Skip to content Skip to navigation

OpenStax-CNX

You are here: Home » Content » Atomic combinations: Shapes of molecules

Navigation

Lenses

What is a lens?

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

This content is ...

Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
  • FETChem display tagshide tags

    This module is included inLens: Siyavula: Chemistry (Gr. 10-12)
    By: Siyavula

    Review Status: In Review

    Click the "FETChem" link to see all content affiliated with them.

    Click the tag icon tag icon to display tags associated with this content.

  • Bookshare

    This module is included inLens: Bookshare's Lens
    By: Bookshare - A Benetech InitiativeAs a part of collection: "FHSST: Grade 11 Physical Science"

    Comments:

    "Accessible versions of this collection are available at Bookshare. DAISY and BRF provided."

    Click the "Bookshare" link to see all content affiliated with them.

  • Siyavula: Physical Science display tagshide tags

    This module is included inLens: Siyavula textbooks: Physical Science
    By: Free High School Science Texts ProjectAs a part of collection: "FHSST: Grade 11 Physical Science"

    Click the "Siyavula: Physical Science" link to see all content affiliated with them.

    Click the tag icon tag icon to display tags associated with this content.

Also in these lenses

  • Siyavula : FET Physics

    This module is included inLens: Siyavula : FET Physics
    By: Nerk van RossumAs a part of collection: "FHSST: Grade 11 Physical Science"

    Click the "Siyavula : FET Physics" link to see all content selected in this lens.

Recently Viewed

This feature requires Javascript to be enabled.

Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.
 

Atomic combinations: Shapes of molecules

Module by: Free High School Science Texts Project. E-mail the author

The Shape of Molecules

Valence Shell Electron Pair Repulsion (VSEPR) theory

The shape of a covalent molecule can be predicted using the Valence Shell Electron Pair Repulsion (VSEPR) theory. This is a model in chemistry that tries to predict the shapes of molecules. Very simply, VSEPR theory says that the valence electron pairs in a molecule will arrange themselves around the central atom of the molecule so that the repulsion between their negative charges is as small as possible. In other words, the valence electron pairs arrange themselves so that they are as far apart as they can be. The number of valence electron pairs in the molecule determines the dhape of that molecule.

Definition 1: Valence Shell Electron Pair Repulsion Theory

Valence shell electron pair repulsion (VSEPR) theory is a model in chemistry, which is used to predict the shape of individual molecules, based upon the extent of their electron-pair repulsion.

VSEPR theory is based on the idea that the geometry of a molecule is mostly determined by repulsion among the pairs of electrons around a central atom. The pairs of electrons may be bonding or non-bonding (also called lone pairs). Only valence electrons of the central atom influence the molecular shape in a meaningful way.

Determining the shape of a molecule

To predict the shape of a covalent molecule, follow these steps:

Step 1:

Draw the molecule using Lewis notation. Make sure that you draw all the electrons around the molecule's central atom.

Step 2:

Count the number of electron pairs around the central atom.

Step 3:

Determine the basic geometry of the molecule using the table below. For example, a molecule with two electron pairs around the central atom has a linear shape, and one with four electron pairs around the central atom would have a tetrahedral shape. The situation is actually more complicated than this, but this will be discussed later in this section.

Table 1: The effect of electron pairs in determining the shape of molecules
Number of electron pairs Geometry
2 linear
3 trigonal planar
4 tetrahedral
5 trigonal bipyramidal
6 octahedral

Figure 1 shows each of these shapes. Remember that the shapes are 3-dimensional, and so you need to try to imagine them in this way. In the diagrams, the thicker lines represents those parts of the molecule that are 'in front' (or coming out of the page), while the dashed lines represent those parts that are 'at the back' (or going into the page) of the molecule.

Figure 1: Some common molecular shapes
Figure 1 (CG11C1_023.png)

The simulation in covalent bonding also allows you to view the molecules in 3-D. The shape of the molecules in this view is the shape of the molecule predicted by VSEPR.

You can also view different molecules and see their shapes at this website. You do not need to know all these molecules, this is simply to give you a feel for what molecules look like.

Exercise 1: The shapes of molecules

Determine the shape of a molecule of O2O2

Solution
  1. Step 1. Draw the molecule using Lewis notation :
    Figure 2
    Figure 2 (CG11C1_024.png)
  2. Step 2. Count the number of electron pairs around the central atom :

    There are two electron pairs.

  3. Step 3. Determine the basic geometry of the molecule :

    Since there are two electron pairs, the molecule must be linear.

Exercise 2: The shapes of molecules

Determine the shape of a molecule of BF3BF3

Solution
  1. Step 1. Draw the molecule using Lewis notation :
    Figure 3
    Figure 3 (CG11C1_025.png)
  2. Step 2. Count the number of electron pairs around the central atom :

    There are three electron pairs.

  3. Step 3. Determine the basic geometry of the molecule :

    Since there are three electron pairs, the molecule must be trigonal planar.

More about molecular shapes

Determining the shape of a molecule can be a bit more complicated. In the examples we have used above, we looked only at the number of bonding electron pairs when we were trying to decide on the molecules' shape. But there are also other electron pairs in the molecules. These electrons, which are not involved in bonding but which are also around the central atom, are called lone pairs. The worked example below will give you an indea of how these lone pairs can affect the shape of the molecule.

Exercise 3: Advanced

Determine the shape of a molecule of NH3NH3

Solution
  1. Step 1. Draw the molecule using Lewis notation :
    Figure 4
    Figure 4 (CG11C1_026.png)
  2. Step 2. Count the number of electron pairs around the central atom :

    There are four electron pairs.

  3. Step 3. Determine the basic geometry of the molecule :

    Since there are four electron pairs, the molecule must be tetrahedral.

  4. Step 4. Determine how many lone pairs are around the central atom :

    There is one lone pair of electrons and this will affect the shape of the molecule.

  5. Step 5. Determine the final shape of the molecule :

    The lone pair needs more space than the bonding pairs, and therefore pushes the three hydrogen atoms together a little more. The bond angles between the hydrogen and nitrogen atoms in the molecule become 106 degrees, rather than the usual 109 degrees of a tetrahedral molecule. The shape of the molecule is trigonal pyramidal.

Group work : Building molecular models

In groups, you are going to build a number of molecules using marshmallows to represent the atoms in the molecule, and toothpicks to represent the bonds between the atoms. In other words, the toothpicks will hold the atoms (marshmallows) in the molecule together. Try to use different coloured marshmallows to represent different elements.

You will build models of the following molecules:

HCl, CH4CH4, H2OH2O, HBr and NH33

For each molecule, you need to:

  • Determine the basic geometry of the molecule
  • Build your model so that the atoms are as far apart from each other as possible (remember that the electrons around the central atom will try to avoid the repulsions between them).
  • Decide whether this shape is accurate for that molecule or whether there are any lone pairs that may influence it.
  • Adjust the position of the atoms so that the bonding pairs are further away from the lone pairs.
  • How has the shape of the molecule changed?
  • Draw a simple diagram to show the shape of the molecule. It doesn't matter if it is not 100% accurate. This exercise is only to help you to visualise the 3-dimensional shapes of molecules.

Do the models help you to have a clearer picture of what the molecules look like? Try to build some more models for other molecules you can think of.

Figure 5

Content actions

Download module as:

Add module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks