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Molecular Geometry Made Easy: VSEPR Theory and How to …
hell Electron-Pair Repulsion Theory (VSEPR)
Electron-Pair Repulsion Theory (VSEPR)
Predicting the Shapes of
There is no direct relationship between the formula of a
compound and the shape of its molecules. The shapes of these
molecules can be predicted from their Lewis structures, however,
with a model developed about 30 years ago, known as the valence-shell
electron-pair repulsion (VSEPR) theory.
The VSEPR theory assumes that each atom in a molecule will
achieve a geometry that minimizes the repulsion between electrons
in the valence shell of that atom. The five compounds shown in
the figure below can be used to demonstrate how the VSEPR theory
can be applied to simple molecules.
There are only two places in the valence shell of the central
atom in BeF2 where electrons can be found. Repulsion
between these pairs of electrons can be minimized by arranging
them so that they point in opposite directions. Thus, the VSEPR
theory predicts that BeF2 should be a linear molecule,
with a 180o angle between the two Be-F bonds.
There are three places on the central atom in boron
trifluoride (BF3) where valence electrons can be
found. Repulsion between these electrons can be minimized by
arranging them toward the corners of an equilateral triangle. The
VSEPR theory therefore predicts a trigonal planar
geometry for the BF3 molecule, with a F-B-F bond angle
BeF2 and BF3 are both two-dimensional
molecules, in which the atoms lie in the same plane. If we place
the same restriction on methane (CH4), we would get a
square-planar geometry in which the H-C-H bond angle is 90o.
If we let this system expand into three dimensions, however, we
end up with a tetrahedral molecule in which the H-C-H bond
angle is 109o28′.
Repulsion between the five pairs of valence electrons on the
phosphorus atom in PF5 can be minimized by
distributing these electrons toward the corners of a trigonal
bipyramid. Three of the positions in a trigonal bipyramid are
labeled equatorial because they lie along the equator of
the molecule. The other two are axial because they lie
along an axis perpendicular to the equatorial plane. The angle
between the three equatorial positions is 120o, while
the angle between an axial and an equatorial position is 90o.
There are six places on the central atom in SF6
where valence electrons can be found. The repulsion between these
electrons can be minimized by distributing them toward the
corners of an octahedron. The term octahedron
literally means “eight sides,” but it is the six
corners, or vertices, that interest us. To imagine the geometry
of an SF6 molecule, locate fluorine atoms on opposite
sides of the sulfur atom along the X, Y, and Z
axes of an XYZ coordinate system.
and Triple Bonds Into the VSEPR Theory
Compounds that contain double and triple bonds raise an
important point: The geometry around an atom is determined by the
number of places in the valence shell of an atom where electrons
can be found, not the number of pairs of valence electrons.
Consider the Lewis structures of carbon dioxide (CO2)
and the carbonate (CO32-) ion, for example.
There are four pairs of bonding electrons on the carbon atom
in CO2, but only two places where these electrons can
be found. (There are electrons in the C=O double bond on the left
and electrons in the double bond on the right.) The force of
repulsion between these electrons is minimized when the two C=O
double bonds are placed on opposite sides of the carbon atom. The
VSEPR theory therefore predicts that CO2 will be a
linear molecule, just like BeF2, with a bond angle of
The Lewis structure of the carbonate ion also suggests a total
of four pairs of valence electrons on the central atom. But these
electrons are concentrated in three places: The two C-O single
bonds and the C=O double bond. Repulsions between these electrons
are minimized when the three oxygen atoms are arranged toward the
corners of an equilateral triangle. The CO32-
ion should therefore have a trigonal-planar geometry, just like
BF3, with a 120o bond angle.
The Role of Nonbonding
Electrons in the VSEPR Theory
The valence electrons on the central atom in both NH3
and H2O should be distributed toward the corners of a
tetrahedron, as shown in the figure below. Our goal, however,
isn’t predicting the distribution of valence electrons. It is to
use this distribution of electrons to predict the shape of the
molecule. Until now, the two have been the same. Once we include
nonbonding electrons, that is no longer true.
The VSEPR theory predicts that the valence
electrons on the central atoms in ammonia and water will point
toward the corners of a tetrahedron. Because we can’t locate the
nonbonding electrons with any precision, this prediction can’t be
tested directly. But the results of the VSEPR theory can be used
to predict the positions of the nuclei in these molecules, which
can be tested experimentally. If we focus on the positions of the
nuclei in ammonia, we predict that the NH3 molecule
should have a shape best described as trigonal pyramidal,
with the nitrogen at the top of the pyramid. Water, on the other
hand, should have a shape that can be described as bent,
or angular. Both of these predictions have been shown to
be correct, which reinforces our faith in the VSEPR theory.
When we extend the VSEPR theory to molecules in which the
electrons are distributed toward the corners of a trigonal
bipyramid, we run into the question of whether nonbonding
electrons should be placed in equatorial or axial positions.
Experimentally we find that nonbonding electrons usually occupy
equatorial positions in a trigonal bipyramid.
