Noble Gases: “Monoatomic Substances” or “Monoatomic Molecular Substances”?

 

Abstract

Recently, views are rising among some teachers that noble gases should be understood as “existing in the form of monoatomic molecules\’\’ instead of just “separate atoms”. Undoubtedly, this has made students confused when trying to understand the properties of noble gases. To evaluate the two notions, this article first reviews historical discussions around the definition of molecules and their enrichment when quantum mechanics was introduced. Based on this, it discusses the advantages and disadvantages of both notions in both fields of chemistry and education, using some of the current secondary school, high school-university transition and undergraduate textbooks. Lastly, after another round of discussion of the educational purposes and the environment in various regions, supported by original investigations of correspondingly local students and research done by other scholars, this article proposes that noble gases should still be classified as monoatomic substances.

Introduction

The definitions of molecules and particle properties of noble gases are treated as thoroughly investigated notions and used widely as fundamental notions in high school chemistry education. The long-used definition is “Monoatomic Substances”, which is more aligned with the higher understanding of chemistry with the help of quantum mechanics, while the newly suggested one is “Monoatomic Molecular Substances”, which is alleged to be more preferable education-wise in terms of the easier integration of noble gases with other related knowledge such as the classification of crystal structures of substances in general.

The first concept of the particular nature of noble gases is adopted in the Singaporean chemistry education system, while the second concept is adopted in the Chinese chemistry education system. It is thus speculated that this difference would lead to some noticeable impacts on the chemistry learning outcome of students in these two chemistry education systems.

To evaluate the value of both notions, it is necessary to first review the notion of “molecule” and later review how noble gases are classified on the particle level.

From Dalton to Avogadro——The history of the concept of \”molecule\”

At the beginning of the 19th century (1805), Dalton systematically introduced his atomic theory to the world,[1] which states that substances are made of small, indivisible atoms. However, it soon met some problems. It was proved to be unable to explain the Law of Combining Volumes of Gases, discovered by Gay-Lussac in 1809, which states that “when gases react together they do so in volume which bears simple whole-number ratio provided that the temperature and pressure of the reacting gases and their products remain constant”.[2]

Avogadro gave a great answer. In his hypothesis, he suggested the notion of a “molecule” and stated that “molecules of gases can consist of several atoms” and that “under the same temperature and pressure, the numbers of molecules in gases of equal volume are the same”.[2] In the problem (Dalton’s atomic theory’s inability to explain the Law of Combining Volumes of Gases) raised above, gases with non-integer reaction coefficients (e.g. H2 and Cl2 in the reaction to form HCl) could be explained as they are both made of “diatomic molecules”. Avogadro’s molecule theory perfectly explained the Law of Combining Volumes of Gases by making “molecules”, instead of the “indivisible atoms”, the ones to bear the non-integer reaction coefficient suggested by the Law of Combining Volumes of Gases.

This gave people a new mindset to see the world of particles and suggests that “molecules” were supposed to act as an intermediate between atoms and substances in some circumstances where substances’ properties cannot be explained directly with theories based on atoms.

The modern effort in defining molecules

As early as 1997, the International Union of Pure and Applied Chemistry (IUPAC) has stated that a molecule is “an electrically neutral entity consisting of more than one atom (n>1). Rigorously, a molecule, in which n>1, must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.”[3] This definition is obtained using quantum mechanics calculations and expressions, which means that a molecule consists of more than one atom binding together to achieve a state with lower energy. This can be seen as a newer version of Avogadro’s definition with the enrichment brought by quantum mechanics.

Some versions of definitions / inferences of molecules in textbooks

However, in real education practice, simplified definitions are given instead of the direct IUPAC definition. There are two main streams.

One stream (stream 1) allows the possibility of saying “monoatomic molecules”. Molecules are introduced to students (or can be inferred) as “the smallest particles that maintain the properties of a substance/have a stable independent existence”.[2, 4, 5] This can be supported by the idea that “if one classifies both individually-existing atoms and entities of atoms as ‘molecules’, one can say that chemistry is to research molecules’ constituents, structure, properties and changes”.[1] This is, in fact, also exhibited in the term “kinetic molecular theory of substances” that is used in place of the “kinetic particle theory” in many books, suggesting that “molecule” carries the equal concept as “particles”. From this perspective, this stream appears to be also acceptable and easy to understand.

