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Lewis Dot Diagrams

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Lewis Dot Diagrams

When drawing chemical species, we often want to know the arrangement of their valence electrons. After all, the number of valence electrons often dictates a species' properties and how it reacts. But drawing out all of the electron shells can get quite tiresome and complicated, especially for larger molecules. Instead, we use drawings called Lewis dot diagrams.

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

  • This article is about Lewis dot diagrams in chemistry.
  • We'll start by looking at what Lewis dot diagrams are before turning our attention to some common examples.
  • We'll then learn how to draw Lewis dot diagrams.
  • Finally, we'll consider the importance of Lewis dot diagrams.

What are Lewis dot diagrams?

Above, we introduced you to the idea of a Lewis dot diagram: a simplified representation of a molecule's valence electrons. Lewis dot diagrams are also known as Lewis structures, Lewis dot structures or electron dot structures. Don't worry if you see any of these terms floating around - they all mean the same thing!

Lewis dot diagrams show the atoms in a molecule, be it a neutral compound or a charged ion. They also show the number, position and arrangement of the atoms' valence electrons, whilst assuming that all inner electron shells are complete. Specifically, they show any bonded pairs or lone pairs of electrons.

Note the following:

  • Electrons are shown as dots. Two dots side by side represent a lone pair of electrons.
  • Bonded pairs of electrons are shown as lines, signifying a covalent bond. A single line represents a single covalent bond with just one bonded pair of electrons, whilst two lines represent a double covalent bond with two bonded pairs of electrons.
  • Lewis dot diagrams are based on the octet rule. Atoms are at their most stable when they have full outer shells and the octet rule states that for most atoms, this occurs when they have eight valence electrons.
  • Electrons are generally spaced out evenly around all four sides of the atom, with two per side.

We have a whole article all about the octet rule and some of the exceptions you'll encounter over at "The Octet Rule".

Want to see Lewis dot diagrams in practice? Let's now look at some examples.

Lewis dot diagram for oxygen

An oxygen molecule consists of two oxygen atoms, joined by a double covalent bond. Each oxygen atom also has two lone pairs of electrons. You can see this in the Lewis dot diagram below.

Lewis Dot Diagrams A Lewis dot diagram for an oxygen molecule StudySmarterA Lewis dot diagram for an oxygen molecule. Anna Brewer, StudySmarter Originals

How many valence electrons does each oxygen atom have? If you count them up, you'll see that each oxygen atom has four electrons from its two lone pairs, and four electrons from its double covalent bond. This adds up to eight valence electrons. This oxygen molecule, therefore, satisfies the octet rule.

Lewis dot diagram for carbon dioxide

Next up: carbon dioxide, CO2. This molecule consists of two oxygen atoms joined to a central carbon atom by double covalent bonds. Again, each oxygen atom also has two lone pairs of electrons.

Lewis Dot Diagrams Lewis Dot Diagram for carbon dioxide StudySmarterA Lewis dot diagram for carbon dioxide. Anna Brewer, StudySmarter Originals

Counting the electrons, we can see that both oxygen atoms have four electrons from their two lone pairs, and four electrons from their double covalent bond. This adds up to eight valence electrons. The carbon atom doesn't have any lone pairs but features two double bonds. Each double bond supplies four electrons, and so carbon also has eight valence electrons.

Lewis dot diagram for water

Let's now look at the Lewis dot diagram for water, H2O.

Lewis Dot Diagrams Lewis Dot Diagram for water StudySmarterA Lewis dot diagram for water. Anna Brewer, StudySmarter Originals

Notice that each hydrogen atom only has two valence electrons, both coming from the covalent bond it shares with oxygen. However, they still have full outer shells. Hydrogen is one of the exceptions to the octet rule since it only needs two valence electrons to be at its most stable.

Lewis dot diagram for methane

Here is another example: the Lewis dot diagram for methane, CH4.

Lewis Dot Diagrams Lewis Dot Diagram for methane StudySmarterA Lewis dot diagram for methane. Anna Brewer, StudySmarter Originals

Lewis dot diagram for carbon

We can also draw Lewis dot diagrams for uncombined atoms that have yet to form a molecule. One example is carbon. Here is its Lewis dot diagram:

Lewis Dot Diagrams Lewis Dot Diagram for carbon StudySmarterA Lewis dot diagram for carbon. Anna Brewer, StudySmarter Originals

Drawing Lewis dot diagrams for simple molecules like oxygen or methane is fairly straightforward. However, it can get a little more tricky for more complex molecules and so there is a set procedure that is helpful to follow. In the next section, we'll look at some more complicated examples of drawing Lewis dot diagrams.

