VSEPR Theory — Definition & Overview

What is VSEPR Theory?

VSEPR stands for valence shell electron pair repulsion. Valence shell electrons are on the outer part of an atom. These electrons can participate in bonding (bonding pairs) or act as non-bonding electrons (lone pairs). These bonding and lone pairs electrons repel each other.

VSEPR theory is where electrons in the valence shell of an atom repel each other, they can affect the geometry of a bonds in a molecule. Molecule geometry describes the location and angles between bonds in regards to the location of electron density (where electrons are located).

Water Molecule and VSEPR Theory

Let's consider a water molecule or $H_2O$! Try drawing a water molecule with bonds represented by black line starting from the oxygen and connecting to hydrogens. A hint is that there are two lone pairs or two non-bonding electron pairs.

The correct way to draw a water molecule is on the right. It is understandable knowing the basics of VSEPR someone would want to draw the water molecule like the left image. However, electron pairs can cause different amounts of repulsion. The lone pairs on the oxygen "push down" the hydrogens to create a bond angle of 104.5°.

AXE Method

The AXE method is a way to characterize molecules and determine geometry. A stands for the central atom. The central atom in the atom which other atoms are bonded to, some molecules will have multiple central atoms. X stands for a ligands or atoms bond to central a central atom. E stands for the lone pairs on the central atom.

For our water example the AXE method abbreviation is AE$_2$X$_2$. All molecules with this same designation will have a bent geometry.

In the chart below, the molecular geometry is given for some types of molecules. Notice that the molecular geometry does not include the lone pairs. If lone pairs are taken into account it is called lone pair geometry. The chart is broken into sections denoted by the solid lines, the electron geometry is bolded for that section.

You have been studying covalent bonding. You may have wondered if different molecules have different shapes. For example, consider carbon monoxide ($CO$), carbon dioxide ($CO_{2}$) and methane ($CH_4$). Carbon monoxide only has one atom attached to the central carbon. Carbon dioxide has two atoms bonded to the central carbon. Methane has four hydrogens bonding with the central carbon. Each of these molecules has a different geometry. To help us approximate molecular geometries, we use VSPER theory. Molecular geometry is important for knowing a molecule's chemical and physical properties.

What does VSEPR stand for?

VSEPR is an acronym for valence shell electron pair repulsion. We pronounce it "ves-per." Two physical chemists developed VSEPR theory. Other chemists had done previous work on molecular geometry. In 1957, Ronald Gillespie and Sir Ronald Sydney Nyholm presented a complete process. They developed VSEPR as an easy way to choose between different shapes.

In the history of chemistry, 1957 is a late development. Schrodinger had already developed his quantum mechanical models. But, VESPR theory did not use quantum mechanics. Instead, VSEPR theory is an approximation of experimental results. It assumes that electrons stay localized. In contrast, in quantum mechanics, the electrons delocalize throughout the whole molecule.

How does VSEPR work?

We know that atoms and molecules want the lowest energy. Lower energy means the molecule is more stable. In the VSEPR model, the molecule's geometry minimizes the repulsion forces between electrons. Minimized electron repulsion means the molecule is at its lowest energy and most stable.

In VSEPR theory, there are three types of interactions that increase energy. Most of the energy is the result of the repulsion between lone electron pairs. When two atoms bond, they share electrons. The electrons' charges distribute between the atoms. In contrast, non-bonded electron pairs are on a single atom. So, the repulsion caused by two lone electron pairs is significant. Repulsion between lone and bonded electron pairs is another source of energy. Bonded electrons are between atoms. But, VSEPR assumes that they have some localized charge. So, the bonded electrons interact with the lone pairs. Finally, the lowest energy repulsion is between two sets of bonded electrons. In the case of bonded electrons, the electrons' charges are the most distributed. This interaction is the weakest of the three.

Steps for Using VSEPR Theory: AXE Method

1. Start by drawing the molecule's lewis dot structure. Make sure you include all the bonds and the lone pairs.
2. Determine the central atom(s).
3. Determine the number of bonds.
4. Determine the number of electron lone pairs.
5. Look up the VSEPR geometry. Use the number of bonds and lone pairs from steps 3 and 4. Your teacher might have you memorize some of the most common geometries. You may not always have the luxury of looking it up.

For VSEPR, a central atom is any atom bonded to two or more atoms. Sometimes a molecule only has one central atom. For example, in the oxoanion, Sulfite ($SO_{3}^{2-}$), only Sulfur is a central atom. Other times there can be multiple central atoms. For example, in ethane ($C_{2}$$H_{6}$), each carbon has bonded four times. Both carbons are central atoms. You can apply VSEPR to each carbon.

When you look up a VSPER chart, you will often see AXE. The A stands for the central atom. X stands for bonded atoms. E stand for electron lone pairs. Let's say you looked up the geometry for water ($H_{2}O$). The AXE chart would be $A$$X_{2}$$E_{2}$. The chart would tell you that this is a bent molecule. There are 104.5 degrees between the hydrogen atoms.

VSEPR Limitations

VSPER theory does have limitations. It does not consider molecular orbital theory nor valence bond theory. From molecular orbital theory, we know that molecular orbitals distribute throughout the molecule. VSPER assumes that bonding electrons stay localized between the atoms. In valence bond theory, different hybrid orbital combinations have unique electron interactions. Overall, VSPER is an excellent approximation. It can get you through most of undergraduate-level chemistry. Eventually, you will need to consider quantum mechanics.

VSEPR theory also assumes that all bonding atoms act the same. But, this cannot be true. For example, consider chloroform ($CHCl_{3}$). The chlorine atoms have larger atomic radii than the hydrogen atom. Chlorine also has p-orbitals. Chlorine is bigger and has more electrons. It is going to exert a larger repulsive force than hydrogen. Chloroform would deviate from the ideal bond angles. When all the bonding atoms are the same and there are no lone pairs, the molecule has ideal bond angles.

Chemistry in the Real World! How do we determine molecular geometries?

We can find molecular geometries by electron diffraction spectroscopy. The basis of this technique is the wave-particle duality. An electron can act as a wave. The wavelength of an electron is comparable to the distances between atoms. In electron diffraction spectroscopy, the chemist heats the sample to its gaseous state. The gas state reduces the intermolecular forces. The chemist then shoots electrons at the specimen. As the electrons hit different parts of the molecule, they scatter. A computer detects the scattered electrons. The computer determines the electrons' paths and what they hit. So, we know the location of the molecule's atoms and bonds. Finally, we can determine the molecular geometries.

Computational chemists can also use computer models to approximate molecular geometries. But, larger molecules have more complicated calculations. So this is only done for smaller molecules.

(Video) VSEPR Theory - Basic Introduction

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor explains the VSEPR repulsion theory. It helps us determine the molecular geometry of covalent molecules. In this first video, he demonstrates the possible geometries and bond angles of molecules that obey the octet rule.

(Video) Molecular Geometry & VSEPR Theory - Basic Introduction

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor continues his explanation of VSEPR theory. In this second video, he demonstrates the possible geometries and bond angles of molecules that disobey the octet rule.