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VSEPR Model

To understand molecular behavior, it is important to be able to describe bonds in terms of their distances, angles, and relative arrangements in space (Figure 1).

Two structures of formaldehyde (C H subscript 2 O) are shown. In the one at the left, the carbon atom is in the middle and the 2 H atoms and the 1 O atom are arranged around the carbon atom in a trigonal planar geometry. There are single bonds between the carbon and the hydrogen atoms and then a double bond between the carbon and oxygen atom. The oxygen atom also has two lone pairs. The bond angles are labeled. The O-C-H bond angles are 122.4 degrees and the H-C-H bond angle is 115.4 degrees. In the structure on the right, we have a ball and stick model that shows the black carbon sphere in the middle attached to two white hydrogen spheres and double bonded to a red oxygen sphere. A chart to the right indicates that the C-H bond length is 1.110 Angstroms and the C double bond O bond length is 1.207 Angstroms.
Figure 1. Formaldehyde depicted by a Lewis structure (left), ball and stick model (center), and a comparison of bond lengths (right).

A bond angle is the angle between any two bonds that include a common atom, usually measured in degrees. A bond distance (or bond length) is the distance between the nuclei of two bonded atoms along the straight line joining the nuclei. Bond distances are typically measured in Ångstroms (1 Å = 10–10 m) or picometers (1 pm = 10–12 m, 100 pm = 1 Å).

Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular geometry, including approximate bond angles around a central atom, of a molecule from an examination of the number of bonds and lone electron pairs in its Lewis structure. The VSEPR model assumes that electron pairs in the valence shell of a central atom will adopt an arrangement that minimizes repulsions between these electron pairs by maximizing the distance between them. The electrons in the valence shell of a central atom form either bonding pairs of electrons, located primarily between bonded atoms, or lone pairs. The electrostatic repulsion of these electrons is reduced when the various regions of high electron density assume positions as far from each other as possible.

VSEPR theory predicts the arrangement of electron pairs around each central atom and, usually, the correct arrangement of atoms in a molecule using only electron pair repulsions. This neglects other important factors which contribute to electronic and molecular geometries such as orbital mixing, π conjugation, hyperconjugation, dipole-dipole interactions, hydrogen-bonding, etc. Many of these other factors will be explored in this course and future chemistry courses.

A Lewis structure is shown. A fluorine atom with three lone pairs of electrons is single bonded to a beryllium atom which is single bonded to a fluorine atom with three lone pairs of electrons. The angle of the bonds between the two fluorine atoms and the beryllium atom is labeled, “180 degrees.”As a simple example of VSEPR theory, let us predict the structure of a gaseous BeF2 molecule. The Lewis structure of BeF2 shows only two electron pairs around the central beryllium atom. With two bonds and no lone pairs of electrons on the central atom, the bonds are as far apart as possible, and the electrostatic repulsion between these regions of high electron density is reduced to a minimum when they are on opposite sides of the central atom. The F-Be-F bond angle is 180° in the BeF2 molecule resulting in a linear geometry.

Figure 2 illustrates this and other electron-pair geometries that minimize the repulsions among regions of high electron density (bonds and/or lone pairs). Two regions of electron density around a central atom in a molecule form a linear geometry; three regions form a trigonal planar geometry; four regions form a tetrahedral geometry; five regions form a trigonal bipyramidal geometry; and six regions form an octahedral geometry.