In the other stream (stream 2), molecules are defined as “an independent structure consisting of two or more atoms chemically bound together”.[6, 7, 8] This is not only supported by the IUPAC definition of molecules[3] but also by the historical view of the notion of “molecule”, which is the view of molecules acting as a bridge linking the atom and substance together. They both propose a basic idea that molecules should be treated as particles of a higher level than atoms, acting as a bridge between “atoms” and “substances” when necessary (when needed to explain the Law of Combining Volume of Gases, for example), and should not be equal to atoms. Therefore, this stream denies the possibility of the notion of “monoatomic molecules”.

From the comparison above one can see that both streams are supported with valid evidence, and the difference between the two is merely whether one would want the definition of “molecules” to be general (stream 1) or detailed (stream 2), which makes little difference for basic chemistry learners who are still in high school. Thus, one should go back to noble gases and discuss which one is preferred.

Back to noble gases themselves

Noble gases are special elements in both chemistry history and modern education. Historically, they formed a new group and completed Mendeleev’s periodic table in the late 19th-century.[1] From an educational perspective, however, their classification becomes quite arguable.

When it comes to the context of crystal structures of substances in general, the main conflicts are concentrated in the notion of “molecular crystals”, also known as “molecular solids”. It is so because in many textbooks, molecular crystals (or molecular solids) are said to “consist of (atoms or) molecules held together by intermolecular forces”.[1, 2, 7, 8] As one can see, the textbook editors have made a great effort to include noble gases in the concept of molecular crystals by trying to include noble gas atoms in this definition. However, whether the effect of this effort is helpful for students is still pending.

Editors of some other textbooks tried to resolve this problem in other ways. A concept of “atomic crystal”, which, according to these textbooks, refers to “crystals which are formed by individual atoms held together by dispersion forces”, was created to fit the noble gases.[6] This can be seen as another way around the confusion of the definition of the idea of “molecules” raised by noble gases. However, the potential confusion between this newly-created concept and the “network covalent crystals” is also significant (especially when in Chinese, where both are called “原子晶体”).

In another effort, the concept of crystal structure comes in the chapter of covalent bonding,[9] where noble gases are naturally left out of the discussion because they are thought to not bond at all. It thus seemed unnecessary to discuss the crystal structures of noble gases. However, this would lead to a vacancy in the concept of the crystal structure of substances in general. Therefore, the suggested concept of noble gases being “monoatomic molecular substances” might be a way to fill up this vacancy. This suggested concept is thus reasonable.

Additionally, in many American textbooks, the notion of “kinetic molecular theory” is used in place of “kinetic particle theory”.[4, 6, 8] Indeed, it can be argued that it might be simply because this notion was historically named so. However, the fact that the editors accepted this notion in their books reflects that they, to some extent, have agreed to make an effort to help their readers to understand chemistry as a whole by using the term “molecule” to represent most of the usual particles studied. Hence, this also supports the rise of the notion of “monoatomic molecules”.

Methods

To gain a deeper insight into which classification of noble gases should be preferred in education, the authors surveyed students in China and Singapore to ask about their understanding and perceptions of the classification of noble gases. In China, the possibility of saying “monoatomic molecules” is allowed, whereas in Singapore, the possibility of saying so is denied. They thus represent both streams of the classification of noble gases.

Due to the COVID-19 pandemic, the survey was an online questionnaire conducted via Wenjuanxing (a Chinese online survey website) for both Chinese and Singaporean students. Both groups first were asked to indicate their chemistry education level out of five ranges: Sec1-Sec2, Sec3-Sec4 (where “Sec” is short for Secondary), JC1-JC2 (where “JC” is short for junior college), Undergraduate (chemistry related subjects), and Postgraduate (chemistry related subjects). The survey had a total of 118 responses from Chinese students (among which 94 were fully completed and fully analysable) and 47 responses from Singaporean students (among which 46 were fully completed and fully analysable). Among the 94 Chinese students surveyed, none were from Grade 7-8 (Sec 1-2) because in China, chemistry is only introduced to students starting from Grade 9 (Sec 3). 78 of the Chinese students were from Grade 9-10 (Sec 3-4), 10 were from Grade 11-12 (JC 1-2), and 6 were undergraduate students in chemistry-related subjects. Among the 46 Singaporean students surveyed, 6 were from Sec 1-2, 24 were from Sec 3-4, 16 were from JC 1-2, but none were from university. For both Chinese and Singaporean respondents, none were postgraduates.

