Periodic Table and the Atomic Shell Model

• Type of physical science: Atomic physics
• Field of study: Nonrelativistic quantum mechanics

The periodic table was developed as a means of correlating chemical data about the elements and about the way they react by placing elements in families according to their chemical similarities. The shell model of the atom provided the theoretical background to explain why the elements fit into the arrangement as they do.

Overview

The periodic table is a reflection of the periodic law and is explained in terms of the shell theory of the electron arrangement around atoms. Separately, each of these items is important to the scientist's view of nature, and together they compose one of the most important bases for modern understanding of the manner in which materials, natural and human-made, behave. Chemists constantly make use of the correlations and regularity embedded in these ideas to make predictions and to guide planning for future experiments.

The formulation of the periodic law drew on the large body of experimental knowledge that had been collected about the elements that were known at the time. Included in this knowledge were boiling points, melting points, and specific gravities of the elements and/or their compounds, atomic (or combining) weights, the atom ratio present in compounds, the extent of metallic behavior of the elements, and the heat effects that were involved when substances reacted with one another. Examination of this wealth of data showed that many elements could be grouped together according to similarity between their properties. This similarity led to the formulation of the periodic law. In modern terms, the law summarizes this behavior by stating that the properties of the elements are periodic functions of their atomic numbers; that is, there is a repetition of properties in a regular fashion when the elements are listed from hydrogen, whose atomic number is one, on to elements with atomic numbers near one hundred. The elements that are gathered by similarities in their properties are said to be in the same group, or family.

There is not one particular shape of the periodic table but many, depending on the type of trends in properties that one wishes to highlight. All tables do, however, have in common the fact that the elements are listed sequentially according to their atomic numbers.

Three-dimensional tables exist that are helical, with the families listed in straight lines parallel to the axis of the helix. Spiral tables show the families along the radii. The most common table in use is a two-dimensional, gridlike arrangement called the long form of the periodic table. This grid is eighteen blocks wide and seven blocks high with a fourteen-block-by-two-block section appended beneath it. The vertical columns are the families, or groups, and the horizontal rows are called periods, being numbered sequentially from the top down.

The elements listed within the appended section are collectively known as the rare earth elements, even though some are not all that rare in the composition of Earth. Reading from the left side, the main body of the table is composed of a two-block-wide section containing the alkali metal family and the alkaline earth family, a ten-block-wide segment containing the transition metals, and a six-block-wide part that contains both the posttransition metals and the nonmetals. The name "transition elements" is apt because these elements fill the space on the table between the most chemically active metals and the nonmetals. In a six-block-wide section, the last two families are the only ones with common names, those being the halogens and the noble gases, in that order. Elements listed in the main body of the table, exclusive of the transition metals, are often referred to as the representative elements. There is a short form table, which has retained some popularity, that treats the transition metals in a different manner, but that table does not reflect the shell theory of the atom as directly.

The shell theory explains that behind the similarities in chemical and physical properties stands the shell nature of the atom's electrical configuration. The negatively charged electrons that surround a given, neutral nucleus are equal in number to the atomic number of the element. They are thought of as existing, both spatially and energetically, in shells or levels starting near the nucleus and continuing outward. Those electrons that are nearest to the nucleus are most strongly attracted toward the positive charge residing at the nucleus, and this attraction decreases with increasing distance from the nucleus. The shells themselves are, in some cases, divided into subshells of slightly differing energies. The existence of the shells and subshells and the manner in which the electrons occupy positions in them is derived from interpretation of emission or absorption spectra. The electrons in the shells or subshells that extend farthest from the nucleus are those that are least tightly held and those that are involved in the process of chemical reaction. This shell is termed the "valence shell" and the electrons there the "valence electrons."

The manner in which electrons occupy the shells and subshells results in a number of electrons in the valence shell of between one and eight in the case of each element. This shell is divided into two subshells, one called an s subshell with a maximum of two electrons, and one called a p subshell with a maximum of six. In the case of the transition metals, an incomplete inner d subshell capable of containing between one and ten electrons exists. The rare earth elements are characterized by having an incomplete f subshell of between one and fourteen electrons. As a consequence of the regularity of electron occupancy of these shells and subshells, all elements that are a part of a particular group have the same valence shell electron arrangement. Because these are the electrons involved when atoms react, this sameness is the cause of the chemical similarities of elements in a family.

