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Electron Affinity and Electronegativity



Electron Affinity 

A neutral atom can accept an electron to form negative ion. In this process, in general, energy is released.

 Electron affinity (EA) of an element: is the amount of energy released when an electron is added to a gaseous atom to form an anion.


The energy involved in the addition of the first electron is called first-electron affinity; the energy involved in the addition of a second electron is called second-electron affinity; and so on. Thus:


The electron affinity of an element measures the ease with which it forms an anion in the gas phase.

Electron affinities are difficult to measure and accurate values are not known for all elements.
They are expressed in kJ mol–1.

Trends in Electron Affinities

The factors that determine the magnitude and sign of electron affinities are similar to those used to explain ionisation energies of elements. In fact, the electron affinity of a neutral atom may be thought of simply as equivalent to the ionisation energy of the singly charged negative ion of the atom.


The first-electron affinities of elements in the Periodic table are expected to show trends analogous to those of ionization energies.

(a) Increase across a Period

The values of electron affinities for Period (2) are listed below:


As we proceed from left to right, the general trend is the increase of electron affinities. Be, N and Ne are exceptions.

Explanation

Elements having relatively stable electronic configurations find it difficult to accept an electron readily. The atom of Be has the configuration 1s2 2s2. The 1s subshell is completely filled and, therefore, the electron being added must go to a subshell of considerably higher energy. This gives rise to negative electron affinity for Be.

The atom of N  (1s2 2s2 , 2px1 ,2py1 ,2pz1)   has half-filled 2p subshells, a condition of extra stability. Therefore the electron affinity of N would be less than expected.

The electron affinity of Neon is low because it has a stable outer-shell octet. Its atom shows little tendency to start a new shell.

(b) Decrease down a Group

The values of electron affinities for halogens (Group VII) are given below.


The electron affinities show a general decrease from top to bottom. This is so because the valence shell is progressively farther from the nucleus. The value for fluorine, however, is out of line as it has a smaller atomic size than that of chlorine.

(c) Second electron affinity negative

The second electron affinity of an element is always negative. This is on account of repulsion
between the electron being added and the already negatively charged atom. For example,

Electronegativity

In a molecule A – B the electrons forming the covalent bond are attracted by atom A as well as by B. This attraction is measured in terms of what we call electronegativity, EN.


Electronegativity : is The attraction exerted by an atom on the electron pair bonding it to another atom by a covalent bond.

It is evident that an atom of high electronegativity will attract the shared electron pair away from one of lower electronegativity. Thus the former atom will acquire a partial negative charge while the other atom will get a partial positive charge.

Electronegativity Values

Using measured values of bond energies, Pauling devised a set of electronegativity values.

He allotted a value of 4 to the most electronegative atom, namely fluorine, and assigned values to the atoms of other elements.

Trend in Electronegativities

The variations in electronegativities of elements in the Periodic table are similar to those of
ionisation energies and electron affinities.

(1) Increase across a Period

The values of electronegativities increase as we pass from left to right in a Period. Thus for Period (2) we have:


This is so because the attraction of bonding electrons by an atom increases with increase of nuclear charge (At. No.) and decrease of atomic radius. Both these factors operate as we move to the right in a Period.

(2) Decrease down a Group

The electronegativities of elements decrease from top to bottom in a Group. Thus for Group VII


The decrease trend is explained by more shielding electrons and larger atomic radius as we travel down a Group.

Importance of Electronegativity

The electronegativities of elements are widely used throughout the study of Chemistry. Their usefulness will be discussed at appropriate places.

The important applications of electronegativities
are listed below.

(1) In predicting the polarity of a particular bond. The polarity of a bond, in turn, shows the
way how the bond would break when attacked by an organic reagent.

(2) In predicting the degree of ionic character of a covalent bond.

(3) In predicting of inductive effects in organic chemistry.

(4) In understanding the shapes of molecules.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.



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