The Periodic Table

“After evolving for nearly 150 years through the work of numerous individuals, the periodic table remains at the heart of the study of chemistry. This is mainly because it is of immense practical benefit for making predictions about all manner of chemical and physical properties of the elements and possibilities for bond formation. Instead of having to learn the properties of the 100 or more elements, the modern chemist, or the student of chemistry, can make effective predictions from knowing the properties of typical members of each of the eight main groups and those of the transition metals and rare earth elements.”

I wasn’t very impressed with this book, but it wasn’t terrible. It didn’t include a lot of new stuff I didn’t already know and it focused in my opinion excessively on historical aspects; some of those things were interesting, for example the problems that confronted chemists trying to make sense of how best to categorize chemical elements in the late 19th century before the discovery of the neutron (the number of protons in the nucleus is not the same thing as the atomic weight of an atom – which was highly relevant because: “when it came to deciding upon the most important criterion for classifying the elements, Mendeleev insisted that atomic weight ordering would tolerate no exceptions”), but I’d have liked to learn a lot more about e.g. some of the chemical properties of the subgroups, instead of just revisiting stuff I’d learned earlier in other publications in the series. However I assume people who are new to chemistry – or who have forgot a lot, and would like to rectify this – might feel differently about the book and the way it covers the material included. However I don’t think this is one of the best publications in the physics/chemistry categories of this OUP series.

Some quotes and links below.

“Lavoisier held that an element should be defined as a material substance that has yet to be broken down into any more fundamental components. In 1789, Lavoisier published a list of 33 simple substances, or elements, according to this empirical criterion. […] the discovery of electricity enabled chemists to isolate many of the more reactive elements, which, unlike copper and iron, could not be obtained by heating their ores with charcoal (carbon). There have been a number of major episodes in the history of chemistry when half a dozen or so elements were discovered within a period of a few years. […] Following the discovery of radioactivity and nuclear fission, yet more elements were discovered. […] Today, we recognize about 90 naturally occurring elements. Moreover, an additional 25 or so elements have been artificially synthesized.”

“Chemical analogies between elements in the same group are […] of great interest in the field of medicine. For example, the element beryllium sits at the top of group 2 of the periodic table and above magnesium. Because of the similarity between these two elements, beryllium can replace the element magnesium that is essential to human beings. This behaviour accounts for one of the many ways in which beryllium is toxic to humans. Similarly, the element cadmium lies directly below zinc in the periodic table, with the result that cadmium can replace zinc in many vital enzymes. Similarities can also occur between elements lying in adjacent positions in rows of the periodic table. For example, platinum lies next to gold. It has long been known that an inorganic compound of platinum called cis-platin can cure various forms of cancer. As a result, many drugs have been developed in which gold atoms are made to take the place of platinum, and this has produced some successful new drugs. […] [R]ubidium […] lies directly below potassium in group 1 of the table. […] atoms of rubidium can mimic those of potassium, and so like potassium can easily be absorbed into the human body. This behaviour is exploited in monitoring techniques, since rubidium is attracted to cancers, especially those occurring in the brain.”

“Each horizontal row represents a single period of the table. On crossing a period, one passes from metals such as potassium and calcium on the left, through transition metals such as iron, cobalt, and nickel, then through some semi-metallic elements like germanium, and on to some non-metals such as arsenic, selenium, and bromine, on the right side of the table. In general, there is a smooth gradation in chemical and physical properties as a period is crossed, but exceptions to this general rule abound […] Metals themselves can vary from soft dull solids […] to hard shiny substances […]. Non-metals, on the other hand, tend to be solids or gases, such as carbon and oxygen respectively. In terms of their appearance, it is sometimes difficult to distinguish between solid metals and solid non-metals. […] The periodic trend from metals to non-metals is repeated with each period, so that when the rows are stacked, they form columns, or groups, of similar elements. Elements within a single group tend to share many important physical and chemical properties, although there are many exceptions.”