The chemistry education levels of respondents were divided in such a manner because they are different stages of chemistry education, and in each stage of chemistry education, the textbooks are different as a result of different education guidelines.

Participants were first asked, “During your chemistry learning progress, has your teacher mentioned the particulate identity of noble gases? If yes, what is it?” They were to choose their answer from the following choices: “No”, “Yes, noble gases are ‘monoatomic substances’” and “Yes, noble gases are ‘monoatomic molecular substances’”.

Participants were then asked, “Which expression of particulate identity of noble gases do you agree with?” They were to choose their answer from the following choices: “Noble gases are ‘monoatomic substances’” and “Noble gases are ‘monoatomic molecular substances’”

Finally, participants were asked, “If possible, could you explain briefly in your answer above?” It was an open-ended question.

It should be noted that due to technical issues, Chinese respondents\’ responses to the third question were unable to be collected in a formally analysable form. Hence, the analysis of Chinese students was conducted without considering the last open-ended question.

Results and Discussion:

From the investigation of chemistry education in China and Singapore, very distinctive results were obtained. When asked about which definition of noble gases they agree with, most Chinese students agreed that noble gases should be classified as monoatomic molecules regardless of the knowledge level of the students (Figure 2.1).

Figure 2.1 Distribution of perceptions in different grades of Chinese students

When it comes to Singaporean students, one can see that in contrast to Chinese students, the majority of students agreed to the definition of noble gases as monoatomic substances, as shown in Figure 2.2.

Figure 2.2 Distribution of perceptions in different grades of Singaporean students

When Singaporean students were asked to further elucidate their choices, those who agreed on the definition of monoatomic molecules failed to give detailed explanations and gave very vague responses, many of which were based on feelings or a lack of knowledge (Figure 2.3). This shows these responses were not well-considered or closely understood by the respondents and indicates that Singaporean students’ perception of the definition of noble gases leans to the side of monoatomic substances, as they were able to give scientific explanations of their choice (Figure 2.4).

Figure 2.3 Responses of Singaporean students who chose “monoatomic molecules” as preference

Among Singaporean students who chose the definition of monoatomic substances as their preference, the responses mainly fell into three types (Figure 2.4): the first type was in terms of general feelings (as shown in yellow), the second type was in terms of the definition of molecules (as shown in green), and the third type was in terms of electron configuration and bonding (as shown in purple). Additionally, there was one response that showed advanced understanding, which was able to evaluate the classification from both perspectives of IUPAC definition and daily usage (as shown in blue).

Figure 2.4 Responses of Singaporean students who chose monoatomic substances as their preference

One can see that the Singaporean students mostly gave detailed and accurate explanations in terms of bonding and the definition of molecules. This result indicates that most of the Singaporean students had a relatively good understanding of the topics that they were currently learning and may therefore be able to consistently apply these concepts in their understanding of related substances.

Evaluation of the two definitions

When evaluating the effect of the two classifications of noble gases (“monoatomic substances” and “monoatomic molecular substances”) on constructing the chemistry learning system of students, one should bear in mind that one of the three methods of classification commonly used in middle school and high school chemistry education is to put substances with similar properties in the same category to develop students’ abilities to deduct the properties of unknown substances using knowledge of the known substances in the same category. This method mainly has two aims, one of which is to help students systematize knowledge. When facing new questions, students need to have the skill to extract useful information and compare the new questions with the previously encountered questions. Putting substances with similar properties in the same category helps students to develop the ability to sort questions into categories and efficiently develop solutions according to each category. The second aim is to lay the foundation of sustainable application of the subject for every student. Unlike higher chemistry education, where courses are more specialized and focused on high-level knowledge, middle and high school chemistry education functions more to build a basic understanding of chemistry concepts and fundamental analytical and problem-solving skills, equipping all students with the ability to scientifically analyse and address problems in future learning. For instance, when students encounter the anomaly of discoloured phenolphthalein solution regaining red colour upon heating, their high school chemistry education is expected to enable them to suggest a hypothesis, design and carry out experiments to test the hypothesis and reflect upon the results and repeat the process for a satisfactory outcome.[10] Through this method of classification, students can actively learn the process of reasoning and deducting instead of simply memorising knowledge.