How do you draw Lewis dot diagrams?

Now that you know what a Lewis dot diagram is and what each part represents, let's practice drawing some more complicated molecules. Here are the steps you should follow. Don't worry if it seems quite long-winded - we'll break each part down for you.

  1. Use the chemical formula of the molecule and a periodic table to work out the molecule's total number of valence electrons. You'll use this number when adding up the electrons later on.
  2. Draw the rough position of the atoms in the molecule. Remember that molecules like to be symmetrical - this should help you to work out which atom is the central atom, and which are outer atoms.
  3. Connect the atoms using single covalent bonds, showing them with straight lines.
  4. Add pairs of electrons to the outer atoms until they all have a full outer shell of electrons. This usually means that they need eight electrons, but there are some exceptions. Remember to count any bonds as a pair of electrons.
  5. Count up how many electrons you've added to your molecule in total, and subtract this from the molecule's number of valence electrons that you worked out earlier. This will give you your number of remaining electrons. Once again, remember to count any bonds as pairs of electrons.
  6. Add any remaining electrons to the central atom.
  7. The central atom should hopefully now have a full outer shell of electrons. If it doesn't, use some of the lone pairs of electrons from the outer atoms to form double covalent bonds with the central atom, until all atoms have full outer shells.

Sound tricky? We'll start off with a basic molecule. Let's look at an example now.

Draw a Lewis dot diagram for ammonia, NH3.

First of all, we need to work out the number of valence electrons in this molecule, ammonia. We can do this by looking at the periodic table. Nitrogen is in group V and so has five valence electrons. Hydrogen is in group I and so has one valence electron. The total number of valence electrons is therefore 5 + 3(1) = 8.

Next, draw the rough position of the atoms in the molecule. In this case, the central atom is nitrogen, and it is surrounded by the three hydrogen atoms. Add a bond between nitrogen and each hydrogen atom, like so.

Lewis Dot Diagrams Lewis Dot Diagram for ammonia StudySmarterAmmonia. Anna Brewer, StudySmarter Originals

Next, add electrons to the outer atoms until they all have full outer shells. You should know that hydrogen can only have two electrons in its outermost shell. Here, each hydrogen atom already has two electrons, namely the bonded pair shared with nitrogen. We, therefore, don't need to add any more electrons to any of the hydrogens. We can move on to the next step: adding any remaining electrons to the central atom, nitrogen.

First, count up how many electrons you've already added to the structure. Here, we've added three bonding pairs, and so we've already added 3(2) = 6 electrons. We calculated earlier that ammonia has eight valence electrons, and so we still need to add 8 - 6 = 2 electrons. We add them to the central atom, nitrogen, as a lone pair.

Lewis Dot Diagrams Lewis Dot Diagram for ammonia StudySmarterAmmonia. Anna Brewer, StudySmarter Originals

Let's count up the electrons around nitrogen. It has three bonding pairs and one lone pair, giving it 3(2) + 2 = 8 electrons. It, therefore, has a full outer shell - we don't need to form any more covalent bonds. Our Lewis dot diagram is complete.

Here's a slightly harder example for you to have a go at.

Draw a Lewis dot diagram for the carbonate ion, CO32-.

Firstly, let's count up the number of valence electrons. Carbon is in group IV, giving it four valence electrons. Oxygen is in group VI, giving it six valence electrons. However, this is a negative ion with a charge of 2-. It, therefore, has two extra electrons. This means that the total number of valence electrons is 4 + 3(6) + 2 = 24.

Next, draw out the different atoms. This molecule consists of a carbon atom surrounded by three oxygen atoms. Add bonds between each oxygen atom and the carbon atom, and you should end up with something like this:

Lewis Dot Diagrams Lewis Dot Diagram for carbonate ion StudySmarterThe carbonate ion. Anna Brewer, StudySmarter Originals

Next up, add electrons to the outer atoms until they all have full outer shells. Each oxygen atom already has two electrons in its outer shell, thanks to the bonded pair it shares with carbon, so we just need to add six more to each.

