Paradoxes and paradigms: elements and compounds—similar names, very different energetics

Semi-systematic and even trivial names are commonly used by the chemical community because of complicity of systematic names. But the differences between the expected and the actual composition are surprising in most cases. In this article, which is Part 1 (of a planned 4 articles), we consider elements from hydrogen (H) to argon (Ar) (Z = 1 to 18) and related compounds chosen for phonetic similarity of sound. A brief overview of the chemical energetics, most generally the enthalpy of formation, is given.


Introduction
As of this writing of this study (Fall, 2022), 118 elements are known, with "elements" uniquely defined by their atomic number (Z), i.e., the number of protons in the nucleus. Species with the same number of protons but a different number of neutrons are recognized as isotopes. This difference in neutron number generally has a very small qualitative effect on the observed chemistry, and different isotopes are not generally distinguished as different elements. We say generally because the three isotopes of hydrogen with 0, 1, and 2 neutrons, respectively, are occasionally called protium, deuterium, and tritium, and all three may be considered different elements. There are a few much more exotic hydrogen-like species called positronium, muonium, and muonic helium that can also be considered isotopes of hydrogen. Some of these names, as is generally the case for the other, more "classical," elements are "sensible" protium, deuterium, and tritium refer to primary (one), second (two), and third (three) forms of elemental hydrogen. Many, indeed most, element names are not "sensible." The suffix "-ium" (and, occasionally, "-um") generally refers to metals and thus to a tendency to form cations by electron loss. But the element (and, indeed, any neutral chemical species) that requires the greatest energy to remove an electron is, surprisingly and disconcertingly, helium (Z = 2) [1,2]. Helium is generally placed in the same column (descending) in the periodic table as neon (Z = 10), argon (Z = 18), krypton (Z = 36), xenon (Z = 54), radon (Z = 86), and so on. Fittingly, the recently discovered "and so on" element "oganesson" with Z = 118 has "-on" as the last two letters in its name, reflecting its home and neighborhood in the periodic table. However, the similarly named elements boron (Z = 5), carbon (Z = 6), silicon (Z = 14), and iron (Z = 26) do not share chemical similarity except that they are all solids under the conditions that most chemists study them or any derived species. Recently, a detailed analysis of the names of elements was published, which showed that "one often cannot put an equal sign between the element discoverer, the name giver and name publisher, the time of the discovery and the time of the naming, as well as between what was in fact discovered and how it was named" [3]. In addition, the names of elements' are not given much space even in classical chemical texts.
By contrast with the number and types of atoms, the number of uniquely characterized compounds (or should we say isolated polyatomic species) exceeds that of the elements by a factor of millions. The proper, systematic names of these compounds are "sensible" in that knowledge of the name enables the reader to deduce unambiguously the composition and even the chemical structure of the compound. With this knowledge, one can gain a chemical understanding of the species of interest. It has to be acknowledged though that the use of proper, systematic naming comes at a price-the proper, systematic names are usually so complicated that most chemists ignore them when writing, reading, listening, and talking about chemistry.
Instead, semi-systematic and even trivial names are generally used by the chemical community. Should we say communities whose common interests and information are obscured by word choice or, more commonly, by the complete absence of the word in another's vocabulary? Organic chemists are but rarely students of "anorganische Chemie," and practitioners of either organic or inorganic chemistry are even more rarely interested in "organ transplants" or "organ recitals." For this study, we ignore the chemical similarities and differences between the element and the word-related compound. Rather, we may ask how "sensible" the names "hydrogen, protium, deuterium, tritium, positronium, muonium, and muonic helium" are. Hydrogen, meaning "water former," is a correct statement both chemically and etymologically, but numerous compounds also contain hydrogen, and they are not named after hydrogen. Millions of compounds also are related to water by combustion-all hydrocarbons are among them. The chemical words "hydrate" and "anhydride" relate to the gain and loss of water. However, there are many chemical names that incorporate "hydr-" and "hydro-," "hydrate," and "anhydride" for which any connection with water and hydrogen are incomplete, confusing, misleading, or even incorrect. Such species include anhydrite, cyclohexanone cyanohydrin, dehydroacetic acid, epichlorohydrin, hydantoin, and ninhydrin.
This study reflects our long-term interest in chemical energetics, etymology, and wordplay. The compounds in the current study were selected based on the similarity of the sound (and/or scientific origin) of the element name. These compounds for which a key energetics quantity (most generally the enthalpy of formation) is known from experiments served as the study subjects. In no case, do we present a compound because that species "only" contains that element. Hydrogen chloride, for example, is not a species we chose for our discussion of either hydrogen or chlorine.
Due to a large number of elements, our paper will be divided into four (4) parts of about the same length. The first (the present) part discusses elements from hydrogen (H) to argon (Ar) (Z = 1 to 18). The second part will discuss potassium (K) through xenon (Xe) (Z = 19 to 54). The third part will present our discussion from cesium (Cs) through rhenium (Rn) (Z = 55 to 86), and the last part will discuss francium (Fr) through oganesson (Og) (Z = 87 to 118) and the diverse isotopes of hydrogen. In all cases, following the name of the element and its atomic number, we list an example of a compound and its formula, generally a more systematic name and the structure (with the exception of ionic salts), the CAS registry number, the numerical value where our preference was the enthalpy of formation value, and a literature reference (author, article title, and full citation to the relevant energy quantity, typically in kJ mol -1 ). In most cases, primary sources were used as the reference source for thermochemical data; however, in some cases, secondary sources, i.e., "Wagman et al. compendium" [4], and "Pedley compendium," [5] were used as preferred sources. The first part of our study of the names and energetics of elements and compounds is presented in Table 1.

Elements and compounds: similar names, very different energetics
Our discussion begins with a sample element, by using the name of the element for which we present compounds with similarly sounding names and their associated information. We end this introduction with the hope that the reader will find this study both enjoyable and educational.

Conclusions
Compounds that have similar phonetic of semi-systematic or trivial names to that of the elements have been studied in terms of their composition, structure, and chemical energetics. Despite similarity of names, the significant differences in expected and actual composition are surprising. However, semi-systematic or trivial names are widely preferred by the chemical community because of complicated nature of systematic names.
Acknowledgements All authors are grateful to the UMBC library science reference staff for supporting our extensive use of SciFinder n .
Author contribution All authors (MPS, KFE, JFL) have contributed equally to writing and reviewing the manuscript.
Funding MPS gratefully acknowledges the Slovenian Research Agency (ARRS Grant P1-0045, Inorganic Chemistry and Technology) for financial support.
Data Availability Not applicable.  [53] References mentioned at the last column of this table are listed at the Further Readings section