74 CHEMISTRY UNIT 3 CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES The Periodic Table is arguably the most important concept in chemistry, both in principle and in practice. It is the everyday support for students, it suggests new avenues of research to After studying this Unit, you will be professionals, and it provides a succinct organization of the able to whole of chemistry. It is a remarkable demonstration of the fact that the chemical elements are not a random cluster of • appreciate how the concept of entities but instead display trends and lie together in families. grouping elements in accordance to An awareness of the Periodic Table is essential to anyone who their properties led to the wishes to disentangle the world and see how it is built up development of Periodic Table. from the fundamental building blocks of the chemistry, the understand the Periodic Law; • chemical elements. • understand the significance of atomic number and electronic Glenn T. Seaborg configuration as the basis for periodic classification; • name the elements with In this Unit, we will study the historical development of the Z >100 according to IUPAC Periodic Table as it stands today and the Modern Periodic nomenclature; Law. We will also learn how the periodic classification • classify elements into s, p, d, f follows as a logical consequence of the electronic blocks and learn their main configuration of atoms. Finally, we shall examine some of characteristics; the periodic trends in the physical and chemical properties • recognise the periodic trends in of the elements. physical and chemical properties of elements; 3.1 WHY DO WE NEED TO CLASSIFY ELEMENTS ? compare the reactivity of elements • We know by now that the elements are the basic units of all and correlate it with their occurrence in nature; types of matter. In 1800, only 31 elements were known. By • explain the relationship between 1865, the number of identified elements had more than ionization enthalpy and metallic doubled to 63. At present 114 elements are known. Of character; them, the recently discovered elements are man-made. • use scientific vocabulary Efforts to synthesise new elements are continuing. With appropriately to communicate ideas such a large number of elements it is very difficult to study related to certain important individually the chemistry of all these elements and their properties of atoms e.g., atomic/ innumerable compounds individually. To ease out this ionic radii, ionization enthalpy, problem, scientists searched for a systematic way to electron gain enthalpy, organise their knowledge by classifying the elements. Not electronegativity, valence of only that it would rationalize known chemical facts about elements. elements, but even predict new ones for undertaking further study. 2021-22 CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES 75 3.2 GENESIS OF PERIODIC chemist, John Alexander Newlands in 1865 CLASSIFICATION profounded the Law of Octaves. He arranged the elements in increasing order of their atomic Classification of elements into groups and weights and noted that every eighth element development of Periodic Law and Periodic had properties similar to the first element Table are the consequences of systematising (Table 3.2). The relationship was just like every the knowledge gained by a number of scientists eighth note that resembles the first in octaves through their observations and experiments. of music. Newlands’s Law of Octaves seemed The German chemist, Johann Dobereiner in to be true only for elements up to calcium. early 1800’s was the first to consider the idea Although his idea was not widely accepted at of trends among properties of elements. By that time, he, for his work, was later awarded 1829 he noted a similarity among the physical Davy Medal in 1887 by the Royal Society, and chemical properties of several groups of London. three elements (Triads). In each case, he noticed that the middle element of each of the The Periodic Law, as we know it today owes Triads had an atomic weight about half way its development to the Russian chemist, Dmitri between the atomic weights of the other two Mendeleev (1834-1907) and the German (Table 3.1). Also the properties of the middle chemist, Lothar Meyer (1830-1895). Working element were in between those of the other independently, both the chemists in 1869 Table 3.1 Dobereiner’s Triads Element Atomic Element Atomic Element Atomic weight weight weight Li 7 Ca 40 Cl 35.5 Na 23 Sr 88 Br 80 K 39 Ba 137 I 127 two members. Since Dobereiner’s relationship, proposed that on arranging elements in the referred to as the Law of Triads, seemed to increasing order of their atomic weights, work only for a few elements, it was dismissed similarities appear in physical and chemical as coincidence. The next reported attempt to properties at regular intervals. Lothar Meyer classify elements was made by a French plotted the physical properties such