Lesson Explainer: VSEPR Chemistry

In this explainer, we will learn how to identify the shape of molecules using VSEPR.

The valence shell electron pair repulsion (VSEPR) model can be used with Lewis-structure diagrams to predict the shapes of different molecules.

Lewis-structure diagrams are used to identify valence electrons in polyatomic molecules, and the VSEPR model is used to understand how these valence electrons are arranged in three-dimensional space. The VSEPR model assumes that there will always be electrostatic repulsion between pairs of valence electrons. The valence electrons will be forced to adopt specific three-dimensional shapes that minimize high-energy repulsive electrostatic interactions.

The following figure shows the Lewis-structures of some simple polyatomic molecules and ions. The red color is used to represent the valence electrons on the central atom and the black color is used to represent the valence electrons on the other atoms.

Definition: Lone pair

Lone pairs are pairs of valence electrons that are not shared in a covalent bond.

The AXE method is used with Lewis-structure diagrams to categorize polyatomic molecules and ions into different groups. Each group has a preferred three-dimensional geometry that can include, for example, the linear or trigonal planar structures. The correct category can be determined by choosing appropriate A, X, and E terms. The A term represents the central atom and the X and E terms represent the bonded and nonbonded electron pairs. The 𝑚 and 𝑛 values are always integers, and they represent the number of bonding and nonbonding electron pairs.

The following figure shows Lewis-structure diagrams and VSEPR geometries for some of the simplest AXE polyatomic molecular categories.

The figure shows that the number of valence electrons determines how they are arranged around the central atom (A). Molecules adopt rather simple linear configurations when there are just two bonded pairs of valence electrons and more complex configurations when there are three or four pairs of bonded or unbonded valence electrons.

Valence electrons always adopt specific three-dimensional structures that minimize the strength of repulsive electrostatic interactions between bonded and nonbonded pairs of electrons. This insight can be used to predict the three-dimensional structures of more complex molecules. The following figure shows the shape of higher-order AXE category molecules.

Less common higher-order AXE molecules are formed as the number of pairs of bonded or unbonded valence electrons increases.

It is useful to move away from generic molecular structures and focus on real molecules instead. Boron trifluoride (BF3) is one of the simplest molecules that we can study to better understand the VSEPR model. The central atom (A) is a single boron atom and it has three bonded valence electron pairs and zero nonbonded electron pairs.

Boron trifluoride molecules preferentially adopt a trigonal planar structure because they are AX3 group molecules. The fluorine atoms minimize the repulsive electrostatic energy in BF3 molecules by moving to the vertices of a triangle that is centered on the boron atom. There is an angle of 120 between each one of the three covalent BF bonds.

Methane molecules (CH4) are slightly more complex and are categorized as AX4 group molecules. Methane molecules contain one central carbon atom and four pairs of bonded valence electrons. The hydrogen atoms minimize the repulsive electrostatic energy in methane molecules by moving to four vertices of a three-dimensional tetrahedral structure that is centered on the carbon atom. There is an angle of 109.5 between each one of the four covalent CH bonds.

Methane and ammonium ions (NH4+) both belong to the AX4 group and they adopt similar tetrahedral-shaped structures. The molecules have four equivalent bonded pairs of valence electrons because dative bonds are fundamentally identical to “normal” two-electron covalent bonds.

Ammonia molecules are more interesting because they have three equivalent bonded pairs of valence electrons and one nonbonded lone pair of electrons. The nonbonded lone pair takes up more space in the valence shell of the nitrogen atom and this pushes the three bonded pairs (NH) closer together. The ammonia molecule ends up having a skewed trigonal pyramidal structure, and there is a relatively small angle of only 107 between the three covalent NH bonds.

Example 1: Understanding How to Classify Molecules with the AX𝑚E𝑛 Method

Using the AXE method, which of the following AXE notations corresponds to a molecule with a trigonal pyramidal shape?

  1. AXE13
  2. AX4
  3. AXE22
  4. AX3
  5. AXE3

Answer

In the AXE method, the central atom is referred to as A. The X and E terms represent bonded and nonbonded electron pairs.

An example of a molecule with a trigonal pyramidal shape would be the ammonia molecule (NH3). The central nitrogen atom has five valence electrons. Three of these valence electrons are used to make covalent bonds with hydrogen atoms. The other two valence electrons do not form any bonds. There are three bonded pairs of valence electrons and just one lone pair of electrons.

This suggests that the NH3 molecule belongs to the AXE3 group. We can use this statement to determine that option E is the correct answer for this question.

Lone pairs of electrons take up the most space in the valence shell of the central atom, and they generate the strongest repulsive electrostatic interactions in the VSEPR model. The strongest repulsive electrostatic interactions are between two lone pairs of electrons, and the weakest repulsive electrostatic interactions are between two bonded pairs of electrons. The combination of one lone pair and one bonded pair has a strength somewhere between these two extremes.

