Using Molecular Modelling Kits to Predict, Explain, Observe, Explain the Relationship between Chemical Formulas and Molecular Shapes – Post #43

 In my grade nine applied science class the students are typically very good at both interpreting a chemical formula and in building molecular models, using the tradition ball and stick kits. However, when it comes to the higher order thinking required to add or subtract an atom from a molecular compound they are not able to visualize how the molecular shape is changed. This semester I decided to use the Predict, Explain Observe, Explain (P.E.O.E.) model that is encouraged by Smarter Science to help the students move forward in their thinking.

Prior to performing this activity the students were taught how to deconstruct a chemical formula. That is, they could articulate that H₂O (water) was a molecule composed of two hydrogen and one oxygen atom whereas H₂O₂ (hydrogen peroxide) was a molecule made up of two hydrogen and two oxygen atoms. Although the students were familiar with the Periodic Table and knew that families or groups represented the vertical columns and atoms in the same family shared chemical and physical properties they were unable to interpret the number of valence electrons each atom has and how this influenced chemical bonding.

1.      Initiate and Plan

To engage the students in this activity I began by distributing the molecular model kits and explaining the reason that different coloured balls had a different number of holes. Together we observed the colour coding of the atoms, their position on the Periodic Table, the number of holes drilled into the model atoms and the number of valence electrons they each had. Most students were very confused at this stage.

2.      Perform and Record

In pairs, the students found the correct coloured atoms to build one molecule of water and built it. It quickly became apparent that unless the oxygen atom was placed in the center the molecule could not exist. It was further observed that the molecule was “bent” or angled, and could not be a straight line.

I distributed the P.E.O.E. template to the students and had them predict how the look (shape, geometry) of an atom of H₂O would change if one atom of oxygen was added, making H₂O₂. When asked to make this prediction on paper most students just drew an oxygen atom to the oxygen atom that was central to the water molecule.  They did not take into account that there were insufficient valence electrons to make this possible. This concept appears to be too abstract for these learners at this stage of their academic career. 

Once the students had completed box 2 of their P.E.O.E. template they were then instructed to build the H₂O₂ molecule. When given the hands-on model to build they immediately realized that their predictions were incorrect, took their water molecule apart and constructed the correct molecule for H₂O₂. 

3.      Analyze and Interpret

When the students compared their models of water to that of hydrogen peroxide they were able to contrast their shapes (bent vs. straight line)

.4.      Communicating

Students used the PE.O.E.  to communicate what they had learned about the relationship between the chemical formula and the look of the molecule. They went on to build a number of various molecules. Initially I supplied them with a list of chemical formulas to build and draw. Subsequently they built molecules ad hoc and determined their chemical formulas. Lots of authentic learning occurred, everyone had fun and my learning outcomes were achieved.

What learning outcomes?

From Strand A of our Ministry document – Scientific Skills and Investigation we demonstrated scientific investigation skills (related to both inquiry and research) in the four areas of skills (initiating and planning, performing and recording, analysing and interpreting, and communicating) (A1). We also selected appropriate instruments and materials for particular inquiries (A1.2). And from the Chemistry unit we demonstrated an understanding of the properties of common elements and simple compounds, and general features of the organization of the periodic table (C3). 

What strategies do you use to help these learners understand that importance of chemical bonding in molecular geometry?