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  Electron Affinity, Chemistry tutorial

INTRODUCTION

We know that the energy necessary to eliminate an electron from an isolated neutral atom in gaseous condition is stated as the ionization energy.

Factors affecting electron affinity:

Factors affecting electron affinities are usually the same with those affecting ionization energies. These factors are:

  • Atomic Radius- When an electron adds on to any atom, the nucleus of the atom holds it through an electrostatic force of attraction that depends upon the effective nuclear charge or size of the atom. The smaller the size of the atom, the greater will be the force of attraction of the nucleus for the additional action added. Thus the higher will be the electron affinity of the atom therefore; more energy will be released in picking up an electron.
  • Electronic Configuration- Electronic configuration of the atom as well plays a vital role in determining the magnitude or sign of electron affinity. Halogens can accomplish a stable noble gas configuration through accepting just one electron. Thus, they have large negative (exothermic) electron affinities. On another hand the noble gases with closed sell ns2np6 configuration, beryllium or magnesium with np2(stable due to filled sub shell) or nitrogen having ns2 np3 (stable due to half filled p sub shell) configurations robustly resist the calculation of any electron. Consequently, the electron affinities of these elements are also zero or have small positive values.
  • Effective Nuclear Charge- The superior the effective nuclear charge, the greater the force of attraction exerted through the nucleus on the added electron or therefore, higher will be the electron affinity of the atom.

Periodicity in Electron Affinity:

We have so far described electron affinity or contain considered the factors which affect it. In this sub division we will study how the electron affinities differ in the provided descriptions.

Trends across periods:

On moving from left to right in a time, the size of an atom reduces or effective nuclear charge amplifies. Both these factors favor an increase in the force of attraction exerted through the nucleus on the extra electron. Therefore, the electron affinity usually increases across a period although irregularly. Therefore electron affinities of alkaline metals contain small negative values indicating their reluctance to form an anion. On another hand, electron affinities of halogens in a time have the highest negative values that reflect their ability to form anions most readily. As explained previous, the electron affinities of noble gases, nitrogen, magnesium and beryllium contain small positive values.

Trend across groups:

We familiar from the earlier section which on moving down the group of s- or p- block elements in the periodic table, the effective nuclear charge remains approximately stable, but there is a common amplify in atomic radius due to raise in the value of the principal quantum number n.

Consequently, the electron affinity usually decreases down any group in the periodic table. This is evident from the values given listed in table. Values of electron affinities of 2nd row non metals that are B, C, N, 0, F are though against the general trend, being smaller than those of analogous elements which is Al, Si, P, S, Cl of period 3. This is apparently an indirect product of the small size of the atoms of these elements that is B, C, N, 0, F. therefore, the crowding of electrons in the smaller outer shell of an atom of an element of period 2 makes common larger outer shell of an atom of an element of period 3. Thus even though an electron added to an atom of an element of period 2 is closer to the nucleus than one added to an atom of an element of period 3, the greater inter electronic repulsion in a smaller shell leads to a lower electron affinity. Repulsion of electrons significantly greater than that in the relatively.

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