What is a major limitation of resonance structures?

Lewis dot structures are a bit like chemical floor plans. They show the rough positions of atoms and electrons in a molecule. However, Lewis dot structures also have their drawbacks. Unlike floor plans, they aren't to scale and they don't accurately represent the geometry of the molecule. After all, it is quite hard to depict a 3D structure in just two dimensions. This is just one limitation of Lewis dot structures.

• This article is about the limitations of Lewis dot structures in chemistry.
• We'll begin by defining Lewis dot structures before looking at a few examples.
• We'll then explore some of the limitations of Lewis dot structures.

Lewis dot structures: Definition

You might have already met Lewis dot structures in the article "Lewis dot diagrams". We'd recommend checking out that article before you read on any further. But before we move on to the main focus of this article, the limitations of Lewis dot structures, let's quickly recap what these diagrams actually are.

Lewis dot structures are also known as Lewis structures, Lewis dot diagrams or electron dot diagrams. These four terms all refer to the same thing: a diagram showing a molecule's atoms, valence electrons and bonding. In these diagrams, electrons are shown as dots and covalent bonds are shown as lines.

Lewis dot structures are built around the idea that atoms obey the octet rule, meaning that they try to have eight electrons in their outer shell. However, as we'll see later on, this isn't always the case.

Examples of Lewis dot structures

It might be helpful to look at a few examples of Lewis dot structures before we discuss their drawbacks. First up, let's consider water, H2O. Here is its Lewis structure:

What does this tell us?

• The molecule consists of two hydrogen atoms joined to a central oxygen atom by single covalent bonds. These are represented by straight lines.
• The oxygen atom has two lone pairs of electrons. These are represented by pairs of dots.

Next, let's consider the Lewis structure of ozone, O3.

What can we infer?

• The molecule consists of three oxygen atoms joined by a single and a double covalent bond.
• The oxygen atoms have varying numbers of lone pairs of electrons.

However, this structure isn't completely accurate. The two bonds between the oxygen atoms are actually equivalent. Instead of there being one single and one double bond, both bonds can be thought of as one-and-a-half bonds. This is because ozone shows resonance. This is an example of one of the limitations of Lewis dot structures. Let's look at these now.

Ozone's resonance is explored in much more detail in the article "Resonance".

Limitations of the Lewis dot structure

As handy as the Lewis dot structure is, it has its limitations. We've already touched on a few of them. Here are some more:

• It doesn't show the length of bonds or the size of atoms.
• It doesn't show the different types of orbitals.
• It doesn't accurately represent resonance.
• It doesn't show geometry.
• It assumes that all atoms follow the octet rule.

Bond length and atom size

First of all, Lewis dot structures aren't scaled diagrams. They don't show the relative sizes of atoms or the lengths of bonds. In Lewis structures, all atoms are shown as being the same size and all bonds are shown as being the same length, whereas, in reality, this isn't the case.

For example, take a look at the Lewis diagram for ethene, C2H4. Ethene consists of two carbon atoms joined by a double covalent bond. Each carbon atom is also bonded to two hydrogen atoms with single covalent bonds. Carbon atoms are much bigger than hydrogen atoms - they have an extra electron shell. In addition, double bonds are much shorter and stronger than single bonds. However, in a Lewis structure, atoms and bonds are shown as being the same size and length.

Orbitals

When atoms bond covalently, their electron orbitals overlap. But before this happens, the atom sometimes alters some of its orbitals to make them all equal. This process is called hybridisation. For example, sp2 orbitals are made by one s orbital and two p orbitals rearranging themselves to form three identical orbitals. We find them in ethene, for example. On the other hand, sp3 orbitals are made by one s orbital and three p orbitals rearranging themselves to form four identical orbitals. We find them in ethane. However, Lewis diagrams don't distinguish between different electron orbitals and show all covalent bonds as being the same.

Resonance

Earlier in the article, we looked at the Lewis dot structure of ozone. It contained an O-O single bond and an O=O double bond. In our Lewis structure, the double bond was on the right-hand side, but we can also draw an equally valid Lewis structure with the double bond on the left.

In actual fact, ozone shows resonance. This means that it can't be accurately represented by either of these two Lewis structures, known as resonance structures. Instead, it takes the form of a hybrid molecule, which is an average of the two resonance structures. Rather than having one O-O single bond and one O=O double bond, it has two equal one-and-a-half bonds. You can clearly see that this is not shown in the Lewis diagrams of ozone's resonance structures. They are an inaccurate representation of ozone, and of resonance in general.

Geometry

Next up, let's consider Lewis structures and geometry. In fact, Lewis dot structures are a poor representation of a molecule's geometry. They don't show any bond angles or positions.

Take water, for example. We've already seen its Lewis structure. Water is a v-shaped molecule, meaning that the angle between its two bonds is 104.5°. But you can't tell this from a Lewis diagram.

Another example is ammonia. It contains a nitrogen atom with three covalent bonds and one lone pair of electrons, making the molecule trigonal pyramidal in shape. If we were to see this molecule in 3D, one of the bonds would stick out towards you, one would point backwards away from you, and the bond angle would be 107°. However, the Lewis structure doesn't show this. Instead, it shows the molecule as flat and planar.

Octet rule

When you draw Lewis diagrams, you have to assign pairs of electrons to atoms. You do this by assuming that all atoms follow the octet rule.

The octet rule is a general rule in chemistry used to predict the bonding between atoms. It states that atoms are at their most stable when they have eight electrons in their outer shell.

This means that we try to make sure that all atoms have eight valence electrons. However, for some atoms and molecules, this isn't the case.

Take boron trifluoride, for example. The central boron atom has just six valence electrons. The conventional rules for drawing Lewis diagrams, based on the octet rule, would tell us that this molecule is unstable. But in reality, boron trifluoride is a stable molecule and is perfectly happy with just six electrons in its outer shell.

Lewis diagrams also tell us that noble gases like xenon can't form any bonds - they already have a eight valence electrons. Actually, this isn't the case. Xenon can form molecules, such as xenon tetrafluoride.

We're done with this article. By now you should be able to draw and interpret Lewis structures for different molecules. You should also be able to describe and explain some of the limitations of Lewis structures.

What makes a resonance structure invalid?

What are the limitations of Lewis Structures?

What is the major limitation of Lewis Structures in depicting molecules?

For which of the following resonance structure is not possible?