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The Periodic Table. Geoff Rayner-Canham
Читать онлайн.Название The Periodic Table
Год выпуска 0
isbn 9789811218507
Автор произведения Geoff Rayner-Canham
Жанр Биографии и Мемуары
Издательство Ingram
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Chapter 2
Selected Trends in Atomic Properties
This book cannot be a comprehensive compilation of all the periodic properties. And it is certainly not intended to be a turgid, endless, boring, collection of tabulated data and graphical plots. In this chapter, there will be a focus just upon four major parameters: electronegativity, ionization energy, electron affinity, and relativistic factors. But first …
What is a chemical element? This question may seem self-evident, but it is not. In Chapter 1, a chemical element was defined, by inference, as an atom with a specific number of protons. Now, entering the world of chemistry, things become murkier. Many electrons have been put to work to produce erudite articles on the subject [1–3].
An atom clearly is a chemical element. It is a minuscule fragment of matter with no color, and no sense of whether it is supposed to be a metal or nonmetal; or have any other bulk properties. It does have an electron configuration, an ionization energy, and an electron affinity. However, to define its electronegativity (discussed in the next section), comparison must be undertaken using a pair of dissimilar atoms.
But most chemists deal with real, visible materials, that is, “elements and compounds,” rather than “atoms and molecules.” Thus, the question arises, if the word “element” is used to describe the identity of an atom, can it also be used to do double duty for a bulk collection of atoms [4]? And, indeed, at what size does a cluster of atoms begin to exhibit bulk properties [5]?
The proposal has been made to use the term “elementary substance” for a bulk substance that does not undergo chemical decomposition into other substances [6]. There are two challenges with adoption of this new terminology: first, the cumbersomeness of a two-word term; and second, the acceptance among, not just chemists, but also the wider community at large. As much as this author, in general, espouses precise and correct terminology wherever feasible, here, the word “element” will continue to do “double duty.”
Electronegativity
It may seem odd that the first parameter in this chapter is not a simple measurable property of an atom. However, the concept of electronegativity underlies much of our interpretation of chemical properties and behavior. Jensen has traced the origins of the electronegativity concept back to the early 1800s, with Berzelius naming the concept “electronegativity” in 1811 [7]. In fact, it was in the late 19th and first half of the 20th centuries when the concept became refined into its modern form [8].
Electronegativity as a Fundamental Property
Leach has produced a comprehensive study of the various parameters that have been used to obtain numerical values for electronegativity. He argued that even though electronegativity is not a single parameter of an atom’s properties, it is a fundamental property itself. Among his conclusions were the following (edited) statements [9]:
•Electronegativity is an extremely successful but ill-defined heuristic concept for the description of central properties of entities in the dappled chemical world.
•Electronegativity is a theoretical construction than a natural property. That is, it is cannot be measured directly.
•Electronegativity’s main applications (descriptions of polarities and bonding modes in substances, depiction of oxidation numbers, explanation of reaction mechanisms and acidity, etc.) are qualitative. In Leach’s view, the mathematization of electronegativity is excessive and tends to lead to apparent scientificity.
•Electronegativity