4.2 Ionic or Electrovalent Bond other factors. The crystal structure of sodium chloride, NaCl (rock salt), for example isFrom the Kössel and Lewis treatment of the shown below.formation of an ionic bond, it follows that the formation of ionic compounds would primarily depend upon: • The ease of formation of the positive and negative ions from the respective neutral atoms; • The arrangement of the positive and negative ions in the solid, that is, the lattice of the crystalline compound. The formation of a positive ion involves ionization, i.e., removal of electron(s) from the neutral atom and that of the negative ion involves the addition of electron(s) to the Rock salt structure neutral atom. In ionic solids, the sum of the electron gain M(g) → M+(g) + e– ; enthalpy and the ionization enthalpy may be Ionization enthalpy positive but still the crystal structure gets X(g) + e– → X – (g) ; stabilized due to the energy released in the Electron gain enthalpy formation of the crystal lattice. For example: the ionization enthalpy for Na+(g) formation M+(g) + X –(g) → MX(s) from Na(g) is 495.8 kJ mol–1 ; while the electron The electron gain enthalpy, ∆egH, is the gain enthalpy for the change Cl(g) + e–→ enthalpy change (Unit 3), when a gas phase Cl– (g) is, – 348.7 kJ mol–1 only. The sum of the atom in its ground state gains an electron. two, 147.1 kJ mol-1 is more than compensated The electron gain process may be exothermic for by the enthalpy of lattice formation of or endothermic. The ionization, on the other NaCl(s) (–788 kJ mol–1). Therefore, the energy hand, is always endothermic. Electron released in the processes is more than the Reprint 2025-26 Chemical Bonding And Molecular Structure 107 energy absorbed. Thus a qualitative measure of the stability of an ionic compound is provided by its enthalpy of lattice formation and not simply by achieving octet of electrons around the ionic species in gaseous state. Since lattice enthalpy plays a key role in the formation of ionic compounds, it is important that we learn more about it. 4.2.1 Lattice Enthalpy The Lattice Enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions. For example, the lattice enthalpy of NaCl is 788 kJ mol–1. This means that 788 Fig. 4.1 The bond length in a covalent kJ of energy is required to separate one mole molecule AB. of solid NaCl into one mole of Na+ (g) and one R = rA + rB (R is the bond length and rA and rB are mole of Cl– (g) to an infinite distance. the covalent radii of atoms A and B respectively) This process involves both the attractive forces between ions of opposite charges in the same molecule. The van der Waals and the repulsive forces between ions of radius represents the overall size of the like charge. The solid crystal being three- atom which includes its valence shell in a dimensional; it is not possible to calculate nonbonded situation. Further, the van der lattice enthalpy directly from the interaction Waals radius is half of the distance between of forces of attraction and repulsion only. two similar atoms in separate molecules in Factors associated with the crystal geometry a solid. Covalent and van der Waals radii of have to be included. chlorine are depicted in Fig. 4.2.

2.1 Arrange the following metals in the order in which they displace each other from the solution of their salts. Al, Cu, Fe, Mg and Zn.

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