fourth p -electron from oxygen than it is, to remove one of the three p -electrons from nitrogen. Problem . The first ionization enthalpy ( ∆ i H ) values of the third period elements, Na, Mg and Si are respectively , and kJ mol – . Predict whether the first ∆ i H value for Al will be more close to or kJ mol – ?
Justify your answer. It will be more close to kJ mol – . The value for Al should be lower than that of Mg because of effective shielding of p electrons from the nucleus by s -electrons. (d) Electron Gain Enthalpy When an electron is added to a neutral gaseous atom (X) to convert it into a negative ion, the enthalpy change accompanying the process is defined as the Electron Gain Enthalpy ( ∆ eg H ).
Electron gain enthalpy provides a measure of the ease with which an atom adds an electron to form anion as represented by equation . . X(g) + e – → X – (g) ( . ) Depending on the element, the process of adding an electron to the atom can be either endothermic or exothermic.
For many elements energy is released when an electron is added to the atom and the electron gain enthalpy is negative. For example, group elements (the halogens) have very high negative electron gain enthalpies because they can attain stable noble gas electronic configurations by picking up an electron. On the other hand, noble gases have large positive electron gain enthalpies because the electron has to enter the next higher principal quantum level leading to a very unstable electronic configuration. It may be noted that electron gain enthalpies have large negative values toward the upper right of the periodic table preceding the noble gases.
The variation in electron gain enthalpies of elements is less systematic than for ionization enthalpies. As a general rule, electron gain enthalpy becomes more negative with increase in the atomic number across a period. The effective nuclear charge increases from