Lewis Dot Diagrams Lewis Dot Diagram for carbonate ion StudySmarterThe carbonate ion. Anna Brewer, StudySmarter Originals

Count up the total number of electrons that you've already added. Here, we have three bonding pairs and nine lone pairs, giving us 3(2) + 9(2) = 24 electrons. We know that the carbonate ion only has 24 valence electrons. Therefore, we can't add more to the central atom, carbon. But looking at this carbon atom, we can see that it doesn't quite have a full outer shell - it only has six valence electrons, and ideally, it needs eight. To solve this, we use a lone pair of electrons from one of the oxygen atoms to form another bonded pair, creating a double bond between carbon and oxygen.

All atoms now have full outer shells. There's just one final thing to do - don't forget to add charge symbols to the molecule, to show that it is a 2- ion! Here is your final Lewis dot diagram.

Lewis Dot Diagrams Lewis Dot Diagram for carbonate ion StudySmarterThe carbonate ion. Anna Brewer, StudySmarter Originals

Why are Lewis dot diagrams important?

So far, we've found out what Lewis dot diagrams are and had a look at some examples for simple molecules. We've also had a go at drawing Lewis dot diagrams for more complicated molecules. But why do we need to know how to do this, and why are Lewis dot diagrams important?

Well, Lewis dot diagrams show us the number and type of valence electrons of each atom in a molecule. With a Lewis dot diagram, we can clearly see how many lone pairs and bonded pairs of electrons an atom has, and whether these are part of a single, double or triple bond. Knowing the arrangement of a molecule's valence electrons is useful for a few different reasons.

  • Firstly, it allows us to predict its shape, alongside the angle between any bonds. It also allows us to predict the bond lengths. This is known as molecular geometry.
  • Secondly, it also helps us to predict how the molecule reacts, as well as its properties.

Molecular geometry is based on something called VSEPR theory and is all about an atom's number of lone and bonded pairs of electrons. You can find out more at "VSEPR Theory".

That's it for this article. You should now feel confident at interpreting and drawing Lewis dot diagrams for a whole variety of different species, as well as explaining the importance of Lewis dot diagrams.

Lewis Dot Diagrams - Key takeaways

  • Lewis dot diagrams are simplified representations of a molecule's valence electrons. They show the molecule's atoms, valence electrons and bonding.
  • Lewis dot diagrams are based on the octet rule. Electrons are shown as dots and covalent bonds are shown as lines.
  • Lewis dot diagrams are important because they help us predict a molecule's geometry.

Frequently Asked Questions about Lewis Dot Diagrams

A Lewis dot diagram is a simplified representation of a molecule's valence electrons. It shows any lone pairs of electrons, as well as those involved in covalent bonds.

To draw Lewis dot diagrams, follow these steps:

  1. Count the number of valence electrons in the molecule.
  2. Draw the rough position of the atoms in the molecule.
  3. Add single covalent bonds to the molecule.
  4. Add electrons to the outer atoms until they all have full outer shells of electrons.
  5. Compare the number of valence electrons you counted earlier to the number that you have already added, and add any remaining electrons to the central atom.
  6. Use some of the lone pairs on the outer atoms to create double covalent bonds with the central atom, until all atoms have full outer shells of electrons. 

Lewis dot diagrams show the arrangement and bonding of a molecule's valence electrons. More specifically, they show any lone pair or bonded pairs of electrons.

The electron dot diagram for CO2 shows two oxygen atoms each joined to a central carbon atom by a double bond, represented by two lines. The oxygen atoms both have two lone pairs of electrons, which are each represented by two dots.

The electron dot diagram for calcium oxide shows the Ca2+ ion next to the oxide ion which is drawn in square brackets, with the 2- charge outside the brackets. There are 4 lone pairs of electrons surrounding the oxygen atom.

Final Lewis Dot Diagrams Quiz

Question

What is a Lewis dot diagram?

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Answer

A Lewis dot diagram is a simplified representation of a molecule's valence electrons.

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Question

True or false? Lewis dot diagrams show all of the electrons in a molecule.

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Answer

False

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Question

What type of bonding do Lewis dot diagrams show?

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Answer

Covalent

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Question

Why are Lewis dot diagrams important?

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Answer

They help us predict a molecule's geometry and how it reacts.

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Question

In Lewis dot diagrams, bonded pairs of electrons are shown as _____.

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Answer

Dots

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Question

In Lewis dot diagrams, lone pairs of electrons are shown as _____. 

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Answer

Dots

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Question

What does VSEPR stand for?

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Answer

Valence shell electron pair repulsion theory

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Question

VSEPR is affected by _____.

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Answer

Valence electron pairs

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Question

What is the effect of lone pairs of electron on the bond angle in VSEPR?