A table with four rows and six columns is shown. The header column contains the phrases, “Number of regions,” “Spatial arrangement,” “Wedge/dash Notation,” and “Electron pair Geometry.” The first row reads: “Two regions of high electron density ( bonds and/or unshared pairs )”, “Three regions of high electron density ( bonds and/or unshared pairs ),” “Four regions of high electron density ( bonds and/or unshared pairs ),” “Five regions of high electron density ( bonds and/or unshared pairs ),” and “Six regions of high electron density ( bonds and/or unshared pairs ).” The second row shows diagrams of orbitals. The first image shows two oval-shaped orbs with an arrow indicating an angle of 180 degrees. The second image shows three oval-shaped orbs with an arrow indicating an angle of 120 degrees. The third image shows four oval-shaped orbs with an arrow indicating an angle of 109.5 degrees. The fourth image shows five oval-shaped orbs with an arrow indicating an angle of 90 and 120 degrees. The fifth image shows six oval-shaped orbs with an arrow indicating an angle of 90 degrees. The third row contains Lewis structures. The first structure shows a beryllium atom single bonded to two hydrogen atoms. The second structure shows a boron atom single bonded to three hydrogen atoms. The third structure shows a carbon atom single bonded to four hydrogen atoms. The fourth structure shows a phosphorus atom single bonded to five fluorine atoms. The fifth structure shows a sulfur atom single bonded to six fluorine atoms. The fourth row contains the phrases “Linear; 180 degree angle,” Trigonal Planar; all angles 120 degrees,” “Tetrahedral; all angles 109.5 degrees,” “Trigonal bipyramidal; angles of 90 degrees and 120 degrees. An attached atom may be equatorial, ( in the plane of the triangle ), or axial, ( above the plane of the triangle ),” and “Octahedral; 90 degrees or 180 degrees.”
Figure 2. The basic electron-pair geometries predicted by VSEPR theory maximize the space around any region of electron density (bonds or lone pairs).

Electron-pair Geometry vs. Molecular Geometry

It is important to note that electron-pair geometry around a central atom is not the same thing as its molecular geometry. The electron-pair geometries shown in Figure 2 describe all regions where electrons are located, bonds as well as lone pairs. Molecular geometry describes the location of the atoms, not the lone pairs. Even though molecular geometry does not describe the location of the lone pairs, the lone pairs still influence the location of the atoms and this influence is still taken into account.

We differentiate between these two situations by naming the geometry that includes all electron pairs the electron-pair geometry. The structure that includes only the placement of the atoms in the molecule is called the molecular geometry. The electron-pair geometries will be the same as the molecular geometry when there are no lone electron pairs around the central atom, but they will be different when there are lone pairs present on the central atom.

For example, the methane molecule, CH4, which is the major component of natural gas, has four bonding pairs of electrons (i.e., four regions of electron density) around the central carbon atom; the electron-pair geometry is tetrahedral, as is the molecular geometry.

Four representations of methane are shown. In the one at the left, the molecular formula is written as C H subscript 4. Then there is a right facing arrow to the Lewis structure. In the Lewis structure, the carbon atom has four single bonds, one to each of four hydrogen atoms. These hydrogen atoms are arranged on top, bottom, left and right of the carbon atom. Then there is a right facing arrow to a structure labeled “Electron-pair geometry: tetrahedral”. In this structure, the carbon is in the center and there is a wedge and dash structure showing the 4 hydrogen atoms at the corners of the tetrahedron. Then there is a right facing arrow to a structure labeled “Molecular geometry: tetrahedral”. This structure is a ball and stick model that shows a screenshot of the geometry as seen on Web M O. There is a black sphere in the middle, then the bonds are represented as sticks and there are white balls at the end of each of 4 sticks, for the 4 hydrogen atoms.

The ammonia molecule, NH3, also has four electron pairs associated with the nitrogen atom (i.e., four regions of electron density), and thus has a tetrahedral electron-pair geometry. One of these regions, however, is a lone pair, which is not included in the molecular geometry, though it does influence the shape of the molecule.

