10.1.1 Valence-Shell Electron-Pair Repulsion Theory

Question 1

Valence-Shell Electron-Pair Repulsion Theory

Answer 1

• Valence shell electron pair repulsion theory (VSEPR) allows for the prediction of molecular geometries (shapes).
• The steric number (SN) describes the number of directions in which an atom has electron density.
• Each steric number is associated with a specific three-dimensional arrangement of valence electron pairs
(electronic geometry).

Question 2

note 1

Answer 2

• Valence shell electron pair repulsion theory (VSEPR) allows for the prediction of molecular geometries (shapes).
• By valence shell electron pair repulsion theory, the shape of a molecule is determined by maximizing the distance (and thus minimizing the repulsion) between valence shell electron pairs.
• The shape of a group of balloons is analogous to the shapes predicted by VSEPR theory. For example, a group of six balloons will adopt the shape of an octahedron in order to maximize the distance between the balloons. Similarly, a molecule with electron density in six directions will adopt an octahedral electronic geometry.
• The steric number (SN) describes the number of directions in which an atom has electron density. The SN of a central atom is the sum of the lone pair electrons and the number of bonded atoms.
• For example, the oxygen atom in water (H 2 O) has two lone pairs and two bonded hydrogen atoms. Therefore, the steric number of the oxygen atom in water is four.
• Double and triple bonds are counted as just one bond in calculating steric number. For example, the carbon atom in carbon dioxide (CO 2 ) has no lone pairs and two bonded oxygen atoms, so its steric number is two.
• The steric number for the central atom in intimidating
  molecules can still be easily determined by careful
  examination. For example, the xenon atom in xenon
  tetrafluoride has two lone pairs and four bonded fluorine atoms, so its steric number is six.

Question 3

note 2

Answer 3

• Each steric number is associated with a specific
  three-dimensional arrangement of electron density (electronic geometry) with characteristic bond angles.
• An atom with a steric number of two has a linear electronic geometry.
• An atom with a steric number of three has a trigonal planar electronic geometry.
• An atom with a steric number of four has a tetrahedral
  electronic geometry.
• An atom with a steric number of five has a
  trigonal bipyramidal electronic geometry.
• An atom with a steric number of six has an octahedral
  electronic geometry.

Question 4

The Lewis Dot structure for water can be drawn in the following manner.

What is the steric number value and predicted geometric shape of water based on the SN value?

Answer 4

4; tetrahedral

Question 5

What is the shape of this molecule?

Answer 5

trigonal planar

Question 6

Which statement about VSEPR geometries is not correct?

Answer 6

The maximum angle measurement for these geometries is 120°.

Question 7

Which of the following does not directly pertain to VSEPR theory?

Answer 7

VSEPR theory is based on the attractive forces that exist between atoms and any lone electron pairs. More specifically, it deals with how the bonding pairs and lone electron pairs are arranged in a predictable geometric manner.

Question 8

Using a balloon to represent electron clouds (either an electron bond or a lone electron pair, which statement is not correct about electron clouds?

Answer 8

The electron clouds in a trigonal bipyramid are all at right angles to one another in order to maximize their bond angles (and distances).

Question 9

VSEPR theory, or Valence Shell Electron Pair Repulsion theory, helps chemists to predict and explain the geometric arrangement of atoms in molecules. It is based on characteristic behaviors of electrons in molecules. Which of the following statements is not correct?

Answer 9

The molecular geometry is dictated by the positioning of the atoms and is not affected by the electron arrangement in the molecule.

Question 10

Use the Lewis Dot structure for POCl3 to calculate the SN value for the central phosphorous atom.

Answer 10

4

Question 11

Which of the following has the greatest SN value for the central atom?

Answer 11

sulfur hexafluoride

Question 12

What is the SN value for the central atom of this molecule?

Answer 12

6

Question 13

Which of the following best defines the term steric number (SN)?

Answer 13

The steric number is equal to the number of atoms in a molecule that are bonded to the central atom plus the number of lone pairs of electrons.