3.3 MODERN PERIODIC LAW AND THE physical and chemical properties of elements PRESENT FORM OF THE PERIODIC and their compounds. TABLE Numerous forms of Periodic Table have We must bear in mind that when Mendeleev been devised from time to time. Some developed his Periodic Table, chemists forms emphasise chemical reactions and knew nothing about the internal structure valence, whereas others stress the electronic of atom. However, the beginning of the 20th configuration of elements. A modern version, century witnessed profound developments the so-called “long form” of the Periodic in theories about sub-atomic particles. In Table of the elements (Fig. 3.2), is the most 1913, the English physicist, Henry Moseley convenient and widely used. The horizontal observed regularities in the characteristic rows (which Mendeleev called series) are X-ray spectra of the elements. A plot of called periods and the vertical columns, (where is frequency of X-rays emitted) groups. Elements having similar outer against atomic number (Z) gave a straight electronic configurations in their atoms are arranged in vertical columns, referredline and not the plot of vs atomic mass. to as groups or families. According to theHe thereby showed that the atomic number recommendation of International Union ofis a more fundamental property of an element Pure and Applied Chemistry (IUPAC), thethan its atomic mass. Mendeleev’s Periodic groups are numbered from 1 to 18 replacingLaw was, therefore, accordingly modified. This the older notation of groups IA … VIIA, VIII,is known as the Modern Periodic Law and IB … VIIB and 0. can be stated as : There are altogether seven periods. The The physical and chemical properties period number corresponds to the highest of the elements are periodic functions principal quantum number (n) of the elements of their atomic numbers. in the period. The first period contains 2 The Periodic Law revealed important elements. The subsequent periods consists of analogies among the 94 naturally occurring 8, 8, 18, 18 and 32 elements, respectively. The elements (neptunium and plutonium like seventh period is incomplete and like the sixth actinium and protoactinium are also found period would have a theoretical maximum in pitch blende – an ore of uranium). It (on the basis of quantum numbers) of 32 stimulated renewed interest in Inorganic elements. In this form of the Periodic Table, Chemistry and has carried into the present 14 elements of both sixth and seventh periods with the creation of artificially produced (lanthanoids and actinoids, respectively) are short-lived elements. placed in separate panels at the bottom*. You may recall that the atomic number 3.4 NOMENCLATURE OF ELEMENTSis equal to the nuclear charge (i.e., number WITH ATOMIC NUMBERS > 100of protons) or the number of electrons in a neutral atom. It is then easy to visualize The naming of the new elements had been the significance of quantum numbers and traditionally the privilege of the discoverer electronic configurations in periodicity of (or discoverers) and the suggested name was elements. In fact, it is now recognized that the ratified by the IUPAC. In recent years this has Periodic Law is essentially the consequence led to some controversy. The new elements of the periodic variation in electronic with very high atomic numbers are so unstable configurations, which indeed determine the that only minute quantities, sometimes only * Glenn T. Seaborg’s work in the middle of the 20th century starting with the discovery of plutonium in 1940, followed by those of all the transuranium elements from 94 to 102 led to reconfiguration of the periodic table placing the actinoids below the lanthanoids. In 1951, Seaborg was awarded the Nobel Prize in chemistry for his work. Element 106 has been named Seaborgium (Sg) in his honour. Classification of Elements and Periodicity in Properties 79 0 B VII This B VI electronic B outer V B state IV recommendations. elements. B ground IUPACthe III and for B 0 1984 II theand numbers B I with IB–VIIB atomic VIII, → their accordance VIII withinIA–VIIA, ← of Elements1-18 A scheme the VII ofnumbered A are VI Table numbering A old groups V Periodic the A The the IV of A replaces III form Longconfigurations.notation IIA 3.2 IA Fig. 80 chemistry a few atoms of them are obtained. Their digits which make up the atomic number and synthesis and characterisation, therefore, “ium” is added at the end. The IUPAC names require highly sophisticated costly equipment for elements with Z above 100 are shown in and laboratory. Such work is carried out with Table 3.5. competitive spirit only in some laboratories in the world. Scientists, before collecting the Table 3.4 Notation for IUPAC reliable data on the new element, at times Nomenclature of Elements get tempted to claim for its discovery. For example, both American and Soviet scientists Digit Name Abbreviation claimed credit for discovering element 104. 