Four representations of ammonia are shown. In the one at the left, the molecular formula is written as N H subscript 3. Then there is a right facing arrow to the Lewis structure. In the Lewis structure, the nitrogen atom has a single bond to each of three hydrogen atoms. There is also a lone pair on the nitrogen. These hydrogen atoms are arranged to the bottom, left and right of the nitrogen atom and the lone pair is on top. Then there is a right facing arrow to a structure labeled “Electron-pair geometry: tetrahedral”. In this structure, the nitrogen is in the center and there is a wedge and dash structure showing the 3 hydrogen atoms at the corners of the tetrahedron. The fourth corner of the tetrahedron is occupied by the lone pair. Then there is a right facing arrow to a structure labeled “Molecular geometry: trigonal pyramidal”. This structure is a ball and stick model that shows a screenshot of the geometry as seen on Web M O. There is a blue sphere in the middle for nitrogen, then the bonds are represented as sticks and there are white balls at the end of each of 3 sticks, for the 3 hydrogen atoms. The lone pair is not represented in this structure.

In the ammonia molecule, the three hydrogen atoms attached to the central nitrogen are not arranged in a flat, trigonal planar molecular geometry, but rather in a three-dimensional trigonal pyramid with the nitrogen atom at the apex and the three hydrogen atoms forming the base. The ideal bond angles in a trigonal pyramid are based on the tetrahedral electron pair geometry. Small bond angle distortions occur due to the lone electron pair on nitrogen. The H–N–H bond angles in NH3 are slightly smaller than the 109.5° angle in a regular tetrahedron.

Figure 3 illustrates the ideal molecular geometries, which are predicted based on the electron-pair geometries for various combinations of lone pairs and bonding pairs. Because wedge and dash structures are often confusing at first, we also provide a typical representation of a wedge and dash structure for each combination. While there are other ways that you could rotate the molecule in space and different placements of the wedges and dashes that would also be considered correct, these depictions have been chosen to be the most straightforward to draw and interpret.

Figure 3.  VSEPR-predicted molecular and electron-pair geometries. The molecular geometries are identical to the electron-pair geometries when there are no lone pairs present. A, central atom; X, peripheral atom; E, unshared electron pair.
Example 1: Predicting Electron-pair Geometry and Molecular Geometry: Lone Pairs on the Central Atom

Predict the electron-pair geometry and molecular geometry of a water molecule.

Solution

Two representations of water are shown. In the Lewis structure on the left, the oxygen atom contains two lone pairs and then has a single bond to each of two hydrogen atoms. The label beneath reads “electron-pair geometry: tetrahedral”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a red sphere in the middle for oxygen and then two white spheres in a bent geometry for the hydrogen atoms. The lone pairs are not represented in this structure. The label beneath it reads “molecular geometry: bent”.The Lewis structure of H2O indicates that there are four regions of high electron density around the oxygen atom: two lone pairs and two chemical bonds:

We predict that these four regions are arranged in a tetrahedral (Figure 3) fashion to minimize the electron repulsion, as shown. Thus, the electron-pair geometry is tetrahedral and the molecular geometry is bent with an angle slightly less than 109.5°. In fact, the bond angle is predicted to be 103.7°.

Check Your Learning

The hydronium ion, H3O+, forms when acids are dissolved in water. Predict the electron-pair geometry and molecular geometry of this cation.

Answer

electron pair geometry: tetrahedral; molecular geometry: trigonal pyramidal

According to VSEPR theory, the terminal atom locations (“X” in Figure 3) are equivalent within the linear, trigonal planar, and tetrahedral electron-pair geometries. It does not matter which X is replaced with a lone pair because the molecules can be rotated to convert positions.

For trigonal bipyramidal electron-pair geometries, however, there are two distinct X positions, as shown in Figure 3: an axial position (if we hold a model of a trigonal bipyramid by the two axial positions, we have an axis around which we can rotate the model) and an equatorial position (three positions form an equator around the middle of the molecule). As shown in Figure 3, the axial position is surrounded by bond angles of 90°, whereas the equatorial position has more space available because of the 120° bond angles. In a trigonal bipyramidal electron-pair geometry, lone pairs or larger atoms or groups tend to occupy equatorial positions rather than axial positions because the equatorial positions are more spacious and minimize electron repulsion.

