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 NH 3 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. Click here to check your answer to Practice Problem 6. Use the Lewis structure of the NO 2 molecule shown in the figure below to predict the shape of this molecule. Click here to check your answer to Practice Problem 7. 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.
The central atom, carbon, contributes four valence electrons, and each hydrogen atom has one valence electron, so the full Lewis electron structure is. There are four electron groups around the central atom.
All electron groups are bonding pairs, so the structure is designated as AX 4. Phosphorus has five valence electrons and each chlorine has seven valence electrons, so the Lewis electron structure of PCl 5 is. There are five bonding groups around phosphorus, the central atom.
All electron groups are bonds, so the structure is designated as AX 5. The molecular geometry of PCl 5 is trigonal bipyramidal , as shown below. The molecule has three atoms in a plane in equatorial positions and two atoms above and below the plane in axial positions. The axial and equatorial positions are not chemically equivalent. The central atom, sulfur, contributes six valence electrons, and each fluorine atom has seven valence electrons, so the Lewis electron structure is.
There are six electron groups around the central atom, each a bonding pair. With only bonding pairs, SF 6 is designated as AX 6. All positions are chemically equivalent, so all electronic interactions are equivalent. However, due to the stereochemical inert pair effect, these molecules are found to be regular octahedral because one of the electron pairs is stereochemically inactive. Introduction The VSEPR model is a powerful tool used by chemists to predict the shapes of molecules; yet like many other theories, it has exceptions and limitations.
VSEPR fails for isoelectronic species Isoelectronic species are elements, ions and molecules that share the same number of electrons. VSEPR fails for transition metal compounds The VSEPR model also fails to predict the structure of certain compounds because it does not take relative sizes of the substituents and stereochemically inactive lone pairs into account. References Cotton, F. Albert, and Geoffrey Wilkinson. Use the link below to answer the following questions:.
Skip to main content. Covalent Bonding. Search for:. What do I do now? What is the basic theory behind the model?
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