 E 2 designation have a bent geometry and a bond angle of less than 109.5o. Examples of AX 2 E 2 molecules are water, and sulfur dioxide.
• The rules for AXE designations can be applied to any
  non-terminal atom in a molecule.

Question 4

What is the F–As–F bond angle in the arsenic trifluoride molecule?

Answer 4

about 109.5º

Question 5

Which of the following is not an AX3 designated geometry?

Answer 5

PCl3

Question 6

What is the AXE designation for aluminum chloride?

Answer 6

AX3

Question 7

Which pair of the compounds listed have the same AXE designation?

Answer 7

BF3 and CO32-

Question 8

Which of the following compounds is listed with an incorrect AXE designation?

Answer 8

NO3-, AX3E

Question 9

What is the correct AXE designation for HCN?

Answer 9

AX2

Question 10

Which of the following compounds would have a 120° bond angle?

Answer 10

NO3–

Question 11

Which of the following compounds has an AX2E designated geometry?

Answer 11

sulfur dioxide

Question 12

Which of the following molecules does not have a 180° bond angle?

Answer 12

SeO2

Question 13

What is the AXE designation for the ammonium ion?

Answer 13

AX4

Question 14

What is the AXE designation for carbon tetrachloride?

Answer 14

AX4

Question 15

What is the expected bond angle for a molecule with an AX2E designation?

Answer 15

120º

Question 16

What is the I–Al–I bond angle in the aluminum iodide molecule?

Answer 16

120º

Question 17

Which of the following pairs of compounds have different bond angles?

Answer 17

Sulfur dioxide and carbon dioxide

Question 18

Which molecule has the AXE designation of AX2E2?

Answer 18

Water