0 nil n The Americans named it Rutherfordium 1 un u whereas Soviets named it Kurchatovium. To 2 bi b avoid such problems, the IUPAC has made 3 tri t recommendation that until a new element’s 4 quad q discovery is proved, and its name is officially 5 pent precognised, a systematic nomenclature be 6 hex hderived directly from the atomic number of 7 sept sthe element using the numerical roots for 8 oct o0 and numbers 1-9. These are shown in 9 enn eTable 3.4. The roots are put together in order of Table 3.5 Nomenclature of Elements with Atomic Number Above 100 Atomic Name according to IUPAC IUPAC Symbol Number IUPAC nomenclature Official Name Symbol 101 Unnilunium Unu Mendelevium Md 102 Unnilbium Unb Nobelium No 103 Unniltrium Unt Lawrencium Lr 104 Unnilquadium Unq Rutherfordium Rf 105 Unnilpentium Unp Dubnium Db 106 Unnilhexium Unh Seaborgium Sg 107 Unnilseptium Uns Bohrium Bh 108 Unniloctium Uno Hassium Hs 109 Unnilennium Une Meitnerium Mt 110 Ununnillium Uun Darmstadtium Ds 111 Unununnium Uuu Rontgenium Rg 112 Ununbium Uub Copernicium Cn 113 Ununtrium Uut Nihonium Nh 114 Ununquadium Uuq Flerovium Fl 115 Ununpentium Uup Moscovium Mc 116 Ununhexium Uuh Livermorium Lv 117 Ununseptium Uus Tennessine Ts 118 Ununoctium Uuo Oganesson Og Classification of Elements and Periodicity in Properties 81 Thus, the new element first gets a be readily seen that the number of elements temporary name, with symbol consisting in each period is twice the number of atomic of three letters. Later permanent name orbitals available in the energy level that is and symbol are given by a vote of IUPAC being filled. The first period (n = 1) starts with representatives from each country. The the filling of the lowest level (1s) and therefore permanent name might reflect the country has two elements — hydrogen (ls1) and helium (or state of the country) in which the element (ls2) when the first shell (K) is completed. The was discovered, or pay tribute to a notable second period (n = 2) starts with lithium and the scientist. As of now, elements with atomic third electron enters the 2s orbital. The next numbers up to 118 have been discovered. element, beryllium has four electrons and has Official names of all elements have been the electronic configuration 1s22s2. Starting announced by IUPAC. from the next element boron, the 2p orbitals are filled with electrons when the L shell is Problem 3.1 completed at neon (2s22p6). Thus there are 8 elements in the second period. The third What would be the IUPAC name and period (n = 3) begins at sodium, and the added symbol for the element with atomic number 120? electron enters a 3s orbital. Successive filling of 3s and 3p orbitals gives rise to the third Solution period of 8 elements from sodium to argon. The From Table 3.4, the roots for 1, 2 and 0 fourth period (n = 4) starts at potassium, and are un, bi and nil, respectively. Hence, the added electrons fill up the 4s orbital. Now the symbol and the name respectively you may note that before the 4p orbital is filled, are Ubn and unbinilium. filling up of 3d orbitals becomes energetically favourable and we come across the so called 3d

3.2 GENESIS OF PERIODIC the periodic recurrence of properties. This CLASSIFICATION also did not attract much attention. The English chemist, John Alexander NewlandsClassification of elements into groups and in 1865 profounded the Law of Octaves. Hedevelopment of Periodic Law and Periodic arranged the elements in increasing orderTable are the consequences of systematising of their atomic weights and noted that everythe knowledge gained by a number of eighth element had properties similar to thescientists through their observations and first element (Table 3.2). The relationship wasexperiments. The German chemist, Johann just like every eighth note that resembles theDobereiner in early 1800’s was the first to first in octaves of music. Newlands’s Law ofconsider the idea of trends among properties Octaves seemed to be true only for elementsof elements. By 1829 he noted a similarity up to calcium. Although his idea was notamong the physical and chemical properties widely accepted at that time, he, for his work,of several groups of three elements (Triads). In was later awarded Davy Medal in 1887 by theeach case, he noticed that the middle element Royal Society, London.of each of the Triads had an atomic weight about