4.1: The Periodic Table- A Brief Introduction

Figure 4-2: The "twin-towered" edifice we know as the Periodic Table of the Elements embeds a wealth of information. Elements here are color coded: metallic elements are blue, nonmetals are pink, and metalloids are purple. Also note the highlighted column labels; elements in columns 1A-8A are referred to as Main Group Elements, those in 1B-8B are referred to as Transition Metals. See Note 1 for comments on this labeling scheme of the vertical columns. For clarity, elements 57-70, and 89-118 are omitted from this version of the Periodic Table.

Philip Ball, in his book Stories of the Invisible [2], discusses how the structure of chemistry parallels that of languages. As part of an argument against the traditional emphasis on the Periodic Table – sacrilege to many in our trade! - he likens atoms to letters and molecules to words. Despite the fundamental importance of letters in any written language, things don’t get interesting until one focuses on words. Letters don’t inspire. Words do. Likewise, molecules have far more variety and can do far more interesting things than atoms. In other words, knowing the elements is fine and dandy, but chemistry is a rich field only because of the richness of the compounds the elements make.

His point is compelling and borne out by the experience of most chemists: we spend far more time thinking about what molecules do (or don’t do, if they stubbornly resist acting how we think they should) than about isolated atoms or pure elements. But, to counter his analogy a bit, one still needs to understand how letters can combine to make words: rules exist and your efforts as a reader and writer are made much easier if you can recognize patterns that provide clues as to what they are. The same is true in chemistry. It is beyond dispute that compounds are vastly richer in their properties than elements: tens of millions of unique compounds have been identified, and the list is growing by hundreds every day, while the number of naturally occurring elements is only ninety. Understanding some simple rules for how individual atoms interact is enormously helpful when trying to make sense of all that richness. 

An intuitively accessible approach to begin understanding the information in the Periodic Table begins with an examination of the physical characteristics of the elements. A cursory look reveals one thing immediately: most of the elements are metallic. Moreover, the metallic elements seem to be spatially segregated from other elements, as illustrated by the color-coding of Figure 4-2. Famed author and neurologist Oliver Sacks noted this separation almost immediately when, as a boy, he visited a wall-sized exhibit of samples of all of the elements at the Science Museum near his home in London. With samples of each element arranged in a display that followed the organization of the Periodic Table, he later described his reaction thusly:

“… I found myself looking at the table in almost geographic terms, as a realm, a kingdom, with different territories and boundaries. Seeing the table as a geographic realm allowed me to rise above the individual elements, and see certain general gradients and trends. Metals had long been recognized as a special category of elements, and now one could see, in a single synoptic glance, how they occupied three-quarters of the realm – all of the west, most of the south – leaving only a smallish area, mostly in the northeast, for the nonmetals. A jagged line, like Hadrian’s Wall, separated the metals from the rest, with a few ‘semimetals’, metalloids – arsenic, selenium – straddling the wall.” [3]

You are undoubtedly familiar with that “twin-towered edifice”, Ball’s words again, that is the Periodic Table. Each element is represented by a tile that shows its symbol and some representative data such as its average atomic mass [4], atomic number, etc. Regardless of the specific details shown on any version of the Periodic Table, however, there is considerable information embedded in it by virtue of its organization alone, so much so that it can be daunting at first. How does one interpret it or recognize what information is there? We certainly do not think memorizing the entire table, or even parts of it, are worthwhile activities, but knowing how to extract meaningful information from it is absolutely necessary. And to do that, it is helpful to consider how it was originally devised.

