Instructor Card #2

August 29, 2018
Flash PointsDane DeSutter

Find the next Flash point in this series here or go to the previous post.

Being able to fluidly translate between diagrams or representations in chemistry is an essential to speaking the language of chemistry. In fact, the majority of diagrams in organic chemistry textbooks use the bond-line formalism, so we should think about how to help students connect their experience with Mechanisms to other resources like their textbook. 

This lesson will have you work with students to explicitly perform the translation between two common representations: ball and stick and bond-line. The overall design of this activity should be flexible to really any puzzle in Mechanisms, here we will look at some puzzles in the Structure pack.

Learning Objective

  • Students should be able to translate between ball and stick representations and bond-line formalisms

Learning Activity

Note to Instructor

  • Assign students three of the introductory puzzles from the Structure pack in Mechanisms (e.g., Essentials > Structure #1, 2, 3)
  • Download the Mechanisms App via the IOS App store or Google Play

Scaffold Learning

On screen you should see a partial ball and stick representation of (2E)-but-2-en-1-ylium. Direct students to tap on each of the carbon atoms to reveal the implicit hydrogen atoms when first opening this and other puzzles. This should expand the structure to give a complete ball and stick representation.

Ball and stick representaiton of (2E)-but-2-en-1-ylium

Before you have students work through translating this into bond-line formalisms, first emphasize the rules of bond-line diagrams:

  • Carbon-carbon bonds are represented as straight lines
  • Carbon is implied by vertices
  • Non-carbon atoms are represented using their atomic symbol from the periodic table
  • Bond lines reproduce the geometric layout of the molecule
  • Hydrogen atoms are always implied unless attached to heteroatoms
  • Non-zero formal charges must be indicated

Model for students how to draw a carbon backbone of the molecule by sketching all the carbon-carbon bonds in the structure. Pay particular attention to whether students indicate a double bond between C-2 and C-3.

Draw students' attention to the terminal carbon bearing the formal charge. Explain that the formal charge on the terminal carbon in Mechanisms should also appear in their diagram.

Ask students to compare the charged carbon to the neutral carbon and propose a reason why one carbon is charged while the other is not (e.g., one carbon is bonded to two hydrogen atoms, the other is bonded to three hydrogen atoms). Indicate that the charged carbon has a formal charge because it has an incomplete octet and that this must be indicated in the bond-line diagram.

Finally, explain that the conventions of the bond-line diagram require that hydrogen atoms connected to the carbon atoms be implicit. Tell students to tap each carbon atom in Mechanisms until all of the hydrogen atoms are hidden.

Continue the exercise with other example structures until students begin to understand how the conventions of bond-line diagrams work. A completed bond line diagram of (2E)-but-2-en-1-ylium is shown below.

Equivalent ball and stick & bond-line representations of (2E)-but-2-en-1-ylium

Challenge Your Students

  • Consider assigning a more complex molecular structure with heteroatoms (e.g., Structure Puzzle #16)
  • Ask students to use Mechanisms to generate a valid resonance contributor to any of the assigned puzzles and sketch a valid bond-line representation of that contributor

How this Activity Targets Learning

This activty is designed to:

  1. Have students directly compare molecular representations used in Mechanisms with those they will frequently see in their textbooks.
  2. Support students as they learn exactly how to translate from the explicit ball and stick representation of a molecule to equivalent bond-line representations. Translation is supported in this activity by making explicit how the features of each representation map to one another.

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