Electron shells students’ understanding

Of course, what the students are really interested in is why thallium is poisonous. Surprisingly, thallium is toxic because it mimics potassium in the body. But why would thallium behave like potassium As we study the periodic table and chemical periodicity, there is no immediate reason to suspect that these two elements would have similar properties. A close look at the electron shell arrangement of thallium and potassium, however, reveals that both form +1 ions. Since Tl+ ions also happen to be similar in size to K+ ions, they are able to replace potassium ions in cellular processes. (Thallium poisoning is treated with a compound called Prussian blue, which binds to +1 ions and thus facilitates their removal from the body.) It is clear then that we cannot understand the toxicity of thallium without studying its atomic structure and electron distribution. But chemistry is only part of the story. The effects of thallium poisoning only make sense if the... 

Summary. It is pointed out that, in order to avoid misconceptions, the introduction of ions is very important ions have been dealt with as basic particles of matter according to Dalton s atomic model (see Chap. 5). In order to understand the charges of ions and the change of ions and atoms by electron transfer, the differentiated atomic model with nucleus and electron shells should be introduced. With the assistance of a clear terminology, it is easy to formulate half-reaction for the oxidation and reduction steps, the number of electrons to be transferred can be clearly recognized. Finally, if mental models -for instance, from involved atoms or ions in Galvanic cells or in batteries - are relayed and drawn by the students themselves, then they could more easily see through the redox processes or even perhaps be able to repeat them independently. In all explanations, one should pay attention that the observations should be done at the substance level, but that the interpretations and discussions of reaction equations should consequently take place at the level of the smallest particles as atoms, ions and molecules. 

The existence of bonding which does not lead to atoms having full electron shells is consequently something of a mystery to many learners. These students are not able to understand - for example - why sulphur would want to go beyond SCI2 or SF2 to give SCI4 or SFe, or why the chlorine atom in AICI3 would want to share an electron pair to form a dative bond. 

There is a very extensive literature for the electronic structure of two-electron atoms and ions, including several articles that have been published in the Journal of Chemical Education. If the chemistry student understands some of the problems involved in describing the behavior of how electrons interact in an atom, he or she will be much better served to understand their behavior in molecules. In this section, we consider only the closed-shell ground state of two-electron systems. 

I have spent a good deal of time asking secondary-age students to tell me about their understanding of various science topics. There are some very common alternative ideas (alternative conceptions or misconceptions) that are found in just about any class. For example, it is very likely that in any upper secondary science or chemistry class you teach, at least some (and in many classes it will be most) of the students think that chemical reactions occur so that individual atoms can fill their electron shells. (If you cannot see what is wrong with that idea, then you may find Chapter 4 quite interesting.)... 

Merchant et al. (2012) investigated the impact of a 3D desktop virtual reality environment on the learning of the valence shell electron pair repulsion (VSEPR) theory in an introductory chemistry class. Their sample consisted of 204 undergraduates enrolled in a chemistry course at a university in the USA. They used 11 -multiple-choice questions to assess the students understanding of molecular angles, molecular geometry, and species identifications. They also constructed 15 items to measure the students self-efficacy for learning VSEPR theory. The students self-efficacy was found to positively relate to their scores on the multiple-choice test. 

Barker [18] additionally comments, that teachers put far too much emphasis on the Octet rule in order to determine formulas and bindings of chemical species. As a result, the students rely on this rule to deduce formulae. During the lesson unit on ionic bonding, teachers often use this rule, in order to show that some atoms fill their shells through electron transfer instead of sharing electrons in covalent bonding. She further points out, that students are not capable of understanding how ion lattices are formed solely based on this explanation [18]. 

It is certainly very important to teach these ideas as theoretical, because although the models are successful and central to modern chemistry, it is not helpful if students think our models of atoms and molecules are precise realistic descriptions. Certainly the models introduced at secondary level fall somewhat short of this. As just one example, the notion that atoms contain shells of electrons should not be taken to imply either that there is any kind of physical shell which contains the electrons (as some students assume), nor that the electrons in a shell can always be considered as equivalent. Students who select chemistry as a subject for further study will soon run into problems if they develop fixed ideas along these lines. It is much better to teach that atoms often behave as though they have electrons arranged in shells, but to warn students that scientists have found this to be a simplification. This approach provides students with a more authentic understanding, avoids over-commitment to the model that might impede more advanced learning, and better reflects the nature of chemistry as a science. 

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