many books, the negative of the enthalpy change for the process depicted in equation . is defined as the ELECTRON AFFINITY (A e ) 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 ∆ eg H = –A e – / R T. Table . Electron Gain Enthalpies * / (kJ mol – ) of Some Main Group Elements Group ∆ eg H Group ∆ eg H Group ∆ eg H Group ∆ eg H H – He + – O – F – Ne + – S – Cl – Ar + K – Se – Br – Kr + Rb – Te – I – Xe + Cs – Po – At – Rn + ability to attract electrons. Approximate values for the electronegativity of a few elements are given in Table .
(a) The electronegativity of any given element is not constant; it varies depending on the element to which it is bound. Though it is not a measurable quantity, it does provide a means of predicting the nature of force that holds a pair of atoms together – a relationship that you will explore later. Electronegativity generally increases across a period from left to right (say from lithium to fluorine) and decrease down a group (say from fluorine to astatine) in the periodic table. How can these trends be explained?
Can the electronegativity be related to atomic radii, which tend to decrease across each period from left to right, but increase down each group ? The