We're going to be honest: we do Physics. That's why this is called The Physics Classroom website. And when we do the Teacher's Notes section for our Concept Builders, we typically have a lot to say ... and a lot of resources to point you to. We're not claiming to be ignorant of chemistry; we just don't have a lot of resources here at The Physics Classroom to point you to. And so this page is going to be a lot shorter than our usual page that accompanies our Physics Concept Builders. That's our honest confession.

Many chemistry courses devote a unit to understanding the bonding in molecules. Covalent bonding and Lewis electron dot structures is typically one topic in such a unit. And often times the flow of the unit leads to a discussion of how the bonding in molecules leads to a prediction of the shape of the molecule. This leads to a well-known model called the Valence Shell Electron Repulsion Theory (VSEPR). The theory is built on the premise that the electrons groups - bonding electrons and lone, unshared electrons - that surround the central atom of the molecule will repel one another and spatially arrange themselves about the central atom so as to minimize the repulsions. The shape of the molecule thus emerges from the number of electrons groups surrounding the central atom and from the number of bonded atoms that are associated with those electron groups. 

The most common student difficulty with this topic is to impulsively jump to conclusions regarding the shape of molecules before drawing a Lewis electron dot diagram and determining the number of electron groups that surround the central atom. The shape of the molecule cannot be determined from the molecular formula. After all, H₂O and CO₂ have the same type of molecular formula but one has a bent molecular shape and the other is linear. One would never know this until they first took the time to count the number of valence electrons, draw a Lewis electron dot diagram and then inspected the central atom to determine the number of electron groups that surround it.

This Concept Builder is designed to challenge this common student difficulty. Three molecular formulas are given; the formulas - like H₂O and CO₂ - are usually of the same type. There are two tasks that students must accomplish. First they must identify the correct Lewis diagram. There are typically four to six answer options to toggle through and select from. Once all three correct Lewis diagrams have been selected, they then apply their understanding of VSEPR in order to identify which molecule has a shape that is different than the others. Each question has a Help Me button that leads to a rich set of information about VSEPR and molecular shape. It's definitely worth encouraging students to study it.

Like all our Concept Builders, this Concept Builder utilizes a variety of strategies to make each student's experience different. The ordering of questions is random. The Question number assigned to each question is scrambled. For instance, two side-by-side students will not have the same question for question number three. And questions are organized into "groups" with questions within the same group being very similar (for instance, they have the same type of chemical compounds) but not identical.

The Concept Builder also keeps track of student progress. It requires that students demonstrate a mastery of questions in each Question Group. If they miss a question from one group, then they will have to answer two consecutive questions correctly in order to demonstrate mastery. Progress is displayed in the progress report on the right side of the Concept Builder. A star indicates a demonstration of mastery. A question with a red background indicates that the student has missed the question. And a question with a yellow background means that the student must get one more question from that Question Group correctly answered in order to obtain a star. When an activity is completed, the student will be awarded a Trophy. This Trophy is displayed on the Main Menu screen. These strategies make the Concept Builder an ideal addition to the 1:1 classroom and other settings in which computers are readily available. 


In order to complete an activity, a student must correctly analyze each question of that difficulty level. If a student's analysis is incorrect, then the student will have to correctly analyze the same or very similar question twice in order to successfully complete the activity. This approach provides the student extra practice on questions for which they exhibited difficulty. As a student progresses through an activity, a system of stars and other indicators are used to indicate progress on the activity. A star is an indicator of correctly analyzing the question. Once a star is earned, that question is removed from the que of questions to be analyzed. Each situation is color-coded with either a yellow or a red box. A red box indicates that the student has incorrectly analyzed the question and will have to correctly analyze it twice before earning a star. A yellow box is an indicator that the question must be correctly analyzed one time in order to earn a star. Once every question of a difficulty level has been analyzed, the student earns a Trophy which is displayed on the Main Menu. This system of stars and trophies allows a teacher to easily check-off student progress or offer credit for completing assigned difficulty levels.

The most valuable (and most overlooked) aspect of this Concept Builder is the Help Me! feature. Each question group is accompanied by a Help page that discusses the specifics of the question. This Help feature transforms the activity from a question-answering activity into a concept-building activity. The student who takes the time to use the Help pages can be transformed from a guesser to a learner and from an unsure student to a confident student. The "meat and potatoes" of the Help pages are in the sections titled "How to Think About This Situation:" Students need to be encouraged by teachers to use the Help Me! button and to read this section of the page. A student that takes time to reflect upon how they are answering the question and how an expert would think about the situation can transform their naivete into expertise.