Lewis dot structures are like the blueprint for a chemical. They give you an idea of where the atoms and electrons are located. However, these structures have some limitations. Unlike blueprints, they don't show the molecule's actual shape and they're not always to scale. It's tough to display a 3D structure on a 2D surface. This is just one example of the limitations of Lewis dot structures.

In this blog post, we'll talk about the limitations of Lewis dot structures in chemistry. First, we'll give you a quick overview of what Lewis dot structures are and provide some examples. Then, we'll dive into some of the limitations you should be aware of.

What are Lewis dot structures?

If you've heard of Lewis dot diagrams before, you might already be familiar with Lewis dot structures. But in case you're not, let's quickly go over what they are. Lewis dot structures are simple diagrams that show a molecule's valence electrons.

Sometimes called Lewis structures, Lewis dot diagrams, or electron dot diagrams, they all refer to the same thing. These diagrams display a molecule's atoms, valence electrons, and bonding. Electrons are represented by dots, and bonds are shown as lines.

The idea behind Lewis dot structures is that atoms follow the octet rule. This means they aim to have eight electrons in their outer shell. However, as we'll discuss later, this isn't always possible. Before we dive into the limitations of Lewis dot structures, we recommend checking out our article on Lewis dot diagrams.

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:

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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.

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From the given information, we can infer that the molecule being discussed is ozone, which is made up of three oxygen atoms connected by one single and one double covalent bond. However, due to resonance, both bonds are equivalent and can be considered as one-and-a-half bonds. This is an example of a limitation of Lewis dot structures.

Moving on to the limitations of Lewis dot structures, it doesn't provide accurate information about the length of bonds, the size of atoms, the different types of orbitals, resonance, and geometry. The Lewis structures don't show the relative sizes of atoms or the lengths of bonds, which can be misleading. For instance, in the Lewis diagram for ethene, carbon atoms and hydrogen atoms are shown as being the same size and length, while in reality, carbon atoms are larger and double bonds are shorter and stronger.

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Orbitals

Hybridization is a process in which atoms rearrange their electron orbitals to form hybrid orbitals of equal energy and shape. These hybrid orbitals are formed by mixing pure atomic orbitals, and the number of hybrid orbitals formed is always equal to the number of atomic orbitals that were mixed together. For example, sp2 orbitals are made by one s orbital and two p orbitals rearranging themselves to form three identical orbitals. These orbitals are found in molecules such as ethene. On the other hand, sp3 orbitals are made by one s orbital and three p orbitals rearranging themselves to form four identical orbitals. These orbitals are found in molecules such as ethane.

However, Lewis diagrams don't distinguish between different electron orbitals and show all covalent bonds as being the same.

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Find out more in "Bond Hybridization" and "Hybrid Orbitals".

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.

The given passage suggests that ozone shows resonance, which means it can't be accurately represented by either of the two Lewis structures. Instead, it takes the form of a hybrid molecule, which is an average of the two resonance structures. This hybrid molecule has two equal one-and-a-half bonds, instead of one O-O single bond and one O=O double bond as shown in the resonance structures. The Lewis diagrams of ozone's resonance structures are an inaccurate representation of ozone and resonance in general. Therefore, it is important to understand the limitations of Lewis structures and use other methods to accurately represent the molecular structure of compounds.

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.

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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

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. When drawing Lewis diagrams, we assume that all atoms follow the octet rule. However, there are exceptions to this rule. For example, boron trifluoride has a central boron atom with just six valence electrons, yet it is still a stable molecule.

Other exceptions to the octet rule include molecules that have an odd number of electrons, molecules in which one or more atoms possess more or fewer than eight electrons, and molecules that simply do not have enough electrons to achieve octets around all atoms.

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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.

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As an AI assistant, I don't have the ability to visit websites. However, I can provide more information on the limitations of the octet rule.

In addition to the exceptions mentioned earlier, there are also some other limitations to the octet rule. For example, the octet rule does not apply to elements in the third row or beyond in the periodic table, such as sulfur and phosphorus. These elements can have expanded octets, meaning they can have more than eight electrons in their outer shell.

Another limitation of the octet rule is that it assumes that all electrons in a molecule are localized between atoms. However, in reality, some molecules have delocalized electrons, which are not confined to any one atom but are spread out over the entire molecule. This is particularly important in molecules with pi bonds, such as benzene.

Overall, while the octet rule is a useful rule of thumb for predicting the bonding in many molecules, it is important to recognize its limitations and use other methods, such as molecular orbital theory, to accurately describe the bonding in more complex molecules.

Limitations of Lewis Dot Structure

Why are Lewis structures limited? 

Lewis structures don't show the relative sizes and lengths of atoms and bonds, the electron orbitals involved in the bond or the molecule's geometry. They also assume that all atoms follow the octet rule and aren't good representations of resonance.

What is one disadvantage of the Lewis structure? 

They don't show the geometry of the molecule.

What are some restrictions in writing the Lewis structure of molecules? 

Rules for drawing Lewis structures typically state that atoms must have eight valence electrons. However, this isn't always the case. for example, xenon in xenon tetrafluoride has twelve valence electrons.

What are the rules for Lewis structures? 

In Lewis diagrams, electrons are shown as dots and covalent bonds are shown as lines. In general, electrons in Lewis structures must be paired and each atom must have eight outershell electrons. However, this isn't always the case.

What are two limitations of the octet rule? 

The octet rule states that atoms are at their most stable when they have the configuration of a noble gas, with eight electrons in their outer shell. However, this isn't always the case. Some molecules feature atoms with incomplete octets, such as boron trifluoride. Some molecules feature atoms with expanded octets, such as xenon in xenon tetrafluoride.

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