To understand why, we have to recognize that nonbonding
electrons take up more space than bonding electrons. Nonbonding
electrons need to be close to only one nucleus, and there is a
considerable amount of space in which nonbonding electrons can
reside and still be near the nucleus of the atom. Bonding
electrons, however, must be simultaneously close to two nuclei,
and only a small region of space between the nuclei satisfies
Because they occupy more space, the force of repulsion between
pairs of nonbonding electrons is relatively large. The force of
repulsion between a pair of nonbonding electrons and a pair of
bonding electrons is somewhat smaller, and the repulsion between
pairs of bonding electrons is even smaller.
The figure below can help us understand why nonbonding
electrons are placed in equatorial positions in a trigonal
If the nonbonding electrons in SF4 are placed in an
axial position, they will be relatively close (90o) to
three pairs of bonding electrons. But if the nonbonding electrons
are placed in an equatorial position, they will be 90o
away from only two pairs of bonding electrons. As a result, the
repulsion between nonbonding and bonding electrons is minimized
if the nonbonding electrons are placed in an equatorial position
The results of applying the VSEPR theory to SF4,
ClF3, and the I3– ion are shown
in the figure below.
When the nonbonding pair of electrons on the sulfur atom in SF4
is placed in an equatorial position, the molecule can be best
described as having a see-saw or teeter-totter
shape. Repulsion between valence electrons on the chlorine atom
in ClF3 can be minimized by placing both pairs of
nonbonding electrons in equatorial positions in a trigonal
bipyramid. When this is done, we get a geometry that can be
described as T-shaped. The Lewis structure of the
triiodide (I3–) ion suggests a trigonal
bipyramidal distribution of valence electrons on the central
atom. When the three pairs of nonbonding electrons on this atom
are placed in equatorial positions, we get a linear
Molecular geometries based on an octahedral distribution of
valence electrons are easier to predict because the corners of an
octahedron are all identical.
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Frequently Asked Questions About how does the vsepr theory explain molecular shape
If you have questions that need to be answered about the topic how does the vsepr theory explain molecular shape, then this section may help you solve it.
How does the VSEPR theory determine the shape of molecules?
According to the valence-shell electron pair repulsion model, or VSEPR model (which is based on the repellent behavior of electron pairs), a molecule will modify its shape in order to keep its valence electron pairs as far apart from one another as possible.
How does the quizlet on molecular shape explain the VSEPR theory?
Because electron pairs repel one another regardless of whether they are in bond pairs or lone pairs, according to the valence-shell electron-pair repulsion (VSEPR) theory, electron pairs will disperse as far apart as possible.
What can we infer from the VSEPR theory?
The shape of a molecule is determined by the location of the nuclei and its electrons, which settle into positions that minimize repulsion and maximize attraction. The electron bond pairs and lone pairs on the center atom will assist us in predicting the shape of a molecule using the VSEPR theory.
How are molecular geometry and bond angles explained by VSEPR?
If you imagine the valence electrons as occupying orbitals that are similar in shape to party balloons, then it is not too difficult to see how different molecules acquire their specific shapes. This theory is known as the Valence Shell Electron Pair Repulsion Theory, or VSEPR Theory.
What theory accounts for a molecule’s shape?
As a result of this arrangement, electrons are found in various atomic orbitals and they are typically associated with different nuclei, according to the molecular orbital theory, which states that each atom tends to combine together to form molecular orbitals.
What scientific theory describes the molecular structure?
The main tenets of the VSEPR theory are as follows: i The number of valence shell electron pairs surrounding the central atom determines the shape of the molecule; ii Valence shell electron pairs repel one another because their electron clouds are negatively charged.
What are the VSEPR theory’s two main tenets?
The valence-shell electron-pair repulsion (VSEPR) model, which predicts the shape of a molecule by taking into account the repulsion between electron pairs, is frequently used in chemistry to predict the three-dimensional arrangement, or geometry, of molecules.
What do electron domains and molecular shape reveal from VSEPR?
It includes the overall shape of the molecule as well as bond lengths, bond angles, torsional angles, and any other geometrical parameters that determine the position of each atom. Molecular geometry is the three-dimensional arrangement of the atoms that make up a molecule.
What are the VSEPR theory’s two main tenets?
One nerve cell in the brain signals another by releasing molecules of a specific shape to go find matching receptor molecules on the surface of the receiving cell. Molecular shape is crucial in biology because it determines how most molecules recognize and respond to each other.
How does the function of a molecular shape determine it?
Covalent molecules have distinctive shapes, known as the molecule’s molecular geometry, as a result of the electron pairs’ mutually negative charges repelling each other and trying to move as far apart as possible in order to stabilize the molecule.
What causes the formation of molecular shapes?
Molecular shapes are crucial in determining macroscopic properties like melting and boiling points, as well as in predicting the ways in which one molecule can react with another. Molecular shapes are defined by the location in three-dimensional space of the nucleus of each atom in a molecule, which is known as molecular geometry.
What can we infer from a molecule’s shape?
For describing ligands interacting with receptors, ion channels, enzymes, transporters, and a variety of other proteins and intricate biological processes, shape is a fundamentally significant molecular feature.
Why is the shape of molecules so significant in biology?
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