Lewis Dot Structures: Resonance

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  • 0:01 Review: Lewis Dot Structures
  • 4:06 Resonance
  • 5:52 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

In this lesson, we'll review Lewis dot structures and how to draw them. Then, learn about resonance and resonance structures for molecules and polyatomic ions. Afterwards, assess your new knowledge with a quiz.

Review: Lewis Dot Structures

Take a stroll on a sunny day and look up at the blue sky. Up high in the atmosphere is a layer of gas made up of ozone molecules. If you can't see it, that's okay! Ozone is a clear gas that protects us from harmful radiation released by the sun.

Ozone (O3) is made up of three oxygens bonded together in a somewhat unique way. To understand what makes the bonding in ozone unique, we will review Lewis dot structures and how to draw them using ozone. Then, we will learn about resonance and resonance structures.

To review, Lewis dot structures are diagrams that show the arrangement of atoms and valence electrons in a molecule. A Lewis dot structure consists of three parts: element symbols, lines and dots. The element symbols represent the nucleus and core electrons of an element, lines represent bonds and dots represent valence electrons.

A single bond is formed when two atoms share two electrons. A single line represents a single bond between two atoms. A double bond is formed when four electrons are shared between two atoms. Two parallel lines represent a double bond between two atoms. Three parallel lines represent a triple bond between two atoms.

When drawing Lewis dot structures, it is important to remember the octet rule. The octet rule is a chemical rule that states all atoms want to have a stable, noble gas electron configuration, consisting of eight valence electrons. The only exception to this rule is hydrogen, who is happy to have two valence electrons. A correct Lewis dot structure fulfills the octet rule for all atoms in the molecule.

Lewis dot structures can also be drawn for polyatomic ions. A polyatomic ion is an ion containing multiple atoms. It is basically a molecule that has gained or lost electrons so that the constituent atoms can achieve the octet rule. Now that we've reviewed, let's draw the Lewis dot structure for ozone (O3).

Step 1:
Sum the number of valence electrons from each atom in the compound. Ozone has three oxygens, each with six valence electrons. We have a total of 18 valence electrons

Step 2:
Make a skeleton structure for your molecule by connecting atoms with single bonds. I'm going to put my three oxygens in a row and connect them with single bonds.

Step 3:
Determine how many electrons remain after making the skeleton structure. I used four electrons to make my skeleton structure, so 14 electrons remain.

Step 4:
Place remaining electrons to satisfy the octet rule for atoms. I'm going to start by placing electrons around the outside oxygens. Each outside oxygen already has access to two electrons in the bond that connects it to the central oxygen, so each outside oxygen only needs six valence electrons. After placing electrons on the outside oxygens, I only have two electrons remaining, so I place them on the central oxygen.

Step 5:
If there aren't enough electrons to satisfy the octet rule for all atoms, try double or triple bonds. My middle oxygen only has access to six electrons, and it needs two more. I'll make a double bond between the central oxygen and the oxygen to the left so that it can achieve the octet rule.

Step 6:
Make sure atoms are satisfied. If you want, draw a circle around each atom that has achieved the octet rule. This Lewis dot structure works for ozone.

But before we call it quits, let's go back to step 5. During step 5, we made a double bond between the left oxygen and the central oxygen. But what if we had made it on the right side instead? Now we have two Lewis dot structures for ozone. This leads us to ask, which structure is the right one?

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