When it comes to considering the transition metals and the rare earth elements, the situation becomes a little different. The difference in the electronic structure of these elements is not in the outer shell of electrons but rather in an interior subshell. This variation on the more general situation results in there being considerable similarity of these elements in the horizontal direction on the table as well as within families. As an example, iron, cobalt, and nickel have many chemical and physical properties that are similar. Other trends in properties—such as the ability to form charged species called ions and the melting and boiling points—that are found on the periodic table are related to the size differences between atoms. This, in turn, depends on the number of shells of electrons present and on the strength of the force attracting the outer shell electrons to the nucleus. Finally, this effect depends on the number of protons in the nucleus, the total number of electrons, and the distance from the nucleus to the valence electrons.

Applications

The periodic table is a premier organizer of information for chemists. The exact nature of the information placed in this organization varies from person to person, depending on their particular areas of interest. The periodic table offers a way to organize, systematize, and condense much of the vast amount of chemical and physical information about the elements and their compounds. As a teaching and learning tool, it is considered invaluable.

An early use of the table was the prediction of undiscovered elements. When the table was developed, there were spaces left blank in recognition of the fact that the similarities of elements in families demanded an element whose properties fit no known element. The element germanium was not discovered until after the periodic table had been constructed, and the information contained in the table aided the search for the element. It was known that some element was missing between silicon and tin in the family headed by carbon. Knowing the types of reactions undergone by silicon and tin, and the types of compounds that they formed, gave direction to the search to find the missing element. By making predictions based on the known values of properties of the surrounding elements, it was possible to predict the atomic weight, specific gravity, and color of the element as well as the specific gravity and melting and boiling points of some of its compounds. Scientists searching for the new element thus had clues both to find the element and to identify it when it was located. The correspondence between the predicted values and the measured values was very good.

This use has limited modern application because any elements to be found are "off the end" of the table and are human-made rather than naturally occurring. Scientists have come to value the table for its correlative rather than its predictive function. Much information about the attributes of an element and its compounds and the reactions they undergo can be kept easily in mind by knowing the family to which the element belongs. Just as people know general information about an animal if they are told that it is a dog, chemists know general information about an element if they are told that it is a halogen.

A closer look at two families will provide a view of the trends and similarities that are embedded in the table. First, it is necessary to deal with the special case of the element hydrogen.

As the simplest element, hydrogen has properties that make it unique. As the element itself, hydrogen behaves as a typical nonmetal, while in chemical compounds it takes the role of a metal when combined with an active nonmetal and the role of a nonmetal when combined with an active metal. Because of this unusual behavior, hydrogen belongs to more than one family and is often shown that way on the tables. This rare behavior is the reason that hydrogen is left out of the following survey of chemical families.

The alkali metal family, neglecting hydrogen, consists of five naturally occurring elements and one human-made, radioactive element. The six elements, in order of increasing atomic weight or number, are lithium, sodium, potassium, rubidium, cesium, and francium.

Francium is prepared only in small amounts, so it is not included in further discussion of this family. None of these elements is found naturally in its elemental form. They are all too reactive.

When produced in the form of the element, each is a soft, silvery-white metal that is a good conductor of electricity. The melting and boiling points of the elements decrease in a fairly regular way, proceeding from the lightest to the heaviest element. The specific gravity follows the reverse trend and increases as the atomic weight increases. The highly reactive nature of the elements is evidenced by the violence of their reaction with water. Lithium is the least reactive and cesium, the most reactive. This activity increase going down the list of the elements illustrates the general trend of reactions with substances other than water. The chloride compounds of all these elements are high-melting solids, which have the property of conducting electricity either when melted or when dissolved in water. All these chloride compounds also are characterized by having one chloride atom present for each atom of the alkali metal. These similarities in chemical reactions extend to reactions with substances other than water and to compounds other than chlorides.

The halogen family consists of the elements fluorine, chlorine, bromine, iodine, and astatine. The last of these is a human-made, radioactive element and will not be discussed further.

All these elements are too reactive to exist in the elemental form in nature. When they are prepared, each of these elements forms a molecule composed of two atoms of the element.

