4.3: Molecular Shapes- The VSEPR Theory
- Page ID
- 288463
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Determine the shape of simple molecules and polyatomic ions.
Molecules have shapes. There is an abundance of experimental evidence for that from their physical properties to their chemical reactivity. Small molecules, molecules with a single central atom, have shapes that can be easily predicted. Valence shell electron pair repulsion (VSEPR) is the theory of molecular shapes. It says that electron pairs, being composed of negatively charged particles, repel each other to get as far away from each other as possible. VSEPR makes a distinction between electron group geometry, which expresses how electron groups (bonds and nonbonding electron pairs) are arranged, and molecular geometry, which expresses how the atoms in a molecule are arranged. However, the two geometries are related.
There are two types of electron groups: any type of bond (single, double, or triple) and lone electron pairs. When applying VSEPR to simple molecules, the first thing to do is to count the number of electron groups around the central atom. Remember that a multiple bond counts as only one electron group.
- How many electron groups are there around the carbon atom in HCN?
- How many of the groups are bonding groups and how many are nonbonding groups?
Solution
- Look at the electrons immediately next to the C atom. It has a single bond and a triple bond. This is two electron groups. Note 1: Carbon always has four bonds, but it does not always have four electron groups. Double and triple bonds each count as one electron group. Note 2: The lone pair on the N atom does not count because the question asked about the carbon atom.
- In HCN carbon has two bonding groups. Any bond (single, double, triple) counts as a bonding group. Lone pairs are nonbonding groups.
- How many electron groups are there around the carbon atom in ethanol? How many of the groups are bonding groups and how many are nonbonding groups?
- How many electron groups are there around the oxygen atom in ethanol? How many of the groups are bonding groups and how many are nonbonding groups?
- Answer
-
- The carbon atom has four electron groups. All four are bonding groups.
- The oxygen atom also has four electron groups, but two are bonding groups and two are nonbonding groups. Note: The C-O bond is shared between the carbon and oxygen atoms and it counts as an electron group for both of them.
Any molecule with only two atoms is linear. A molecule whose central atom contains only two electron groups orients those two groups as far apart from each other as possible: 180° apart. When the two electron groups are 180° apart, the atoms attached to those electron groups are also 180° apart, so the overall molecular shape is linear. An example is carbon dioxide, CO2:
Figure \(\PageIndex{1}\) Carbon dioxide Lewis Structure and 3D ball-and-stick model.
A molecule with three electron groups orients the three groups as far apart as possible. They adopt the positions of an equilateral triangle: 120° apart and in a plane. The shape of such molecules is trigonal planar. An example is formaldehyde, CH2O, in Figure \(\PageIndex{2}\). The central C atom has three electron groups around it because the double bond counts as one electron group. The three electron groups repel each other to adopt a trigonal planar shape.
Figure \(\PageIndex{2}\) Formaldehyde, CH2O, Lewis Structure and 3D ball-and-stick model.
A formaldehyde molecule will not be a perfect equilateral triangle because the C–O double bond is different from the two C–H bonds, but both planar and triangular describe the appropriate approximate shape of this molecule.
A molecule with four electron groups about the central atom orients the four groups in the direction of a tetrahedron, as shown in Figure \(\PageIndex{3}\) Tetrahedral Geometry. If there are four atoms attached to these electron groups, then the molecular shape is also tetrahedral. Methane (CH4) is an example.
You don't need to be an artist to communicate the 3D shapes of molecules. Figure \(\PageIndex{4}\) illustrates the standard convention of displaying a three-dimensional molecule on a two-dimensional surface. The straight lines are in the plane of the page, the solid wedged line is coming out of the plane toward the reader, and the dashed wedged line is going out of the plane away from the reader.
NH3 is an example of a molecule whose central atom has four electron groups but only three of them are bonded to surrounding atoms.
Although the electron groups are oriented in the shape of a tetrahedron, from a molecular geometry perspective, the shape of NH3 is trigonal pyramidal.
H2O is an example of a molecule whose central atom has four electron groups but only two of them are bonded to surrounding atoms.
Although the electron groups are oriented in the shape of a tetrahedron, the shape of the molecule is bent or angular. A molecule with four electron groups about the central atom but only one electron group bonded to another atom is linear because there are only two atoms in the molecule.
Figure \(\PageIndex{7}\) illustrates several representations of the water, ammonia, and methane molecules. The dash-wedge structures clearly show the bonds and chemical symbols. The ball-and-stick models look similar to plastic model kits that are commonly used in chemistry classes. The space-filling models in which atoms are not perfect spheres and touch one another rather than being attached by sticks are the most realistic.
| Number of Electron Groups on Central Atom | Number of Bonding Groups | Number of Lone Pairs | Electron Geometry | Approximate Bond Angles | Molecular Shape |
|---|---|---|---|---|---|
| 2 | 2 | 0 | linear | 180° | linear |
| 3 | 3 | 0 | trigonal planar | 120° | trigonal planar |
| 3 | 2 | 1 | trigonal planar | 120° | bent |
| 4 | 4 | 0 | tetrahedral | 109.5° | tetrahedral |
| 4 | 3 | 1 | tetrahedral | 109.5° | trigonal pyramidal |
| 4 | 2 | 2 | tetrahedral | 109.5° | bent |
What is the molecular shape of each molecule?
- PCl3
- NOF
Solution
The first step is to draw the Lewis structure of the molecule. For PCl3, the electron dot diagram is as follows:
The lone electron pairs on the Cl atoms are omitted for clarity. The P atom has four electron groups with three of them bonded to surrounding atoms, so the molecular shape is trigonal pyramidal.
The electron dot diagram for NOF is as follows:
The N atom has three electron groups on it, two of which are bonded to other atoms. The molecular shape is bent.
What is the molecular shape of CH2Cl2?
- Answer
-
Tetrahedral
Ethylene (C2H4) has two central atoms. Determine the geometry around each central atom and the shape of the overall molecule.
- Answer
-
Trigonal planar about both central C atoms
When there are multiple central atoms we determine the molecular geometry for each of them individually.
- What are the bond angles and molecular geometry around the carbon atom in ethanol?
- What are the bond angles and molecular geometry around the oxygen atom in ethanol?
Solution
- The C atom has four electron groups so it has bond angles of approximately 109.5°. All four groups are bonding groups so the molecular geometry is tetrahedral.
- The O atom also has four electron groups so it has bond angles of approximately 109.5°. However, two group are bonding groups and two groups are nonbonding so its molecular geometry is bent.
The Lewis structure for alanine, an amino acid, is shown below.
List the molecular geometries and bond angles for each of the four atoms in the middle (N, C, C, O).
- Answer
-
- N has three single bonds and one lone pair. It's molecular geometry is trigonal pyramidal and its angles are approximately 109.5°.
- The C on the left has four single bonds. It's molecular geometry is tetrahedral and its angles are approximately 109.5°.
- The C on the right has two singles bonds and a double bond. That is three bonding electron groups resulting in a molecular geometry of trigonal planar. Its bond angles are approximately 120°.
- The O atom on the right has two single bonds and two lone pairs so its molecular geometry is bent and its bond angles are approximately 109.5°.
Summary
The approximate shape of a molecule can be predicted from the number of electron groups and the number of surrounding atoms.
Contributors and Attributions
- TextMap: Beginning Chemistry (Ball et al.)
Henry Agnew (UC Davis)