Limitations of Lewis Dot Structure

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.

Limitations of Lewis Dot Structure Limitations of Lewis Dot Structure

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Table of contents
    • 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 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, what actually are Lewis structures?

    Lewis dot structures are simplified representations of a molecule's valence electrons.

    Lewis dot structures are also known as Lewis structures, Lewis dot diagrams, dot and cross diagrams and electron dot diagrams. These five 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:

    Water Lewis Structure, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 1. Water Lewis dot diagram.

    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.

    Ozone Lewis Structure, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 2. Ozone Lewis dot diagram.

    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.

    Limitations of Lewis structures: 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.

    Ethene Lewis Structure, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 3. Ethene. Source: commons.wikimedia.org

    Limitations of Lewis structures: 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.

    Ethene vs Ethane, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 4. Ethene, left, and ethane, right. Source: commons.wikimedia.org

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

    Limitations of Lewis structures: 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.

    Resonance structures of ozone, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 5. Resonance structures of ozone.

    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.

    Limitations of Lewis structures: 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.

    Water Lewis Structure, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 6. Water 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.

    Lewis structure and geometry of ammonia, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 6. Ammonia, shown with its Lewis structure, left, and geometry, right. Modified from: commons.wikimedia.org

    Limitations of Lewis structures: 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.

    Boron trifluoride, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 7. Boron trifluoride.

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

    Xenon tetrafluoride Lewis Structure, Limitations of Lewis Dot Structure, Ionic and Molecular Compounds, StudySmarterFig. 8. Xenon tetrafluoride Lewis dot diagram. Image credits: commons.wikimedia.org

    Visit "The Octet Rule" to see some more exceptions to the octet rule.

    Advantages and disadvantages of Lewis structures

    Here's a table to help you understand the advantages and disadvantages of Lewis structures:

    AdvantagesDisadvantages
    Simplicity: Easy to draw and understand, providing a straightforward way to depict bonding.Geometry: Doesn't depict the shape of the molecule or the size of atoms and cannot show hybridisation.
    Visualization: Provides a visual representation of how electrons are distributed around atoms.Resonance: Cannot accurately represent molecules with delocalized electrons, like benzene.
    Predictability: Helps in predicting the type of bond (ionic/covalent), bond order, and bond polarity.Approximation: Assumes that electrons are localized between two atoms which is not always the case.
    Insights: Gives an idea about the stability of a molecule and its reactivity.Assumptions: It assumes that all molecules follow the octet rule.

    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.

    Limitations of the Lewis Structure - Key takeaways

    • Lewis dot structures are simplified representations of a molecule's valence electrons. They show the molecule's arrangement of atoms, valence electrons and bonding.
    • Lewis dot structures have their limitations:
      • They don't show the relative sizes and lengths of atoms and bonds.
      • They don't show the electron orbitals involved in the bond.
      • They don't show resonance accurately.
      • They don't show the molecule's geometry.
      • They assume that all atoms follow the octet rule.
    Frequently Asked Questions about 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.

    Test your knowledge with multiple choice flashcards

    True or false? Lewis structures show the relative lengths of bonds.

    True or false? Lewis structures show the arrangement of atoms and electrons in a molecule.

    True or false? Lewis structures show the geometry of a molecule.

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