Shapes of Molecules

Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of negatively charged electrons, tend to repel one another. This concept is called the valence shell electron pair repulsion (VSEPR) theory.

Linear Shaped Molecules

The two covalent bonds in BeCl2 stay as far from each other as possible, ending up 180° apart from each other. The result is a linear molecule:

The BeCl2 molecule has two bonds from the central Be atom to each of the two outer Cl atoms. The Be-Cl bonds are arranged 180 degrees apart in a linear shape.A ball and stick model of the BeCl2 molecule. Be is green. Cl is white. The Be-Cl bonds are arranged 180 degrees apart.

If you hold your arms as far apart from each other, they will be 180° apart, just like the bonds in a linear molecule.

The name of the linear shape comes from the observation that all atoms in a linear molecule are aligned in a line.

Planar Triangle Shaped Molecules

The three covalent bonds in BF3 repel each other to form 120° angles in a plane, in a shape called planar triangle:

The BF3 molecule has three bonds from the central B atom to each of the three outer F atoms. The B-F bonds are arranged 120 degrees apart in a planar triangle shape.

The trigonal planar shape is flat and somewhat like the shape of a fidget spinner.

Some textbooks and sources will use the name “trigonal planar” for this shape. Planar triangle and trigonal planar are the same shape.

Note: The molecules BeCl2 and BF3 actually violate the octet rule; however, such exceptions are rare and will not be discussed furtherin this text.

Try sticking three toothpicks into a marshmallow or a gumdrop and see if you can find different positions where your “bonds” are farther apart than the planar 120° orientation.

Tetrahedral Shaped Molecules

The four covalent bonds in CH4 arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of 109.5°. CH4 is said to have a tetrahedral shape:

The molecular structure of the methane molecule, CH4, is shown with a tetrahedral arrangement of the hydrogen atoms. Each C-H bond is arranged 109.5 degrees from all other bonds. A bond drawn as a wedge indicates the bond is oriented in front of the page. A dashed bond indicates the bond is oriented behind the page.
The molecular structure of the methane molecule, CH4, is shown with a tetrahedral arrangement of the hydrogen atoms.

Three-dimensional molecular shapes like this one are often drawn using the wedge and dash notation, in which solid lines represent bonds in the plane of the page, solid wedges represent bonds coming up out of the plane, and dashed lines represent bonds pointing behind the plane.

Summary of Molecular Shapes

Atoms Around Central Atom (no lone pairs on central atom) General Molecular Formula Shape Example
2 AB2 Linear BeCl2
3 AB3 Planar Triangle BF3
4 AB4 Tetrahedral CH4

Shapes of Molecules with Double or Triple Bonds

In determining the shapes of molecules, it is useful to first determine the Lewis diagram for a molecule. The shapes of molecules with double or triple bonds are determined by treating the double or triple bonds as one bond. Thus, formaldehyde (CH2O) has a shape similar to that of BF3. The C=O double bond counts as one bond. The CH2O molecule, therefore, has three bonds (two C-H and one C=O) and is a planar triangle shape:

The lewis structure of CH2O is shown. The central carbon is connected to two hydrogen atoms by two single bonds, one single bond to each hydrogen. The central carbon is connected to an oxygen atom by a double bond. The outer atoms (two H and one O) are separated by angles of 120 degrees. The overall shape is planar triangle.A ball and stick model of CH2O. The central C atom is black, the two outer hydrogen atoms are white, and the outer oxygen atom is red. The lewis structure of CH2O is shown. The central carbon is connected to two hydrogen atoms by two single bonds, one single bond to each hydrogen. The central carbon is connected to an oxygen atom by a double bond. The outer atoms (two H and one O) are separated by angles of 120 degrees. The overall shape is planar triangle.

