Using molecular models: Avoid chemical formulae misconceptions

Using models to explain abstract concepts in chemistry is a very important teaching and learning tool, when students are trying to envisage what is happening at the sub-microscopic level. However, it is equally important that students understand what the models are actually trying to represent and appreciate the limitations of the models. If students fail to understand the limitations of the models it can reinforce or lead to further misconceptions.

Many of the simple models, such as coloured beads or plasticine can only be used to count the number of atoms and therefore only represent the chemical formula without giving away any other information.

Chemical Jigsaws can be used to represent ionic compounds.

When using this model students often confuse the ‘anion’ and ‘cation’ but the model does help students to understand that the overall charge on a compound is zero.

The above Chemical Jigsaw could be used to represent calcium chloride, CaCl₂ (Ca²⁺, 2x Cl^-). However, this model can also help to reinforce another common misconception. Students often think that the formula of an ionic compound represents a molecule (of calcium chloride etc) rather than that the formula indicates the ratio of positive and negative ions in a lattice structure.

So which is best; Chemical Jigsaws or Molymods?

It depends on the context of the lesson and your intended learning outcomes. They both have strengths and limitations, as shown in the table below.

Another common difficulty that students often encounter, is understanding that the relative molecular (formula) mass is simply the sum of the relative atomic masses of the component elements. This can cause some students problems because they cannot physically weigh and compare the individual particles. Using formula cards can help to overcome this barrier to learning.

The relative molecular mass (M_r) = sum of A_r values of the component elements

M_r (H₂O) = 1 + 1 + 16

= 18

Student activity

Follow the downloads for the ’Formula card student activity’ and ’Formula card template’ below.

Both elements of the student activity are adapted from Beyond Appearances: Students’ misconceptions about basic chemical ideas [2nd Edition (2004); Vanessa Kind, School of Education, Durham University].