Coming back to noble gases, the distinctive results obtained from students in China (where the possibility of saying “monoatomic molecules” is allowed) and Singapore (where the possibility of saying “monoatomic molecules” is denied) can illustrate the advantages and disadvantages of the two classifications of noble gases discussed above. By comparing the general trends in both sample countries, one can discover that the chemistry education in China focuses on the objective of researching molecules’ constituents, structure, properties and changes as well as “the application of chemistry knowledge in social life and scientific technology”.[11] In addition, the examination system in China focuses very little on areas that require a deeper understanding of the nature of chemistry, such as written scientific argumentation, which is “a means of articulating evidence, warrants, and claims; reflecting on proposed ideas; and critiquing the scientific work of others”.[12] These two features are typical for education systems in stream 1. Considering these features, it is not surprising that middle school and high school students in stream 1 mostly agree with the idea of classifying noble gases as monoatomic molecules, which helps them better understand and memorise the properties of noble gases and other related concepts such as crystal structures.

As opposed to the high school chemistry syllabus generally adopted in stream 1, in the chemistry syllabus of GCE A/O level provided by the Ministry of Education (MOE) in Singapore, the topic of the classification of crystal structures is absent, while bonding structure and molecule definitions are given much more emphasis in both secondary school and junior college. In this case, since students in stream 2 (where the possibility of saying “monoatomic molecules” is denied) are not required to encounter the possibly confusing concepts of crystal structure and its classification, it is understandable that this stream tends to give students the classification of noble gases derived from the IUPAC definition of molecules, as the two aims of helping students systematize knowledge and laying the foundation of sustainable application of the subject of chemistry for every student will be achieved.

Despite that the understanding of noble gases as monoatomic molecules is already “enough” for middle school and high school students in stream 1 to address chemistry problems, it is striking that the result of the investigation shows that the majority, although a smaller portion as compared to middle school and high school students, of college chemistry students who should have already learned the “accurate definition” of molecules suggested by IUPAC to strengthen their understanding of the microstructure of substances from the quantum mechanics’ view also define noble gases as monoatomic molecules (Figure 2.5).

Figure 2.5 Composition of Chinese college students’ responses

This result suggests the continuity of misconceptions. College students (represented by the results obtained from Chinese college students) may tend to stick to their knowledge of molecules as what they were taught in middle school and high school. These pre-implanted concepts would impede the renewal of knowledge when higher level and more advanced concepts of molecules are required in their further study of chemistry. This study suggests that this continual misconception would likely not apply to Singaporean (stream 2) students, as they have been taught the most accurate definition of noble gases as monoatomic substances from the beginning.

This continuity of misconceptions is also the result of constructivism, which means the persistence of misconceptions. When individuals encounter new knowledge, their first reaction will be to incorporate it into their pre-existing understanding framework rather than to alter the framework itself. This may cause students to be unable to correct their originally wrong understanding. When a contradiction occurs, they may change the newly acquired knowledge to fit their pre-existing misconceptions.[13] Such effect will be especially obvious when students have reinforced their misconceptions about the nature of noble gases and the definition of molecules through repetitive practice and exams. Constructivism theory also states that in the sense of students’ cognitive features, knowledge should be discovered as an integrated whole, and there shouldn’t be compartments and divisions in the teaching of concepts.[14] This means students would have higher acceptance of a concept that is consistent throughout the whole learning process, and in turn better actively construct a system of knowledge. Therefore, if the concept of noble gases as monoatomic molecules is first introduced to students in middle school and high school, it would easily lead to the difficulty of university chemistry students to reverse their previous misconceptions and construct a consistent system of higher chemistry knowledge. In this case, the understanding of the structure of substances is instead based on the quantum mechanics calculations and the IUPAC definition of the molecule as “an electrically neutral entity consisting of more than one atom (n>1) and corresponding to a depression on the potential energy surface that is deep enough to confine at least one vibrational state” is adopted.

Limitations and Future Directions

As the result of a relatively immature investigation method, one must be aware of the small sample sizes (118 responses from Chinese students and 47 responses from Singaporean students, among which there were only 6 Chinese college students and 0 Singaporean college students surveyed) of the original research carried out in this article. The generalisability of the study is thus likely greatly affected, potentially leading to an inaccurate or not thoroughly considered conclusion. It is hoped that in the future more well-rounded studies and investigations could be carried out with larger sample sizes of each education level category so that the results obtained on this topic of the classification of noble gases can be scientifically responsible and conclusive.

