Chapter 2
B.sc 1st year Book
(Page 6)

The exception to the vertical trend of ionization energy

There are some exceptions to the vertical trend of ionization energy. These exceptions are found in the case of those elements having an atomic number more than 72 and related to the fourth and fifth p and d-series of the periodic table. it was observed that the ionization potential of the elements in the same group increases from top to bottom in the case of 73 Ta to 82 which are greater than 41 Nb to 50 Sn placed before them in the same subgroup as shown in table 2.10.


The reason for such type of abnormal behavior ( ( e. an increase of I I1) is due to the effect of lanthanide contraction as shown by Nb to Ta; Mo to W; Te to Re: Ru to Os: …….Sn to Pb. This occurs due to the increase of nuclear charge without a corresponding increase in atomic size through lanthanide elements. In addition from B to Al there is a decrease in the values of I.E but the values for the remaining elements Ga, In and TI do not continue this trend as shown below:
Electron affinity is a reverse process to ionization potential
Electron affinity is a reverse process to ionization potential and is related to the formation of an anion by the addition of an extra electron to an isolated gaseous atom (or the addition of more electrons in succession).
   It is defined as the amount of energy released when an extra electron is added to an isolated gaseous atom or an ion in its ground state in the formation of an anion .ft is represented as ‘EA’.
A(g)+θ−⟶A−(g)+EA (energy released) 
Unit: The electron affinity is measured in electron volts or kilo calories mot-1 or kilo joule mol−1, Hence, it is represented either byeV or Kcalmol−1 or kJmol−1,.
Since energy is released by the addition of an extra electron to a gaseous neutral atom, the process is exothermic. Hence, it is represented with a negative sign placed before it. Thus, the greater the energy released in the process of taking up the extra electron, the greater. is the electron affinity. The electron affinity values of various elements belonging to different groups of the periodic table are given in table 2.11.

Table 2.11: Electron affinities of gaseous atoms (eV)
Successive electron affinities:

Like the second and higher ionization energies, the second and higher electron affinities are also possible. However, when an electron is added or put to a neutral isolated atom a negatively charged ion is formed. But the addition of a second electron is opposed by the Coulombic force of repulsion and more energy is required. Thus, the second electron affinity of an atom of the clement may be defined as the amount of energy supplied to add an electron to the uni-negative ion in the formation of a di-negative ion.
i.e.1 A−(9)+e−+Energy ⟶A2−(g) (Endothermic process)
Hence, the second EA has always been a positive sign.
Let us consider the case of oxygen and sulfur to form an anion Oand S by accepting
an electron. In these cases, energy is released and the value of EA is Veracruz dxt whan the second electron is added to O−and S ions, the energy is absorbed in the formation of O2− and SZ−ions and the value of EA is positive.
released and absorbed energy
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