Figure 4-3. Dmitri Mendeleev, the Russian chemist who first articulated the Periodic Law and whose Periodic Table of the Elements has been used by chemists for 150 years. (Image credit: Public Domain, retrieved from https://commons.wikimedia.org/wiki/F...evCab.jpg#file)

The Periodic Table was first laid out in its current form by Russian chemist Dmitri Mendeleev in the late 1860s [5]. His was not the first attempt to meaningfully organize the elements, but his wealth of knowledge and audacity in predicting not only the existence of undiscovered elements, but many of their physical and chemical properties as well, made other chemists take notice; his was not only a scheme for organizing existing knowledge, but a powerful tool for predicting the properties of compounds never before prepared. Mendeleev arranged the elements according to two criteria: increasing atomic mass and by their chemical and physical properties. Doing so revealed what is sometimes referred to as the Periodic Law: the pattern of chemical and physical properties displayed by a given row of elements on the Periodic Table is repeated by the elements in subsequent rows. What this means is best explained using examples. In rows beyond the first, which has only hydrogen and helium, the elements are ordered in such a way that they become “less metallic” as their atomic masses increase. This may seem a peculiar notion at first because we have used the terms “metal” and “non-metal” in a manner that suggests a simple dichotomy, but these terms are more relative than you may have anticipated. There is a continuum of properties between what we consider to be metallic and nonmetallic, just as there is a continuum between polar and nonpolar character as we described in Chapter 1. Specifically, there are elements that appear metallic but do not share many of the properties of metals. Silicon for example, has a metallic reflectivity but is a poor conductor of electricity; it is a semiconductor, having conductivity in between those of conductors and insulators. Such elements are metalloids, those that form the “Hadrian’s Wall” boundary, as Oliver Sacks described them, between the metals and nonmetals. 

We'll use the second row of the Periodic table to illustrate Mendeleev’s idea, starting start with lithium on the far left. It is a very soft and highly active metal, meaning that it easily reacts by losing an electron to form the Li⁺ cation. Sitting to its right is the harder and less metallic beryllium, where “less metallic” means less reactive in that it does not form ions as readily. To the right of beryllium is boron, a metalloid that can exist in a number of forms, none of which have metallic conductivity. Continuing on, we encounter carbon, nitrogen, oxygen, fluorine, a spectacularly reactive non-metal (it violently reacts to readily form fluoride anions, F^-), and finally to an inert gas, neon. Likewise, the third row begins with a very soft, highly reactive metal, sodium, and the elements show a similar decrease in metallicity; next to sodium is the harder, less reactive calcium, then aluminum, silicon, the metalloid we described above, and then the nonmetals phosphorus, sulfur and chlorine, another highly reactive non-metal, before ending with yet another noble gas, argon.

Beyond these physical characteristics, Mendeleev also examined patterns in the compounds that elements made, finding many striking patterns. For example, consider the oxides formed by the elements in the second, third and fourth rows of the periodic table. The formulas of these are shown below in Table 4-1. Notice that when you express the formula of the oxides as E₂O_x, where E is the elemental symbol of the element oxygen is combined with, the E:O ratio increases from 2:1 to 2:7 regularly across each row, meaning that the chemical properties are in some ways mirroring the physical properties of the elements. This pattern repeats itself with each row. The elements in column 4A, for example are CO₂, SiO₂ and GeO₂. (This is a 1:2 ratio, or can be viewed as a 2:4 ratio instead; the latter is shown in the table below to emphasize the trend across the table.) To Mendeleev, this realization was startling and he felt he had revealed a previously hidden dimension to Creation. Clearly, there must be some fundamental principles at work that caused the very soft metals to form cations with a +1 charge, while the harder, less reactive metals that were slightly heavier form cations with a +2 charge, etc. Mendeleev, who died in 1907, never learned the underlying explanation of what causes those patterns as they are grounded in quantum mechanics, which was not developed until the 1920s, about 60 years after his version of the Periodic Table was published. 