half way between the atomic weights of The Periodic Law, as we know it today the other two (Table 3.1). Also the properties owes its development to the Russian chemist, of the middle element were in between those Dmitri Mendeleev (1834-1907) and the of the other two members. Since Dobereiner’s German chemist, Lothar Meyer (1830-1895). Table 3.1 Dobereiner’s Triads Atomic Atomic Atomic Element Element Element weight weight weight Li 7 Ca 40 Cl 35.5 Na 23 Sr 88 Br 80 K 39 Ba 137 I 127 relationship, referred to as the Law of Triads, Working independently, both the chemists in seemed to work only for a few elements, it was 1869 proposed that on arranging elements in dismissed as coincidence. The next reported the increasing order of their atomic weights, attempt to classify elements was made by a similarities appear in physical and chemical French geologist, A.E.B. de Chancourtois in properties at regular intervals. Lothar Meyer 1862. He arranged the then known elements plotted the physical properties such as in order of increasing atomic weights and atomic volume, melting point and boiling made a cylindrical table of elements to display point against atomic weight and obtained Table 3.2 Newlands’ Octaves Element Li Be B C N 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 76 chemistry a periodically repeated pattern. Unlike classification if the order of atomic weight Newlands, Lothar Meyer observed a change was strictly followed. He ignored the order in length of that repeating pattern. By 1868, of atomic weights, thinking that the atomic Lothar Meyer had developed a table of the measurements might be incorrect, and placed elements that closely resembles the Modern the elements with similar properties together. Periodic Table. However, his work was not For example, iodine with lower atomic weight published until after the work of Dmitri than that of tellurium (Group VI) was placed Mendeleev, the scientist who is generally in Group VII along with fluorine, chlorine, credited with the development of the Modern bromine because of similarities in properties Periodic Table. (Fig. 3.1). At the same time, keeping his While Dobereiner initiated the study of primary aim of arranging the elements of periodic relationship, it was Mendeleev who similar properties in the same group, he was responsible for publishing the Periodic proposed that some of the elements were Law for the first time. It states as follows : still undiscovered and, therefore, left several gaps in the table. For example, both gallium The properties of the elements are and germanium were unknown at the time a periodic function of their atomic Mendeleev published his Periodic Table. weights. He left the gap under aluminium and a gap Mendeleev arranged elements in horizontal under silicon, and called these elements rows and vertical columns of a table in order Eka-Aluminium and Eka-Silicon. Mendeleev of their increasing atomic weights in such a predicted not only the existence of gallium and way that the elements with similar properties germanium, but also described some of their occupied the same vertical column or group. general physical properties. These elements Mendeleev’s system of classifying elements were discovered later. Some of the properties was more elaborate than that of Lothar predicted by Mendeleev for these elements Meyer’s. He fully recognized the significance and those found experimentally are listed in of periodicity and used broader range of Table 3.3. physical and chemical properties to classify the elements. In particular, Mendeleev relied The boldness of Mendeleev’s quantitative on the similarities in the empirical formulas predictions and their eventual success and properties of the compounds formed by made him and his Periodic Table famous. the elements. He realized that some of the Mendeleev’s Periodic Table published in 1905 elements did not fit in with his scheme of is shown in Fig. 3.1. Table 3.3 Mendeleev’s Predictions for the Elements Eka-aluminium (Gallium) and Eka-silicon (Germanium) Eka-aluminium Gallium Eka-silicon Germanium Property (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 E Cl3 GaCl3 ECl4 GeCl4 Classification of Elements and Periodicity in Properties 77 SERIES AND earlier GROUPS published IN Table PeriodicELEMENTS THE OF Mendeleev’s 3.1SYSTEM Fig. PERIODIC 78 chemistry

4.11 Explain giving reasons: (i) Transition metals and many of their compounds show paramagnetic behaviour. (ii) The enthalpies of atomisation of the transition metals are high. (iii) The transition metals generally form coloured compounds. (iv) Transition metals and their many compounds act as good catalyst.