Theoretically, we can come up with three possible arrangements for the three bonds and two lone pairs for the ClF3 molecule (Figure 4). The stable structure is the one that puts the lone pairs in equatorial locations, giving a T-shaped molecular geometry.

Four sets of images are shown and labeled, “a,” “b,” “c,” and “d.” Each image is separated by a dashed vertical line. Image a shows a six-faced, bi-pyramidal structure where the central vertical axis is labeled, “Axial,” and the horizontal plane is labeled, “Equatorial.” Image b shows a pair of diagrams in the same shape as image a, but in these diagrams, the left has a chlorine atom in the center while the right has a chlorine atom in the center, two fluorine atoms on the upper and lower ends, and one fluorine in the left horizontal position. Image c shows a pair of diagrams in the same shape as image a, but in these diagrams, the left has a chlorine atom in the center while the right has a chlorine atom in the center and three fluorine atoms in each horizontal position. Image d shows a pair of diagrams in the same shape as image a, but in these diagrams, the left has a chlorine atom in the center while the right has a chlorine atom in the center, two fluorine atoms in the horizontal positions, and one in the axial bottom position.
Figure 4. (a) In a trigonal bipyramid, the two axial positions are located directly across from one another, whereas the three equatorial positions are located in a triangular arrangement. (b–d) The two lone pairs (red lines) in ClF3 have several possible arrangements, but the T-shaped molecular geometry (b) is the one actually observed, consistent with the larger lone pairs both occupying equatorial positions.

Two representations of trifluorochloride are shown. In the Lewis structure on the left, the chlorine atom is in the middle and has 2 lone pairs. There are three fluorine atoms arranged around the chlorine atom, one on top, one on bottom, and one to the right. There are single bonds between the Cl and F atoms. The lone pairs are shown on each fluorine atom to complete the octet. The label beneath the structure reads “electron-pair geometry: trigonal bipyramidal”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a dark green sphere in the middle for the chlorine atom. Then there are light green spheres to the top, right, and bottom of the chlorine atom for each of the fluorine atoms. The bond angles are shown. The bond angle between the F on top to the Cl to the F on bottom is 180.7 degrees. The bond angle between the F on top to the Cl to the F to the right is 87.1 degrees. The bond angle between the F on bottom to the Cl to the F on the right is 87.1 degrees.When a central atom has two lone electron pairs and four bonding regions, we have an octahedral electron-pair geometry. The two lone pairs are on opposite sides of the octahedron (180° apart), giving a square planar molecular geometry that minimizes lone pair-lone pair repulsions (Figure 3).

Predicting Electron-pair Geometry and Molecular Geometry

The following procedure uses VSEPR theory to determine the electron pair geometries and the molecular geometries:

  1. Write the Lewis structure of the molecule or polyatomic ion.
  2. Count the number of regions of electron density (lone pairs and bonds) around the central atom. A single, double, or triple bond counts as one region of electron density.
  3. Identify the electron-pair geometry based on the number of regions of electron density: linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral (Figure 3).
  4. Use the number of lone pairs to determine the molecular geometry (Figure 3). If more than one arrangement of lone pairs and chemical bonds is possible, choose the one that will minimize repulsions, remembering that lone pairs occupy more space than multiple bonds, which occupy more space than single bonds. In trigonal bipyramidal arrangements, repulsion is minimized when every lone pair is in an equatorial position. In an octahedral arrangement with two lone pairs, repulsion is minimized when the lone pairs are on opposite sides of the central atom.

The following examples illustrate the use of VSEPR theory to predict the molecular geometry of molecules or ions that have no lone pairs of electrons. In this case, the molecular geometry is identical to the electron pair geometry.