Mendeleev looked at many compounds beyond oxides and a host of physical properties as well. What emerged were compellingly self-consistent patterns, wherein elements in the same column share many physical and chemical properties. To name just a few such examples:

• Metals in Group 1A (now called the alkali metals) are all very soft and highly reactive [6], forming highly water-soluble and crystalline compounds with elements in Group 7A (called the halogens) in ratios of 1:1 (e.g., NaCl, KBr, LiF, NaI);
• Elements in Group 2A (called the alkaline earth metals) are harder and less reactive than alkali metals and form compounds with the halogens in ratios of 1:2 (e.g., CaCl₂, MgBr₂);
• Elements in Group 7A (the halogens) are all highly reactive nonmetals that all combine with hydrogen to make acids with the formula HX (e.g., HF, HCl, HBr, and HI).

The power of Mendeleev's "periodic system" (as he referred to it) was that it provided insights to make predictions of compounds and elements that had not been discovered yet. For example, germanium was discovered in 1886, fifteen years after Mendeleev predicted not only its existence, but many of its chemical and physical and properties, a selection of which is summarized in the table below. [7]

So accurate were some of his predictions, that the value of his "periodic system" (as he referred to it), was quickly accepted by chemists and other scientists. The impact of his achievement is difficult to overstate, and the development of the Periodic Table is considered one of the most important advances in science. To mark the 150th anniversary of his publication, the year 2019 was dubbed the International Year of the Periodic Table by the United Nations General Assemble and UNESCO. The purpose was "to recognize the importance of the Periodic Table of Chemical Elements as one of the most important and influential achievements of modern science reflecting the essence not only of chemistry, but also of science. physics, biology and other basic science disciplines."

Figure 4-4. 2019 was proclaimed the International Year of the Periodic Table of the Elements by the United Nations, an occasion marked by many countries on postage stamps, such as this block that was printed in Moldova. (Image source: https://www.stamps.kg/products/2019-...-elements.html)

Notes and References

[1] There are three common ways of labeling columns, and the one in Figure 4-2 is not preferred by IUPAC. Therefore you will probably see other versions of the table from other sources, including this one that we recommend you download and print for easy reference. Our choice in using this labeling scheme is based on the simple fact that, for the "A" columns at least, there is a direct connection between the labels and the number of electrons that will participate in bonding; this is also similar to the labeling scheme employed by Mendeleev. The column labels recommended by IUPAC go from 1 through 18, which obscures some useful relationships. We will therefore refer to the version shown in Figure 4-2 to teaching fundamental bonding concepts, but be aware that you will likely encounter the IUPAC recommended version elsewhere.

[2]  Stories of the Invisible: A Guided Tour of Molecules, Philip Ball, Oxford University Press, 2001.

[3] Uncle Tungsten, Oliver Sacks, Knopf (2001) p. 191. A similar geographical treatment of the elements was employed by Peter Atkins in The Periodic Kingdom: A Journey into the Land of Chemical Elements (1995). 

[4] Average atomic mass is defined as the weighted average of the masses of the isotopes of a given element.  Thus chlorine, which is roughly 75% chlorine-35 and 25% chlorine-37 has an average atomic mass of roughly 35.5 amu. 

[5] A brief but wonderful account of Mendeleev's life and approach to developing the Periodic Table can be found in the chapter entitled "Mendeleev's Garden" in Oliver Sacks' Uncle Tungsten mentioned in Footnote 3, above.

[6] Be careful though.  It is easy to overstate the similarities of elements in the same columns; while all of the alkali metals do indeed react readily with water, the rates at which they do so varies tremendously, with the reaction becoming increasingly vigorous as you go down the column. Lithium reacts fairly slowly, gently fizzing away as bubbles of hydrogen gas evolve; sodium reacts faster and potassium explodes on contact with water. This video demonstrates this trend. The reactions of rubidium and cesium are even more energetic; see https://www.youtube.com/watch?v=D4pQz3TC0Jo&t=194s this video demonstration includes some high-speed footage that is quite impressive, and this one compares the various alkali metals.

[7] The information in Table 4-2 is adapted from Greenwood and Earnshaw's "Chemistry of the Elements, 2nd ed", Elsevier, 1997.