3D Marshmallow Models Lab Activity
Adapted from the California Learning Lab
Today’s Question
How do the bonded and non-bonded electron pairs on the central atom of a molecule determine the 3D shape of a molecule?
Your Task
You will use marshmallows and toothpicks to construct models of molecules with different numbers of electron pairs and bonds to a central atom and compare how those electron pairs determine the 3D shape of the molecule.
Data and Information
Model
Use candy and toothpicks (or other similar objects) to represent atoms and electron groups to make 3D molecules.
1. Build a marshmallow model kit at home.
● The kit should contain marshmallows or a similar type of flexible candy in 2 different colors. At least three of one color and 9 of another color. You could also use clay, Play-Doh, etc. These items represent the atoms in the model.
● You will also need at least 9 toothpicks or other short stick-like objects (cut straws, spaghetti, etc.) that can be inserted into the candy. These items represent the electron groups (bonding or non-bonding) in the model.
2. Each of the molecules in this exercise represents some common 3D molecular shapes formed around a central atom “A”. Additional atoms will be represented by “B”. Choose one color marshmallow for the central atom “A” and another color for the surrounding atoms “B”.
3. Each toothpick represents one electron group which could be either a single, double or triple bond to atom B OR a lone pair (LP) of electrons (E). The physical number of bonds (like there are two bonds in a double bond) is not relevant to examination of the 3D shape of the molecule. Each single, double, or triple bond counts as ONE electron group.
3D Marshmallow Models Lab Activity
Adapted from the California Learning Lab
Instructions
Reminder: One toothpick = one electron group.
Part I, Construction of Electronic Geometries
1. For each of the molecules below, construct your electronic geometry models using one toothpick to represent an electron group (E) by placing the toothpicks as far apart as possible. Draw the geometric sketch of the 3D structures you build on the Report Sheet.
1. AE2 (180 degrees between two toothpicks)
2. AE3 (120 degrees between three toothpicks)
3. AE4 (109 degrees between four toothpicks = slightly larger than a right angle between each pair of toothpicks)
2. Examine each of the models you built above (AE2, AE3, and AE4) and assign it the correct electronic geometry from the choices below. Add your choices to the Student Note Sheet.
1. Tetrahedral
2. Linear
3. Trigonal planar
Information
The electronic geometry of a molecule describes the 3Dshape of all the electron groups around the central atom. These electron groups, represented by toothpicks in this model, may include bonds to other atoms (B) AND lone pairs of electrons.
Part II
1. Take the models you constructed above and add marshmallows or candies as “B” atoms onto the toothpicks. You should have two “B” atoms on AB2 (which will replace AE2 because these electron pairs are now being shared), three “B” atoms on AB3, (replacing AE3) and four “B” atoms on AB4 (replacing AE4). Since all of your electron pairs (toothpicks) are connected to atoms, the molecular and electron geometries are the same. Note this detail on your Report Sheet.
2. Now remove one “B” candy from the AB3 and AB4 molecules but leave the toothpicks and draw the geometric sketch these new structures on your Student Report Sheet. The molecular and electron geometries will now be different.
3. Examine the shapes you have drawn of the geometric sketch above and add the names of the electronic geometries and the molecular geometries from the list you used in Part I.
3D Marshmallow Models Lab Activity
Adapted from the California Learning Lab
4. From AB4 remove one additional “B” candy and draw the geometric sketch on your Student Report Sheet.
5. Now examine all the data you have collected and organize it into a table that makes sense to you and will be easy to use to assign both electron and molecular geometries. Use the patterns that emerge to help organize this table.
6. Assign each of the models you built its correct molecular geometry name below and fill in the corresponding table that you make. Hint: there are no electron geometries that are bent.
1. Trigonal planar
2. Bent
3. Trigonal pyramidal
7. Create a table on the Student Report Sheet include the following:
1. the total electron groups present around the central atom A (the toothpicks)
2. the total electron groups between atoms (toothpicks but no B)
3. Organize each of your models (AE2, Ab2, etc. ) on the table, grouping them by electron pair groups, then molecular geometries.
Part III
Now that you understand VSEPR Theory and Molecular Geometry for the following molecules draw the Lewis Structure, determine its electron pair geometry, it’s molecular geometry/shape, provide the bond angle, and determine whether the molecule is overall polar.
This is the last page of your Report Sheet.
Submit your completed Report Sheet to the appropriate Canvas assignment page on the due date.
Bringing it Together: Applying the “Rules”
Examine the “rules” you have developed in each row of Table 1 for determining 3D electronic and molecular geometry and draw a schematic or flowchart you can use in an explanation to determine shape starting from the formula of the molecule. Consider how you can group the rules together.
These rules are based on a chemical concept known as VSEPR Theory:
Valence Shell Electron Pair Repulsion
Given the relationships you developed between electron groups and shape, explain how you think VSEPR theory determines molecular shape and include if this candy model provides strengths or suggest areas of improvement.