“There have been quite literally over 1,000 periodic tables published in print […] One of the ways of classifying the periodic tables that have been published is to consider three basic formats. First of all, there are the originally produced short-form tables published by the pioneers of the periodic table like Newlands, Lothar Meyer, and Mendeleev […] These tables essentially crammed all the then known elements into eight vertical columns or groups. […] As more information was gathered on the properties of the elements, and as more elements were discovered, a new kind of arrangement called the medium-long-form table […] began to gain prominence. Today, this form is almost completely ubiquitous. One odd feature is that the main body of the table does not contain all the elements. […] The ‘missing’ elements are grouped together in what looks like a separate footnote that lies below the main table. This act of separating off the rare earth elements, as they have traditionally been called, is performed purely for convenience. If it were not carried out, the periodic table would appear much wider, 32 elements wide to be precise, instead of 18 elements wide. The 32-wide element format does not lend itself readily to being reproduced on the inside cover of chemistry textbooks or on large wall-charts […] if the elements are shown in this expanded form, as they sometimes are, one has the long-form periodic table, which may be said to be more correct than the familiar medium-long form, in the sense that the sequence of elements is unbroken […] there are many forms of the periodic table, some designed for different uses. Whereas a chemist might favour a form that highlights the reactivity of the elements, an electrical engineer might wish to focus on similarities and patterns in electrical conductivities.”

“The periodic law states that after certain regular but varying intervals, the chemical elements show an approximate repetition in their properties. […] This periodic repetition of properties is the essential fact that underlies all aspects of the periodic system. […] The varying length of the periods of elements and the approximate nature of the repetition has caused some chemists to abandon the term ‘law’ in connection with chemical periodicity. Chemical periodicity may not seem as law-like as most laws of physics. […] A modern periodic table is much more than a collection of groups of elements showing similar chemical properties. In addition to what may be called ‘vertical relationships’, which embody triads of elements, a modern periodic table connects together groups of elements into an orderly sequence. A periodic table consists of a horizontal dimension, containing dissimilar elements, as well as a vertical dimension with similar elements.”

“[I]n modern terms, metals form positive ions by the loss of electrons, while non-metals gain electrons to form negative ions. Such oppositely charged ions combine together to form neutrally charged salts like sodium chloride or calcium bromide. There are further complementary aspects of metals and non-metals. Metal oxides or hydroxides dissolve in water to form bases, while non-metal oxides or hydroxides dissolve in water to form acids. An acid and a base react together in a ‘neutralization’ reaction to form a salt and water. Bases and acids, just like metals and non-metals from which they are formed, are also opposite but complementary.”

“[T]he law of constant proportion, [is] the fact that when two elements combine together, they do so in a constant ratio of their weights. […] The fact that macroscopic samples consist of a fixed ratio by weight of two elements reflects the fact that two particular atoms are combining many times over and, since they have particular masses, the product will also reflect that mass ratio. […] the law of multiple proportions [refers to the fact that] [w]hen one element A combines with another one, B, to form more than one compound, there is a simple ratio between the combining masses of B in the two compounds. For example, carbon and oxygen combine together to form carbon monoxide and carbon dioxide. The weight of combined oxygen in the dioxide is twice as much as the weight of combined oxygen in the monoxide.”

“One of his greatest triumphs, and perhaps the one that he is best remembered for, is Mendeleev’s correct prediction of the existence of several new elements. In addition, he corrected the atomic weights of some elements as well as relocating other elements to new positions within the periodic table. […] But not all of Mendeleev’s predictions were so dramatically successful, a feature that seems to be omitted from most popular accounts of the history of the periodic table. […] he was unsuccessful in as many as nine out of his eighteen published predictions […] some of the elements involved the rare earths which resemble each other very closely and which posed a major challenge to the periodic table for many years to come. […] The discovery of the inert gases at the end of the 19th century [also] represented an interesting challenge to the periodic system […] in spite of Mendeleev’s dramatic predictions of many other elements, he completely failed to predict this entire group of elements (He, Ne, Ar, Kr, Xe, Rn). Moreover, nobody else predicted these elements or even suspected their existence. The first of them to be isolated was argon, in 1894 […] Mendeleev […] could not accept the notion that elements could be converted into different ones. In fact, after the Curies began to report experiments that suggested the breaking up of atoms, Mendeleev travelled to Paris to see the evidence for himself, close to the end of his life. It is not clear whether he accepted this radical new notion even after his visit to the Curie laboratory.”

