Electronegativity and Electron Affinity

– In this subject, we will discuss the difference Between Electronegativity and Electron Affinity

Electronegativity and Electron Affinity

Electron Affinity 

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

– The 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 the ionization energies of elements.

– In fact, the electron affinity of a neutral atom may be thought of simply as equivalent to the ionization 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:

Electronegativity and Electron Affinity

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

– 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  (1s22s2 , 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.

Electronegativity and Electron Affinity

– 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,


– 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 and Electron Affinity

– 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.

Electronegativity and Electron Affinity

Trend in Electronegativity

– The variations in electronegativities of elements in the Periodic table are similar to those of ionization 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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