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Answer

Lone pairs of electron repel other electron pairs more strongly than bonded pairs. This reduces the bond angle.

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Question

What is the basic shape of a molecule with two pairs of electrons?

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Answer

Linear

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Question

What is the basic shape of a molecule with three pairs of electrons?

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Answer

Trigonal planar

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Question

What is the basic shape of a molecule with four pairs of electrons?

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Answer

Tetrahedral

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What is the basic shape of a molecule with five pairs of electrons?

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Answer

Trigonal bipyramidal

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Question

What is the basic shape of a molecule with six pairs of electrons?

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Answer

Octahedral

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Question

Which of these shapes include a bond angle of 120°?

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Answer

Trigonal planar

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Question

Which of these shapes include a bond angle of 90°?

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Answer

Trigonal bipyramidal

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Which of these shapes contain four pairs of electrons?

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Answer

Tetrahedral

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Question

Which of these shapes contain four bonded pairs of electrons?

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Answer

Tetrahedral

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Question

Which of these shapes contain one lone pair of electrons?

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Answer

Trigonal pyramidal

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Question

What is resonance?

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Answer

Resonance is a way of describing bonding in chemistry. It describes how several equivalent Lewis structures contribute to one overall hybrid molecule.

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Question

Compare isomerism and resonance.

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Answer

Isomers differ in their arrangement of atoms. Resonance structures have the same arrangement of atoms but different arrangements of electrons.

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Question

True or false? Molecules that show resonance spend half of their time as one resonance structure and half of their time as the other.

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Answer

False

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Question

True or false? All of the bonds in ozone are equal.

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Answer

True

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Question

Explain why ozone has no dominant resonance structure.

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Answer

The resonance structures of ozone have the same formal charges, meaning that they are equivalent. Therefore, they contribute equally to the overall hybrid molecule.

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Question

Which of the following symbols is used to show resonance?

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Answer

A double headed arrow

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Question

What is the octet rule?

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Answer

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, giving them the electron configuration of a noble gas.

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Question

How many valence electrons do most atoms try to achieve, according to the octet rule?

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Answer

2

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Question

According to the octet rule, atoms try to achieve the electron configuration of _____.

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Answer

A halogen

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Question

Which molecules feature an incomplete octet?

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Answer

BF3

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Question

Which of the following groups loses an electron in order to achieve an octet?

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Answer

Group I

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Question

Which of the following groups gains an electron in order to achieve an octet?

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Answer

Group VII

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Question

Why does hydrogen disobey the octet rule?

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Answer

Hydrogen's outermost electron shell has room for just two electrons. This means that it is stable with two valence electrons instead of eight.

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Question

Which of the following elements can disobey the octet rule?

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Answer

Al

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Question

Why can't the compound NCl5 exist?

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Answer

The central nitrogen atom would have ten valence electrons. However, nitrogen is in period 2 and only has room for eight electrons in its outer shell.

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Elements in _____ and beyond can form expanded octets.

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Answer

Period 3

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Question

Explain why some atoms can form expanded octets.

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Answer

Elements in period 3 and beyond can form expanded octets because the extra valence electrons go into a d-subshell. Elements in periods 1 and 2 can't form an expanded octet because they don't have a d-subshell available. 

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Question

Which of the following have a complete octet?

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Answer

O2-

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Question

What are Lewis structures?

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Answer

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

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

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Answer

False

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Question

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

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Answer

True

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

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Answer

False

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Question

Give three limitations of Lewis structures.

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Answer

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

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Question

Which is more useful at predicting the geometry of a molecule?

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Answer

VSEPR theory

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Question

Give an example of an element that doesn't follow the octet rule when drawing Lewis structures.

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Answer

  • Hydrogen
  • Helium
  • Compounds of boron, aluminium and xenon can also disobey the octet rule.

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Question

What is the shape of a water (H2O) molecule predicted by VSEPR?

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Answer

It is a V shape.

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Question

What is the shape of a  carbon tetrafluoride (CF4) molecule?

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Answer

It is tetrahedral.

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Question

What are the three exceptions to the octet rule?

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Answer

1. Odd number of electrons

2. Incomplete octets

3. Expanded octets

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Question

Which other elements, apart from hydrogen, always form incomplete octets with just two outer shell electrons?

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Answer

Helium and lithium

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Question

Give 2 examples of molecules that have an odd number of electrons.

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Answer

1. Nitric oxide

2.Nitrogen dioxide

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Question

True or false? Resonance is caused by delocalized pi bond electrons.

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Answer

True

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