Chemists have several rules that help them understand bond angles. They state, for example, that the XAX angle will be approximately 2.5 smaller for every extra pair of nonbonded electrons that are counted on any one quaternary bonded central A atom. AX4 molecules have XAX bond angles of 109.5 and AXE3 and AXE22 molecules have XAX bond angles of 107 and 104.5.

Example 2: Determining the Molecular Shape of a Nitrite Molecule

The Lewis structure of a nitrite ion (NO2) is shown in the diagram.

NOO

What is the shape of this molecule?

  1. Tetrahedral
  2. Trigonal pyramidal
  3. Bent
  4. Trigonal planar
  5. Linear

Answer

The central atom of a nitrite ion is a nitrogen atom. Nitrogen has five valence electrons, three of which are involved in the formation of covalent bonds, leaving two as a lone electron pair.

In the VSEPR model, double bonds are considered equivalent to single bonds, meaning that there are essentially two bonded pairs of valence electrons and just one lone pair of electrons.

A stronger repulsive electrostatic interaction exists between a lone pair and a bonded pair of electrons than between two bonded pairs of electrons. This distorts the linear shape associated with an AX configuration and creates a bent or v-shaped arrangement corresponding to AXE in AXE notation.

We can use these statements to determine that option C is the correct answer for this question.

Definition: Bond Angle

Bond angles describe the time-averaged angles between two covalent bonds.

Water molecules are able to dissolve polar alcohol and carboxylic acid solutes because they have a nonlinear or “bent” structure. Water molecules have an asymmetrical distribution of electron density and they have a permanent dipole moment that runs from the plane of the hydrogen atoms through to the oxygen atom lone pairs. Lewis-structure diagrams and the VSEPR model can be used to understand the shape of water molecules.

The Lewis-structure diagram for water molecules shows that the central oxygen atom has six valence shell electrons. Two of these valence electrons are used to create two covalent (OH) bonds and the other four electrons are grouped as two lone pairs.

Water has the AXE22 group classification, and this means that the valence electrons will adopt a skewed tetrahedron structure. The two lone pairs take up more space in the oxygen valence shell than the bonded pairs, and the HOH angle will always end up being smaller than 109.5. The HOH bond angle usually has a value of 104.5 because there are two electron lone pairs and each electron lone pair decreases the XAX bond angle by approximately 2.5 from 109.5.

Example 3: Comparing the Bond Angles of Simple Molecules

Put the following molecules in order of increasing bond angle.

  1. Water
  2. Carbon dioxide
  3. Sulfur dioxide
  4. Chlorine trifluoride

  1. 1, 3, 2, 4
  2. 2, 1, 4, 3
  3. 3, 2, 1, 4
  4. 4, 1, 3, 2
  5. 2, 3, 4, 1

Answer

In order to correctly answer this question, we must first use the AXE model and assign the correct notation to each of the four molecules.

Water has two bonding pairs of electrons and two lone pairs and would have the notation AXE22. This corresponds to a nonlinear, or bent, shape with a bond angle of 104.5 caused by the increased repulsion from the two sets of lone pairs.

Carbon dioxide is two bonding pairs of electrons and no lone pairs would have the notation AX2. This corresponds to a linear shape with a bond angle of 180.

Sulfur dioxide has two bonding pairs of electrons and one lone pair, AXE2. This corresponds to another configuration of the bent shape with an angle of around 118.

Chlorine trifluoride has three bonding pairs and two lone pairs, AXE32. For chlorine trifluoride, this corresponds to a T-shaped configuration, with bond angles of 90.

By combining this information, we can see that molecule 4, chlorine trifluoride, has the smallest bond angle, followed by molecule 1, water. The third-largest bond angle corresponds to the molecule 3 and the largest bond angle is that of carbon dioxide, molecule 2. The order of increasing bond angles is therefore 4, 1, 3, 2, and the correct answer is D.

The oxygen atom has two electron-rich lone pairs, and this partly explains why it can form hydrogen bonds with other molecules. The other reason is that oxygen atoms are highly electronegative. The oxygen atom attracts a disproportionate amount of electron density from the two OH covalent bonds and the water molecule becomes highly polarized. The hydrogen atoms end up being depleted of electron density (𝛿+) and the oxygen atoms become enriched with electron density (𝛿). The VSEPR model shows that the water molecule will always adopt a skewed tetrahedron structure. There will always be an electric dipole moment that runs from the plane of the hydrogen atoms through to the oxygen atom lone pairs. Water molecules will also have a permanent dipole moment and they will always be able to form hydrogen bonds with other molecules.

Definition: Electronegativity

Electronegativity quantifies the tendency of an atom to attract a bonding pair of electrons.

Let us summarize what we have learned in this explainer.

Key Points

  • Lewis-structure diagrams show bonded and nonbonded electron pairs.
  • The VSEPR model can predict the shape of polyatomic molecules and ions.
  • Each AXE method category has an associated molecular geometry.
  • Lone pairs have stronger repulsive electrostatic interactions than bonded pairs.
  • Water molecules have a permanent dipole moment and can form hydrogen bonds.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.