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90 chemistry Table 3.7 Electron Gain Enthalpies* / (kJ mol ) of Some Main Group Elements Group 1 ∆egH Group 16 ∆eg Group 17 ∆eg Group 0 ∆eg H – 73 He + 48 Li – 60 O – 141 F – 328 Ne + 116 Na – 53 S – 200 Cl – 349 Ar + 96 K – 48 Se – 195 Br – 325 Kr + 96 Rb – 47 Te – 190 I – 295 Xe + 77 Cs – 46 Po – 174 At – 270 Rn + 68 negative electron gain enthalpies because they can attain stable noble gas electronic Problem 3.7 configurations by picking up an electron. Which of the following will have the most On the other hand, noble gases have large negative electron gain enthalpy and positive electron gain enthalpies because the which the least negative? P, S, Cl, F. electron has to enter the next higher principal quantum level leading to a very unstable Explain your answer. electronic configuration. It may be noted that electron gain enthalpies have large negative Solution Electron gain enthalpy generally values toward the upper right of the periodic becomes more negative across a table preceding the noble gases. The variation in electron gain enthalpies of period as we move from left to right. Within a group, electron gain enthalpy elements is less systematic than for ionization becomes less negative down a group. enthalpies. As a general rule, electron gain However, adding an electron to the enthalpy becomes more negative with increase 2p-orbital leads to greater repulsion in the atomic number across a period. The than adding an electron to the larger effective nuclear charge increases from left to 3p-orbital. Hence the element with right across a period and consequently it will most negative electron gain enthalpy is be easier to add an electron to a smaller atom chlorine; the one with the least negative since the added electron on an average would electron gain enthalpy is phosphorus. be closer to the positively charged nucleus. We should also expect electron gain enthalpy to (e) Electronegativity become less negative as we go down a group because the size of the atom increases and A qualitative measure of the ability of an atom in a chemical compound to attract shared the added electron would be farther from the electrons to itself is called electronegativity. nucleus. This is generally the case (Table 3.7). However, electron gain enthalpy of O or Unlike ionization enthalpy and electron gain F is less negative than that of the succeeding enthalpy, it is not a measureable quantity. However, a number of numerical scales of element. This is because when an electron is added to O or F, the added electron goes to electronegativity of elements viz., Pauling the smaller n = 2 quantum level and suffers scale, Mulliken-Jaffe scale, Allred-Rochow significant repulsion from the other electrons scale have been developed. The one which present in this level. For the n = 3 quantum is the most widely used is the Pauling scale. level (S or Cl), the added electron occupies Linus Pauling, an American scientist, in 1922 a larger region of space and the electron- assigned arbitrarily a value of 4.0 to fluorine, electron repulsion is much less. the element considered to have the greatest * In many books, the negative of the enthalpy change for the process depicted in equation 3.3 is defined as the ELECTRON AFFINITY (A ) of the atom under consideration. If energy is released when an electron is added to an atom, the electron affinity is taken as positive, contrary to thermodynamic convention. If energy has to be supplied to add an electron to an atom, then the electron affinity of the atom is assigned a negative sign. However, electron affinity is defined as absolute zero and, therefore at any other temperature (T) heat capacities of the reactants and the products have to be taken into account in ∆ H = –A – 5/2 RT. eg e Reprint 2026-27