“While chemists had been using atomic weights to order the elements there had been a great deal of uncertainty about just how many elements remained to be discovered. This was due to the irregular gaps that occurred between the values of the atomic weights of successive elements in the periodic table. This complication disappeared when the switch was made to using atomic number. Now the gaps between successive elements became perfectly regular, namely one unit of atomic number. […] The discovery of isotopes […] came about partly as a matter of necessity. The new developments in atomic physics led to the discovery of a number of new elements such as Ra, Po, Rn, and Ac which easily assumed their rightful places in the periodic table. But in addition, 30 or so more apparent new elements were discovered over a short period of time. These new species were given provisional names like thorium emanation, radium emanation, actinium X, uranium X, thorium X, and so on, to indicate the elements which seemed to be producing them. […] To Soddy, the chemical inseparability [of such elements] meant only one thing, namely that these were two forms, or more, of the same chemical element. In 1913, he coined the term ‘isotopes’ to signify two or more atoms of the same element which were chemically completely inseparable, but which had different atomic weights.”

“The popular view reinforced in most textbooks is that chemistry is nothing but physics ‘deep down’ and that all chemical phenomena, and especially the periodic system, can be developed on the basis of quantum mechanics. […] This is important because chemistry books, especially textbooks aimed at teaching, tend to give the impression that our current explanation of the periodic system is essentially complete. This is just not the case […] the energies of the quantum states for any many-electron atom can be approximately calculated from first principles although there is extremely good agreement with observed energy values. Nevertheless, some global aspects of the periodic table have still not been derived from first principles to this day. […] We know where the periods close because we know that the noble gases occur at elements 2, 10, 18, 36, 54, etc. Similarly, we have a knowledge of the order of orbital filling from observations but not from theory. The conclusion, seldom acknowledged in textbook accounts of the explanation of the periodic table, is that quantum physics only partly explains the periodic table. Nobody has yet deduced the order of orbital filling from the principles of quantum mechanics. […] The situation that exists today is that chemistry, and in particular the periodic table, is regarded as being fully explained by quantum mechanics. Even though this is not quite the case, the explanatory role that the theory continues to play is quite undeniable. But what seems to be forgotten […] is that the periodic table led to the development of many aspects of modern quantum mechanics, and so it is rather short-sighted to insist that only the latter explains the former.”

“[N]uclei with an odd number of protons are invariably more unstable than those with an even number of protons. This difference in stability occurs because protons, like electrons, have a spin of one half and enter into energy orbitals, two by two, with opposite spins. It follows that even numbers of protons frequently produce total spins of zero and hence more stable nuclei than those with unpaired proton spins as occurs in nuclei with odd numbers of protons […] The larger the nuclear charge, the faster the motion of inner shell electrons. As a consequence of gaining relativistic speeds, such inner electrons are drawn closer to the nucleus, and this in turn has the effect of causing greater screening on the outermost electrons which determine the chemical properties of any particular element. It has been predicted that some atoms should behave chemically in a manner that is unexpected from their presumed positions in the periodic table. Relativistic effects thus pose the latest challenge to test the universality of the periodic table. […] The conclusion [however] seem to be that chemical periodicity is a remarkably robust phenomenon.”

Some links:

Periodic table.
History of the periodic table.
Jöns Jacob Berzelius.
Valence (chemistry).
Equivalent weight. Atomic weight. Atomic number.
Rare-earth element. Transuranium element. Glenn T. Seaborg. Island of stability.
Old quantum theory. Quantum mechanics. Electron configuration.
Benjamin Richter. John Dalton. Joseph Louis Gay-Lussac. Amedeo Avogadro. Leopold Gmelin. Alexandre-Émile Béguyer de Chancourtois. John Newlands. Gustavus Detlef Hinrichs. Julius Lothar Meyer. Dmitri Mendeleev. Henry Moseley. Antonius van den Broek.
Diatomic molecule.
Prout’s hypothesis.
Döbereiner’s triads.
Karlsruhe Congress.
Noble gas.
Einstein’s theory of Brownian motion. Jean Baptiste Perrin.
Quantum number. Molecular orbitals. Madelung energy ordering rule.
Gilbert N. Lewis. (“G. N. Lewis is possibly the most significant chemist of the 20th century not to have been awarded a Nobel Prize.”) Irving Langmuir. Niels Bohr. Erwin Schrödinger.
Ionization energy.
Synthetic element.
Alternative periodic tables.
Group 3 element.

December 18, 2017 - Posted by | Books, Chemistry, Medicine, Physics

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