Molecules With Lone Pairs Around Central Atom

Molecules with lone (nonbonding) electron pairs around the central atom have a shape based on the position of the atoms, not the lone electron pairs. The lone electron pairs do repel the bonds, however, leading to smaller bond angles between the bonds than if the lone electron pairs were not present. For example, NH3 has one lone electron pair and three bonded electron pairs (the three pairs of bonded electron are in the three N-H bonds). These four electron pairs repel each other and adopt a tetrahedral arrangement. However, the shape of the molecule is described in terms of the positions of the atoms, not including the lone electron pairs. Thus, NH3 is said to have a pyramidal shape, not a tetrahedral one. A pyramidal shape looks like a short camera tripod:

The lewis structure of NH3 is shown. The central nitrogen atom is connected to three hydrogen atoms by three single bonds. There is also a lone pair of non bonded electrons on the central nitrogen atom. All three N-H bonds and the lone pair are arranged at 109.5 degree angles from each other. The lone pair is not visible in the shape, so the shape is pyramidal.A ball and stick model of NH3 is shown. The central nitrogen atom is blue. The outer hydrogen atoms are white. The central nitrogen atom is connected to three hydrogen atoms by three single bonds. There is also a lone pair of non bonded electrons on the central nitrogen atom. All three N-H bonds and the lone pair are arranged at 109.5 degree angles from each other. The lone pair is not visible in the shape, so the shape is pyramidal.

Similarly, H2O has two lone pairs of electrons around the central oxygen atom and two bonded electron pairs (the two pairs of bonded electrons are in the two O-H bonds). Although the four electron pairs adopt a tetrahedral arrangement, the shape of the molecule is described by the positions of the atoms only. The shape of H2O is bent with an approximate 109.5° angle.

The lewis structure of H2O is shown. The central oxygen atom is connected to two hydrogen atoms, each by one single bond. There are also two pairs of nonbonding electrons on the central oxygen atom. Both O-H bonds and both lone pairs are arranged at 109.5 degree angles form each other. The overall shape of the atoms is a bent shape. The lone pairs are not visible in actuality. Only the O and H atoms are visible.

A ball and stick model of H2O. The oxygen is red and the two hydrogen atoms are white. The O-H bonds are arranged at a 109.5 degree angle from each other resulting in a bent shape.

The bent shape looks like an upside down letter “V”.

Steps to Determine Molecular Shape

Step 1: Determine the Lewis structure.

Step 2: Count the number of bonds (a double/triple bond counts as one) and lone pairs around the central atom.

Step 3: Use the following table to determine the molecular shape:

The table summarizes the shapes identified in this section. If a molecule has two bonds and 0 lone pairs, it is linear. If a molecule has three bonds and 0 lone pairs, it is planar triangle. If a molecule has two bonds and one lone pair, it is bent with a 120 degree angle. If a molecule has four bonds, it is tetrahedral. If a molecule has three bonds and one lone pair, it is pyramidal. If a molecule has two bonds and two lone pairs, it is bent.
The molecular geometry depends on the number of bonds and lone pairs around the central atom, A.

Concept Review Exercises

What is the geometry of the ammonium ion, NH4+? Its Lewis structure is shown below. How is this different from ammonia, NH3?

The lewis structure of ammonia is shown. The central nitrogen atom is connected to four hydrogen atoms each by one single bond.

Solutions

In ammonium ion, the central atom N has 4 bonds and no lone pair’. Hence, this is tetrahedral.

The shape of a generic tetrahedral molecule is shown. The central atom is bonded to four outer atoms by four bonds. Each bond is arranged at a 109.5 degree angle from all other bonds.

In ammonia (NH3), shown below, N has 3 bonds and one lone pair.

The lewis structure of NH3 is shown. The central nitrogen is bonded to three hydrogen atoms, each by one single bond. The central nitrogen also has a nonbonding lone pair of electrons.

Hence, the shape of this molecule is pyramidal.

The shape of a generic pyramidal molecule is shown. The central atom is connected to three outer atoms by single bonds. The central atom also has one lone pair of nonbonding electrons. All bonds and the lone pair of electrons are arranged at a 109.5 degree angle from each other. The lone pair is not included in the overall shape.

Attributions

This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD, Openstax which is licensed under CC BY 4.0. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction

This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J Scott, Saylor which is licensed under CC BY-NC-SA 4.0. Access for free at http://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/index.html

 

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