as atomic geologist, A.E.B. de Chancourtois in 1862. He volume, melting point and boiling point arranged the then known elements in order of against atomic weight and obtained a increasing atomic weights and made a periodically repeated pattern. Unlike cylindrical table of elements to display the Newlands, Lothar Meyer observed a change in periodic recurrence of properties. This also did length of that repeating pattern. By 1868, not attract much attention. The English Lothar Meyer had developed a table of the Table 3.2 Newlands’ Octaves Element Li Be B CN O F At. wt. 7 9 11 12 14 16 19 Element Na Mg Al Si P S Cl At. wt. 23 24 27 29 31 32 35.5 Element K Ca At. wt. 39 40 2021-22 76 CHEMISTRY elements that closely resembles the Modern weights, thinking that the atomic Periodic Table. However, his work was not measurements might be incorrect, and placed published until after the work of Dmitri the elements with similar properties together. Mendeleev, the scientist who is generally For example, iodine with lower atomic weight credited with the development of the Modern than that of tellurium (Group VI) was placed Periodic Table. in Group VII along with fluorine, chlorine, bromine because of similarities in properties While Dobereiner initiated the study of periodic relationship, it was Mendeleev who (Fig. 3.1). At the same time, keeping his was responsible for publishing the Periodic primary aim of arranging the elements of Law for the first time. It states as follows : similar properties in the same group, he proposed that some of the elements were still The properties of the elements are a undiscovered and, therefore, left several gaps periodic function of their atomic in the table. For example, both gallium and weights. germanium were unknown at the time Mendeleev published his Periodic Table. He left Mendeleev arranged elements in horizontal the gap under aluminium and a gap under rows and vertical columns of a table in order silicon, and called these elements Eka- of their increasing atomic weights in such a Aluminium and Eka-Silicon. Mendeleev way that the elements with similar properties occupied the same vertical column or group. predicted not only the existence of gallium and Mendeleev’s system of classifying elements was germanium, but also described some of their more elaborate than that of Lothar Meyer’s. general physical properties. These elements He fully recognized the significance of were discovered later. Some of the properties periodicity and used broader range of physical predicted by Mendeleev for these elements and and chemical properties to classify the those found experimentally are listed in elements. In particular, Mendeleev relied on Table 3.3. the similarities in the empirical formulas and properties of the compounds formed by the The boldness of Mendeleev’s quantitative elements. He realized that some of the elements predictions and their eventual success made did not fit in with his scheme of classification him and his Periodic Table famous. if the order of atomic weight was strictly Mendeleev’s Periodic Table published in 1905 followed. He ignored the order of atomic is shown in Fig. 3.1. Table 3.3 Mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and Eka-silicon (Germanium) Property Eka-aluminium Gallium Eka-silicon Germanium (predicted) (found) (predicted) (found) Atomic weight 68 70 72 72.6 Density / (g/cm3) 5.9 5.94 5.5 5.36 Melting point /K Low 302.93 High 1231 Formula of oxide E2O3 Ga2O3 EO2 GeO2 Formula of chloride ECl3 GaCl3 ECl4 GeCl4 2021-22 CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES 77 Mendeleev’s Periodic Table published earlier Fig. 3.1 PERIODIC SYSTEM OF THE ELEMENTS IN GROUPS AND SERIES 2021-22 78 CHEMISTRY 3.3 MODERN PERIODIC LAW AND THE Numerous forms of Periodic Table have PRESENT FORM OF THE PERIODIC been devised from time to time. Some forms TABLE emphasise chemical reactions and valence, We must bear in mind that when Mendeleev whereas others stress the electronic developed his Periodic Table, chemists knew configuration of elements. A modern version, nothing about the internal structure of atom. the so-called “long form” of the Periodic Table However, the beginning of the 20th century of the elements (Fig. 3.2), is the most convenient witnessed profound developments in theories and widely used. The horizontal rows (which about sub-atomic particles. In 1913, the Mendeleev called series) are called periods and English physicist, Henry Moseley observed the vertical columns, groups. Elements having regularities in the characteristic X-ray spectra similar outer electronic configurations in their of the elements. A plot of ν (whereν is atoms are arranged in vertical columns, frequency of X-rays emitted) against atomic referred to as groups or families. According number (Z ) gave a straight line and not the to the recommendation of International Union of Pure and Applied Chemistry (IUPAC), the plot of ν vs atomic mass.