What is the molecular geometry of a molecule that has 3 electron pairs on the central atom all of them are bonds?

The content that follows is the substance of General Chemistry Lecture 33. In this lecture we introduce the shapes and reasoning behind VSEPR.

Valence Shell Electron Pair Repulsion

Valence shell electron pair repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. The theory centers around the idea that the clouds of electrons that surround nuclei and flow in the bonds that link atoms together repulse each other. Therefore the structures that form from the repulsions will be those that maximize the distances between the atoms and their bonds.

The Theory uses the letter A to represent central atoms and X to represent peripheral atoms and E to represent lone pairs to describe the structure of the molecule.

For Example: The ClO3- Ion has the following structure:

There is one central atom (Cl) and 3 peripheral Oxygen atoms and one lone pair of electrons so the VSEPR code would be AX3E.

Electron Pair Geometry and Molecular Geometry

Electron Pair Geometry (EPG) is the arrangement of the electrons both bonded and lone pairs around a central atom. The Molecular Geometry (MG) is the shape we can "see" of the molecule. This means that the Molecular Geometry only describes the bonds of the molecule and does not describe the lone pair electrons. Electrons do not have sufficient mass for us to be able to see them in the various spectroscopies used to determine molecular structures. We know they are present due to the maintenance of the Electron Pair Geometry but since we can't see them we give a separate name to the structures that have lone pairs present.

Let's go through each of the possible arrangements of electrons around a central atom and discuss both the EPG and MG for each.

AX2

A Central atom with only two other atoms bound to it will form a linear structure in order to maximize the distance between its bonds:

The angle between the bonds is therefore 180 degrees. If you were to remove one of the atoms forming an AXE type of conformation the structure would remain linear as there is no other structure that would increase the distance between bond and lone pair.

AX3 and AX2E

When there are 3 bonds or lone pairs to the central atom, the EPG and the MG is called Trigonal Planar:

The bond angles that maximize the distance between the atoms is 120o much like a Mercedes Benz symbol.

If we take away an atom and the lone pair of electrons remains, the EPG structure remains trigonal planar but we can no longer see one of the "legs" of the trigonal planar structure and thus we have to rename the MG to reflect the new look:

Note that while the structure (MG) that we see does not show the electron pair (Far right above) the shape of the molecule indicates that they are present since if they were not there the stucture with only two bonds would straighten out into a linear form. Because the electrons are present they keep the structure "bent" down as shown and thus this is what the Molecular Geometry of an AX2E structure is called: Bent.

If we were to remove another bond leaving the lone pair there again, the EPG again would remain trigonal planar but all we would see is the single bond and thus the MG would be linear.

AX4, AX3E, and AX2E2

When there are 4 bonds to the central atom, AX4, the EPG and MG are called Tetrahedral:

As with the previous structures, as we remove bonds but leave the lone pair electrons in place, the EPG stays the same but the MG changes. For a structure with 3 bonds and 1 lone pair, the EPG is still Tetrahedral but the MG becomes Trigonal Pyramidal (AX3E):

Note that because the electrons in the lone pair are less structured, they influence the structure by repulsing the lower bonds such that the angles are slightly smaller than the normal 109.5 degrees.

If we remove another bond and leave the lone pair of electrons again we find the MG structure is Bent (AX2E2).

Note that again that the presence of lone pairs not in bonds creates repulsion that narrows the bond angle from 109.5 to 104.5.

Any further removal of bonds will leave a linear molecular geometry.

We continue the discussion of VSEPR in the next lecture for those atoms that can exceed the octet rule.

So in summary:

And for practice:

Learning Objective

1. Determine the shape of simple molecules.

Molecules have shapes. There is an abundance of experimental evidence to that effect—from their physical properties to their chemical reactivity. Small molecules—molecules with a single central atom—have shapes that can be easily predicted.

The basic idea in molecular shapes is called valence shell electron pair repulsion (VSEPR)The general concept that estimates the shape of a simple molecule.. It basically 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 groupsA covalent bond of any type or a lone electron pair.: 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.

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. Examples include BeH2 and CO2:

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 BF3:

Some substances have a trigonal planar electron group distribution but have atoms bonded to only two of the three electron groups. An example is GeF2:

From an electron group geometry perspective, GeF2 has a trigonal planar shape, but its real shape is dictated by the positions of the atoms. This shape is called bent or angular.

A molecule with four electron groups about the central atom orients the four groups in the direction of a tetrahedron, as shown in Figure 9.3 "Tetrahedral Geometry". If there are four atoms attached to these electron groups, then the molecular shape is also tetrahedral. Methane (CH4) is an example.

Figure 9.3 Tetrahedral Geometry

Four electron groups orient themselves in the shape of a tetrahedron.

This diagram of CH4 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.

Double or triple bonds count as a single electron group. CH2O has the following Lewis electron dot diagram.

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:

(The lone electron pairs on the O atom are omitted for clarity.) The 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.

What is the approximate shape of each molecule?

Solution

The first step is to draw the Lewis electron dot diagram of the molecule.

1. 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.

2. 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.

Test Yourself

What is the approximate molecular shape of CH2Cl2?

Answer

Tetrahedral

Table 9.3 "Summary of Molecular Shapes" summarizes the shapes of molecules based on their number of electron groups and surrounding atoms.

Table 9.3 Summary of Molecular Shapes

Key Takeaway

• The approximate shape of a molecule can be predicted from the number of electron groups and the number of surrounding atoms.

Exercises

1. What is the basic premise behind VSEPR?

2. What is the difference between the electron group geometry and the molecular geometry?

3. Identify the electron group geometry and the molecular geometry of each molecule.

4. Identify the electron group geometry and the molecular geometry of each molecule.

5. Identify the electron group geometry and the molecular geometry of each molecule.

6. Identify the electron group geometry and the molecular geometry of each molecule.

7. What is the geometry of each species?

8. What is the geometry of each species?

9. What is the geometry of each species?

   1. COF2
   2. C2Cl2 (both C atoms are central atoms and are bonded to each other)

10. What is the geometry of each species?

    1. CO32−
    2. N2H4 (both N atoms are central atoms and are bonded to each other)

Answers

1. Electron pairs repel each other.

3. 1. electron group geometry: tetrahedral; molecular geometry: bent
   2. electron group geometry: tetrahedral; molecular geometry: tetrahedral

5. 1. electron group geometry: linear; molecular geometry: linear
   2. electron group geometry: tetrahedral; molecular geometry: tetrahedral

9. 1. trigonal planar
   2. linear and linear about each central atom