Like the second and higher ionisation energies , the second and higher electron affinities are also possible. After addition of one electron, the atom becomes negatively charged and the second electron is added to a negatively charged ion. The addition of second electron is opposed by the coulombic force of repulsion and energy has to be supplied for the addition of the second electron. If an atom has spontaneous tendency, i.e., a positive tendency , to gain electron, then conventionally , its electron gain enthalpy is said to be negative and if the atom is reluctant to gain an electron, i.e., it has a negative tendency to gain an electron and is forced to accept it, its electron gain enthalpy is positive. Thus in the case of oxygen, the first electron gain enthalpy is negative since 141 kJ is released when one mole of oxygen atoms get converted to O- ions. In other words oxygen atom has positive tendency to accept electron. However, the second electron gain enthalpy is positive since 770 kJ of energy has to be supplied to convert 1 mol of O- ions to O2- ions.
Similarly in the case of sulphur, while first electron gain enthalpy is negative since 200 kJ of energy is released when 1 mole of S atoms get converted to S- ions and second electron gain enthalpy is positive since 590 kJ of energy has to be supplied to convert 1 mole of S- ions to S2- ions.
Thermodynamically , the energy released is given a negative sign and energy absorbed is given a positive sign. Accordingly, when a species has a positive electron affinity , DH , accompanying the addition of an electron to the species, isnegative and if it has a negative electron affinity, DH is positive. Thus for the reaction Cl + e ® Cl- , while electron affinity is positive , while electron gain enthalpy is negative. In various calculations involving DH, the value of electron affinity of chlorine would be taken as -349 kJmol-1and not +349 kJmol-1.
Factors on which Electron Gain Enthalpy depends
The important factors upon which electron gain enthalpy depends are briefly discussed.
i) Atomic size : As the size of an atom increases , the distance between its nucleus and the incoming electron also increases. Consequently, the incoming electron experiences less attraction towards the nucleus of the atom. Therefore , electron gain enthalpy becomes less negative down the group.
ii) Nuclear charge : With increase in the nuclear charge, force of attraction between the nucleus and and incoming electron increases and so is the value of electron gain enthalpy. Thus, the electron gain enthalpy becomes more negative with increase in nuclear charge.
iii) Symmetry of electronic configuration : The symmetry of electronic configuration has very important role to play. The atoms with symmetrical configuration (having filled and half-filled orbitals in the same sub-shell) do not have any urge to take up extra electrons because their configuration will become unsymmetrical or less stable. In case these are made to accept electrons, energy will be needed and electron gain enthalpy will be positive. For example , noble gas elements have positive electron gain enthalpies.
Variation of Electron gain enthalpy
Along a period
As a general rule, electron gain enthalpy becomes more negative with increase in atomic number across a period. The effective nuclear charge increases as we go from left to right across a period and consequently it will be easier to add an electron to smaller atom since the added electron on an average would be closer to the positively charged nucleus.
The trends in electron gain enthalpy values within a period are irregular indicating that atomic size is not the only criterion for determining electron gain enthalpy. Thus electron gain enthalpy of Be is positive (+66 kJmol-1) is while that of Li is negative (-60 kJmol-1). Similarly , electron gain enthalpy of nitrogen is positive(+31 kJmol-1) while that of oxygen although atomic size of oxygen is less is negative (-141 kJmol-1) .
Along a group
We should also expect electron gain enthalpy to become less negative as we go down a group because the size of the atom increases and the added electron would be farther from the nucleus. This is generally the case (Table above). However, electron gain enthalpy of O or F is less than that of succeeding element. This is because when electron is added to O or F, the added electron goes to smaller n = 2 quantum level and suffers significant repulsion from the other electrons present in this level. For the n = 3 quantum level ( S or Cl) , the added electron occupies a larger region of space and the electron-electron repulsion is much less.
Some typical trends in Electron Gain Enthalp
Halogens have high negative electron gain enthalpy. This is due to their strong tendency to gain an additional electron to change into s2p6 configuration.