Example 2: Predicting Electron-pair Geometry and Molecular Geometry: CO2 and BH3

Predict the electron-pair geometry and molecular geometry for each of the following:

  1. carbon dioxide, CO2
  2. borane, BH3

Solution

Three separate resonance structures for CO2 are displayed left to right. The left structure has the central carbon double bonding to each of the two periphery oxygen atoms. The formal charges for each of these atoms is zero. The central resonance structure has the central carbon triple bonding to the oxygen on the left, and single bonding to the oxygen on the right. The formal charges for these atoms are +1, 0, and -1 going left to right. The rightmost resonance structure has the central carbon triple bonding to the oxygen on the right and single bonding to the oxygen on the left.(a) We write the Lewis structure of CO2 as:

This shows us two regions of high electron density around the carbon atom—each double bond counts as one region, and there are no lone pairs on the carbon atom. Using VSEPR theory, we predict that the two regions of electron density arrange themselves on opposite sides of the central atom with a bond angle of 180°. The electron-pair geometry and molecular geometry are identical, and CO2 molecules are linear.

Two representations of B H subscript 3 are shown. In the Lewis structure on the left, the boron atom is in the middle. There are three hydrogen atoms arranged around the boron atom in a trigonal planar geometry. There are single bonds between the B and H atoms. The label beneath the structure reads “electron-pair geometry: trigonal planar”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a light pink sphere in the middle for the boron atom. Then there are white spheres arranged in a trigonal planar geometry for the hydrogen atoms. The label beneath the structure reads “molecular geometry: trigonal planar”.(b) We write the Lewis structure of BH3 as:

Thus we see that BH3 contains three bonds, and there are no lone pairs of electrons on boron. The arrangement of three regions of high electron density gives a trigonal planar electron-pair geometry. The B–H bonds lie in a plane with 120° angles between them. BH3 also has a trigonal planar molecular geometry.

Check Your Learning

Carbonate, CO32−, is a common polyatomic ion found in various materials from eggshells to antacids. What are the electron-pair geometry and molecular geometry of this polyatomic ion?

Two representations of carbonate are shown. In the Lewis structure on the left, the carbon atom is in the middle surrounded by 3 oxygen atoms. There are single bonds to each of two oxygen atoms and then a double bond between the carbon and a third oxygen atom. The lone pairs are shown on the oxygen atoms to complete the octets. There are brackets around the entire structure and a superscript negative 2. The label reads “electron-pair geometry: trigonal planar”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a black sphere in the middle for the carbon atom. Then there are three red spheres arranged in a trigonal planar geometry around the carbon. The label beneath the structure reads “molecular geometry: trigonal planar”.Answer

The electron-pair geometry is trigonal planar and the molecular geometry is trigonal planar. Due to resonance, all three C–O bonds are identical and the bond angles are all 120°. Whether they are single, double, or an average of the two, each bond counts as one region of electron density.

Example 3: Predicting Electron-pair Geometry and Molecular Geometry: Ammonium

Two of the top 50 chemicals produced in the United States, ammonium nitrate and ammonium sulfate, both used as fertilizers, contain the ammonium ion. Predict the electron-pair geometry and molecular geometry of the NH4+ cation.

Solution

A Lewis structure shows a nitrogen atom single bonded to four hydrogen atoms. The structure is surrounded by brackets with a superscripted positive sign.We write the Lewis structure of NH4+ as:

Two representations of ammonium are shown. In the Lewis structure on the left, the nitrogen atom is in the middle surrounded by 4 hydrogen atoms. There are single bonds between the nitrogen and each hydrogen atom. The hydrogen atoms are arranged in a tetrahedral geometry. There are brackets around the entire structure and a superscript plus sign. The label reads “electron-pair geometry: tetrahedral”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a blue sphere in the middle for the nitrogen atom. Then there are four white spheres arranged in a tetrahedral around the nitrogen. The label beneath the structure reads “molecular geometry: tetrahedral”.We can see that NH4+ contains four bonds from the nitrogen atom to hydrogen atoms and no lone pairs. We expect the four regions of high electron density to arrange themselves so that they point to the corners of a tetrahedron with the central nitrogen atom in the middle (Figure 3). Therefore, the electron pair geometry of NH4+ is tetrahedral, and the molecular geometry is also tetrahedral.

