Experiences

Molecular geometry

Elena Margheri

Italy

Vocational School

Chemistry

Teaching in class, Online Learning

Direct

A good comprehension of molecular geometry is very important to understand the chemical reactivity and the physical properties of substances and it is mandatory for a fruitful approach to organic chemistry in higher technical classes.
Through VSEPR theory, students learn that the arrangement around a central atom is due to the number of electron pairs, repulsing each other because of their negative charge. This repulsion determines the actual molecular geometry.
Approaching this issue, in my experience, students often find 3 kinds of difficulties:
1. they do not realize that the molecules drawn on their textbook or on a blackboard have a three-dimensional structure;
2. they forget that not only bond electron pairs, but all the electron pairs around the central atom, repulse each other and stay at the maximum distance;
3. among the different geometries, the tetrahedral one is the hardest to be understood. We built, in classroom, a paper tetrahedron and we used the “stick and balls” models; however, they found it difficult to locate the central atom and the correct bond angles.
The use of molecular drawing programs was useful; in fact, they make the visualization in the space of molecules easy, showing their three-dimensional structure under different perspectives. I found also some useful web sites that allow students themselves to build different molecules, not only simple ones, and to observe peripheral atoms bound to the central one as well as non-bonding-electron pairs, in their orbitals.
The use of these programs catches the interest of the students, because they can build molecules, foreseeing and then confirming molecular geometry, and, above all, they can play with their colleagues, competing in finding the right geometry.
In my opinion, ludic activity is an important instrument to attain both comprehension of an argument and, above all, a good disposition of mind towards the study of a subject. Competing with colleagues, joking and defusing mistakes can contribute to build “empathy” among students and it is another way to bring out mind speed, aptitude and team working skills. It is a teacher’s work to develop these abilities in young students, looking for different activities besides the lab one.