Fluorine and chlorine are gases at room temperature and pressure, bromine is a liquid, and iodine is a solid. The trend in melting and boiling behavior and density is clear. The reactions of fluorine and chlorine are quite vigorous, but those of the remainder become less so proceeding down the list, showing the trend in their reactivities.

Even the most completely stocked chemical storeroom will not have every chemical. A scientist in need of a particular chemical, however, can use the periodic properties shown by the table to select possible substitutions. By making a choice from the elements either directly above or below the one in question, the scientist is assured of a substance whose reactions and properties are quite close to those of the missing substance. It should be made clear, however, that this is an approximate substitution and not an exact one. Just as there are variations from one dog to another, there are variations from one element to another. Within the same group, the greatest similarity is between adjacent elements, but there are subtle differences that could be important in some applications. For example, calcium chloride is water soluble but calcium fluoride is not, even though chlorine and fluorine are adjacent elements in the halogen family.

The periodic table provides a quick and easy guide, but a sound chemical background is needed to make use of it to good advantage.

Context

The periodic law and its representation as the periodic table preceded by years the development of the shell theory of the atom that so clearly illuminates the necessity of periodic behavior of the elements.

While Dmitry Ivanovich Mendeleyev is usually credited with the origin of the periodic table, others contributed. In France, A. E. Beguyer de Chancourtois presented a helical table in 1863 based on sixteen groups of elements. In this same time period, Gustavus Henrichs, teaching at the University of Iowa, used a spiral form to convey chemical similarities between elements. In England, John Newlands was the first to rely on the element's chemical properties to set the order of elements, rather than follow the original order of increasing atomic weights. The "order numbers" that he assigned in the 1860s were later shown to be the same as the atomic numbers, first determined by Henry Moseley in 1913. Julius Lothar Meyer, in the late 1860s, in Germany, was the first to recognize that there should be gaps left in any arrangement to be filled later by elements that had not been discovered. Mendeleyev's records show his original table as being created in February 1869, as an aid for presenting the elements in an elementary text that he was preparing. Since that time, the form has passed through several changes and many elements—natural and human-made—have been added to the table, but the principal idea has remained.

At the beginning of the twentieth century, a new understanding of atoms began to take shape. The idea that electrons associated with atoms occupied certain definite energy levels surrounding the nucleus was successfully put forward by Niels Bohr. In his model, electrons orbited the nucleus in much the same way as planets orbit the sun. Each orbit was constrained to contain not more than a certain number of electrons. Thus, the concept of a shell model began.

Further experiments showed the inadequacies of the Bohr model, and the process of modifying the theory was begun. Before much had been done, however, atomic physics was set reeling by the introduction of a new model. In the new model, the electron was treated mathematically, not as a particle but as a wave, by methods that have come to be known as quantum mechanics, or wave mechanics. Even with this great change of view, the results still predicted that the electrons could have only relatively few allowed energy values. These energy states are the shells and subshells that contain the electrons in regions of space that are rather loosely located and are called orbitals. The nature of the shells has been changed drastically, and modifications of the theory continued to be made, but the idea of shells and subshells and the periodic law remained.

On December 20, 2017, the United Nations General Assembly and the United Nations Educational, Scientific and Cultural Organization (UNESCO) proclaimed 2019 the International Year of the Periodic Table of Chemical Elements. As 2019 was the 150th anniversary of the periodic table, the UN wanted to recognize its importance as one of the most influential developments in modern science, and to honor its history, researchers, sustainable development, and impact.

Principal terms

ATOM: the smallest particle of an element that retains the chemical properties of the element and that is composed of a nucleus surrounded by shells of electrons

ELEMENT: matter composed of only one kind of atom

FAMILY: elements that have the same outer shell electron configuration and thus have similar chemical and physical properties

ORBITAL: a region in space surrounding the nucleus within which there is a high probability of an electron being located

PERIOD: the name given to horizontal rows on the periodic table in which the elements all have the same arrangement of outer shell electrons

PERIODIC LAW: similarities in the chemical and physical properties of the elements recur in a regular fashion when the elements are listed in order of their atomic numbers

SHELL: describes both the region surrounding the nucleus in which electrons are located and the discrete energy states that are available to electrons

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