Additionally, there is another possible limitation, as the range in which the survey was conducted was very likely limited to the authors’ own circle of friends. Although the respondents consist of students from various grades and schools of different education quality, it is subject to a certain degree of lack of randomness. Survey companies might be employed in the future to distribute the survey to a wider range for more accurate results.

Conclusion

In conclusion, the way of classifying noble gases’ particulate properties reflects how the definition of “molecule” is expected to serve students in an education system.

“Monoatomic molecular substances” (stream 1) boosts students’ understanding of chemistry as an entity and helps them link microscopic features of noble gases with their macroscopic properties, whereas “monoatomic substances” (stream 2) ensures the accurate understanding of one of the most important particles for every high school student—molecules.

The importance of the latter (specific understanding of molecules) overrides the former’s possible benefits of understanding chemistry as an entity. “Monoatomic substances” is preferred not only because of its accuracy but also its alignment with the guiding mindset of chemistry. It is therefore both scientifically and educationally advisable to leave noble gases as an exception when classifying substances, unless a theory that is able to include them without interfering with the current theory system can be developed. Thus, this article proposes that noble gases should continue to be taught as “monoatomic substances” instead of changing the concept into “monoatomic molecular substances”.

References:

1. Beijing Normal University, Huazhong Normal University, and Nanjing Normal University, eds. Inorganic Chemistry. 4th ed. 2 vols. Beijing: Higher Education Press, 2003.

2. Hua Tongwen, Bian Jiang, Wang Yingxia, and Chen Jingzu, eds. Principles of General Chemistry. 4th ed. Beijing: Peking University Press, 2013.

3. A. D. McNaught, and A. Wilkinson. Compendium of Chemical Terminology(the ‘Gold Book’). 2nd ed. Blackwell Scientific Publications, Oxford, 1997. https://doi.org/10.1351/goldbook.

4. Kenneth W. Whitten. General Chemistry. 6th ed. Cengage Learning, 2000.

5. Institute of curriculum and teaching materials. Chemistry(9th grade). Vol. 1. 2 vols. Textbook for compulsory education. Beijing: People’s Education Press, 2012.

6. Silberberg, Martin S. Principles of General Chemistry. 2nd ed. Dubuque, IA: McGraw-Hill, 2010.

7. Ebbing, Darrell D, and Steven D Gammon. General Chemistry. 9th ed. Boston, Mass.: Houghton Mifflin Co., 2009.

8. Chang, Raymond, and Jason Scott Overby. General Chemistry: The Essential Concepts. 6th ed. New York, NY: McGraw-Hill, 2011.

9. Tan Yin Toon, Chen Ling Kwong, John Sadler, and Emily Sadler. GCE O Level Chemistry Matters. 2nd ed. Singapore: Marshall Cavendish Education, 2013.

10. Zhu Zhengde. (2016). Experimental study on the abnormal phenomenon of phenolphthalein test solution turning red when heated. Reference of Chemistry Teaching in Middle Schools (Z1),56-58. doi:CNKI:SUN: ZHhJ.0.2016-Z1-019.

11. The Ministry of Education of China Examination Centre, ‘2019 Syllabus for China’s College Entrance Exam’, 2019, https://gaokao.chsi.com.cn/zyk/zybk/specialityDetail.action?specialityId=73383819.

12. Yore, Larry D., Marilyn K. Florence, Terry W. Pearson, and Andrew J. Weaver. ‘Written Discourse in Scientific Communities: A Conversation with Two Scientists about Their Views of Science, Use of Language, Role of Writing in Doing Science, and Compatibility between Their Epistemic Views and Language’. International Journal of Science Education 28, no. 2–3 (15 February 2006): 109–41. https://doi.org/10.1080/09500690500336601.

13. Piaget J. The Origins of Intelligence in Children. New York: International University Press, 1952. http://www.bxscience.edu/ourpages/auto/2014/11/16/50007779/Piaget%20When%20Thinking%20Begins10272012_0000.pdf.

14. Glover, John A., and Royce R. Ronning, eds. Historical Foundations of Educational Psychology. Boston, MA: Springer US, 1987. https://doi.org/10.1007/978-1-4899-3620-2.

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