Check Your Learning

Identify a molecule with trigonal bipyramidal molecular geometry.

Answer

Any molecule with five electron pairs around the central atoms including no lone pairs will be trigonal bipyramidal. PF5 is a common example.

The next several examples illustrate the effect of lone pairs of electrons on molecular geometry.

Example 4: Predicting Electron-pair Geometry and Molecular Geometry: SF4

Sulfur tetrafluoride, SF4, is extremely valuable for the preparation of fluorine-containing compounds used as herbicides (i.e., SF4 is used as a fluorinating agent). Predict the electron-pair geometry and molecular geometry of a SF4 molecule.

Solution

A Lewis diagram depicts a sulfur atom with one lone pair of electrons single bonded to four fluorine atoms, each with three lone pairs of electrons.The Lewis structure of SF4 indicates five regions of electron density around the sulfur atom: one lone pair and four bonding pairs:

We expect these five regions to adopt a trigonal bipyramidal electron-pair geometry. In the typical fashion, the lone pair occupies one of the equatorial positions, which leads to a trigonal bipyramidal electron-pair geometry and the molecular geometry is that of a seesaw (Figure 3).
Four representations of sulfur tetrafluoride are shown. In the one at the left, the molecular formula is written as S F subscript 4. Then there is a right facing arrow to the Lewis structure. In the Lewis structure, the sulfur atom is in the middle with one lone pair to the left. The fluorine atoms are arranged on top, bottom and to the right. There are two fluorine atoms on the right of the sulfur, one has a wedge and one has a dash. These are all single bonds between sulfur and fluorine. There is also an image of the sulfur atom at the center of an octahedron, showing that there is a fluorine on top and bottom in the axial positions and then two in the equatorial positions around the center. The label beneath says “electron-pair geometry: trigonal bipyramidal”. Then there is a right facing arrow to a structure that is a ball and stick model that shows a screenshot of the geometry as seen on Web M O. There is a yellow sphere in the middle for sulfur, then the bonds are represented as sticks and there are light green balls at the end of each of 4 sticks, for the 4 fluorine atoms. The bond angle between the F on top the S and the F on bottom is labeled as 188.45 degrees. The label beneath reads “molecular geometry: seesaw”.

Check Your Learning

Predict the electron pair geometry and molecular geometry for molecules of XeF2.

Answer

The electron-pair geometry is trigonal bipyramidal. The molecular geometry is linear.

Example 5: Predicting Electron-pair Geometry and Molecular Geometry: XeF4

Of all the noble gases, xenon is the most reactive, frequently reacting with elements such as oxygen and fluorine. Predict the electron-pair geometry and molecular geometry of the XeF4 molecule.

Solution

A Lewis structure depicts a xenon atom with two lone pairs of electrons that is single bonded to four fluorine atoms, each with three lone pairs of electrons.The Lewis structure of XeF4 indicates six regions of high electron density around the xenon atom: two lone pairs and four bonds:

Two diagrams are shown and labeled, “a” and “b.” Diagram a shows a xenon atom in the center of an eight-sided octahedral shape. Diagram b shows the same image as diagram a, but this time there are fluorine atoms located at the four corners of the shape in the horizontal plane. They are connected to the xenon by single lines.These six regions adopt an octahedral arrangement (Figure 3), which is the electron-pair geometry. To minimize repulsions, the lone pairs should be on opposite sides of the central atom. The five atoms are all in the same plane and have a square planar molecular geometry.

