The enthalpy of hydration AH®yd of an ion (i.e. the enthalpy change which accompanies the dissolution of one mole of the ideal gaseous ion in an infinite volume of water at a temperature of 298°K and a pressure of 1 atm) appears in the thermodynamic analysis of several important topics in general and inorganic chemistry (1)(2)(3)(4)(5). Available tabulations of experimental hydration enthalpies cover only about 20 or 30 cations and half a dozen anions. Hydration enthalpies for unstable ions, calculated from empirical formulas, are useful in the rationalization of periodic trends in chemical behavior (4,5). The purpose of this paper is to present a comprehensive tabulation of experimental hydration enthalpies, and to discuss the calculation of hydration enthalpies for unstable ions. Cations Experimental Hydration EnthalpiesHydration enthalpies of ions (6-10) are readily extracted from tabulations of thermodynamic data (11, 12) from the enthalpies of formation AHf of single aqueous ions. These are usually expressed on a scale relative to the hydrated proton H+ (aq), and refer to a standard state of unit molality and unit activity at 298°K. For example, A Hf for Mg2+ is -462 kJ mole-1. This means that AH°for the process Mg(s) + 2H+(aq) -Mg2+(aq) + H3(g) is equal to -462 kJ mole-1 at 298°K. Thus AHf for Mg2+(aq) is the sum of the standard enthalpy changes Mg(s) -* Mg(g) AHi Mg(g) -*• Mg2+(g) AH2 Mg2+(g) -Mg2+(aq) AHa 2H+(aq) -2H+(g) AH4

A numerical scale of acidity / basicity for oxoacid salts.

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It is argued that the terms valence, covalence, hypervalence, oxidation state, and coordination number are often confused and misused in the literature. It is recommended that use of the term valence, and its associated terminology, should be restricted to simple molecular main group substances and to some oxoacids and derivatives, but avoided in both main group and transition element coordination chemistry.