Valence-Shell Electron-Pair Repulsion Theory (10.1.1) Flashcards
• Valence shell electron pair repulsion theory (VSEPR) allows for the prediction
of molecular geometries (shapes).
• 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.
• 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).
• Each steric number is associated with a specific three-dimensional arrangement of
valence electron pairs (electronic geometry).
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 (H2O) 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 (CO2) 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.
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 (H2O) 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 (CO2) 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.
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.
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.
Which of the following does not directly pertain to VSEPR theory?
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. (D)
This statement is almost correct. The only error, though, is serious. In the first sentence, it says that “VSEPR theory is based on the attractive forces that exist between atoms and any lone electron pairs.” VSEPR theory is based on the repulsive forces (from like negative charges) between covalent bonds and lone electron pairs in the molecule.
Using a balloon to represent electron clouds (either an electron bond or a lone electron pair, which statement is not correct about electron clouds?
The electron clouds in a trigonal bipyramid are all at right angles to one another in order to maximize their bond angles (and distances). (C)
In this orientation, the “equator” or middle plane has three electron clouds spread about at equal angles of 120° from one another. However, each of these clouds is at a right angle to the north and south pole (location) electron clouds.
Use the Lewis Dot structure for POCl3 to calculate the SN value for the central phosphorous atom.
4 (A)
What is the SN value for the central atom of this molecule?
6 (D)
Which statement about VSEPR geometries is not correct?
The maximum angle measurement for these geometries is 120°. (C)
Which of the following has the greatest SN value for the central atom?
sulfur hexafluoride (B)
What is the shape of this molecule?
trigonal planar (B)
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?
The molecular geometry is dictated by the positioning of the atoms and is not affected by the electron arrangement in the molecule. (B)
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?
4; tetrahedral (C)
Which of the following best defines the term steric number (SN)?
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. (C)