Two representations of Xenon tetrafluoride are shown. In the Lewis structure on the left, the xenon atom is in the middle with two lone pairs. There is a single bond to each of 4 fluorine atoms arranged on top, bottom, left and right. The octets of the fluorine atoms have been filled with lone pairs. The label beneath reads “electron-pair geometry: octahedral”. The structure on the right is a ball and stick model that is a screenshot of the geometry as seen on Web M O. There is a teal sphere in the middle for the xenon atom. Then there are four light green spheres arranged in a square planar geometry. The label beneath the structure reads “molecular geometry: square planar”.

Check Your Learning

In a certain molecule, the central atom has three lone pairs and two bonds. What will the electron pair geometry and molecular geometry be?

Answer

electron pair geometry: trigonal bipyramidal; molecular geometry: linear

Molecular Geometry for Multi-center Molecules

When a molecule or polyatomic ion has only one central atom, the molecular geometry completely describes the shape of the molecule. Larger molecules do not have a single central atom, but are connected by a chain of interior atoms that each possess a “local” geometry. The way these local structures are oriented with respect to each other also influences the molecular shape, but such considerations are largely beyond the scope of this introductory discussion. For our purposes, we will only focus on determining the local structures.

Example 6: Predicting Structure in Multicenter Molecules

A Lewis structure depicts a nitrogen atom with one lone pair of electrons that is single bonded to two hydrogen atoms and a carbon atom, which is, in turn, single bonded to two hydrogen atoms and another carbon atom. This carbon atom is double bonded to an oxygen atom with two lone pairs of electrons and single bonded to an oxygen that has two lone pairs of electrons and a single bond to a hydrogen atom.The Lewis structure for the simplest amino acid, glycine, H2NCH2CO2H, is shown here. Predict the local geometry for the nitrogen atom, the two carbon atoms, and the oxygen atom with a hydrogen atom attached:

Solution

Consider each central atom independently. The electron-pair geometries:

  • nitrogen––four regions of electron density, tetrahedral
  • carbon (CH2)––four regions of electron density, tetrahedral
  • carbon (CO2)—three regions of electron density, trigonal planar
  • oxygen (OH)—four regions of electron density, tetrahedral

A Lewis structure depicts a nitrogen atom with one lone pair of electrons that is single bonded to two hydrogen atoms and a carbon atom. The atoms described are drawn with bonds that indicate a three-dimensional, tetrahedral shape around the nitrogen atom. The carbon is, in turn, single bonded to two hydrogen atoms and another carbon atom, and again, a tetrahedral, three dimensional configuration is indicated by the types of bonds. This second carbon atom is double bonded to an oxygen atom and single bonded to an oxygen that has two lone pairs of electrons and a single bond to a hydrogen atom.

The local structures:

  • nitrogen––three bonds, one lone pair, trigonal pyramidal
  • carbon (CH2)—four bonds, no lone pairs, tetrahedral
  • carbon (CO2)—three bonds (double bond counts as one region of electron density), no lone pairs, trigonal planar
  • oxygen (OH)—two bonds, two lone pairs, bent (109°)

Check Your Learning

A Lewis structure depicts a nitrogen atom with two lone pairs of electrons that is single bonded to two hydrogen atoms and a carbon atom, which is, in turn, single bonded to a hydrogen atom, a methyl group and another carbon atom. This carbon atom is single bonded to an oxygen atom with two lone pairs of electrons and single bonded to an oxygen that has two lone pairs of electrons and a single bond to a hydrogen atom.Another amino acid is alanine, which has the Lewis structure shown here. Predict the electron-pair geometry and local structure of the nitrogen atom, the three carbon atoms, and the oxygen atom with hydrogen attached:

Answer:

electron-pair geometries:

  • nitrogen—tetrahedral
  • carbon (CH)—tetrahedral
  • carbon (CH3)—tetrahedral
  • carbon (CO2)—trigonal planar
  • oxygen (OH)—tetrahedral

local structures:

  • nitrogen—trigonal pyramidal
  • carbon (CH)—tetrahedral
  • carbon (CH3)—tetrahedral
  • carbon (CO2)—trigonal planar
  • oxygen (OH)—bent

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