Electron
affinity is defined as the amount of energy released when an electron is added
to the valence shell of the isolated gaseous species (which may be an atom or an
ion or a molecule). The positive value of the electron affinity indicates that
the process, i.e. X(g) +e → X-, is exothermic and the
negative value indicates the process to be an endothermic one. Thus, the
convention accepted in defining the electron affinity apparently contradicts
the established convention in thermodynamics. It happens so because in the
given definition the term, 'energy release' has been coined but in all electron
affinity processes energy release may not occur and in some cases energy
absorption may occur.
Thus,
F + e →
F-, ΔH = -328 KJmol-1 and EA = 328 KJmol-1
N + e →
N-, ΔH = 20.3 KJmol-1 and EA = -20.3 KJmol-1
From the
definition, it appears that the electron affinity is the reverse process of the
ionization process, i.e.
X-
→ X + e, IE(X-) = ΔH = EA(X).
In fact,
the ionization process (which accumulates positive charge) and electron
affinity (which accumulates negative charge) are related in the same way in
the relation involving the enthalpy change (ΔH) of the process and the amount
of charge accumulation (q) on the species. By neglecting the higher terms, we
get the following quadratic equation:
ΔH = αq
+ βq2
Where,
ΔH (thermodynamic convention) = ∑IE or ∑EA, and q is the charge on the species.
ΔH relates with EA when q is –ve, and ΔH relates with IE when q is +ve. The
magnitude of β is large for the non-polarisable and small species
while it is small for the large and polarizable species. For example,
β(F) > β(Cl) > β(Br)
As in
the case of ionization energies, the successor electron affinity is such as EA1,
EA2, EA3.... can also be defined. EA2 leads to
the di-negative species while EA3 produces the tri-negative species.
It is always more difficult to place an additional electron to a species which
is already bearing a negative charge compared to the process involving a
neutral species. This is why; higher electron affinity is always
thermodynamically less favourable. In fact, EA2 for all species
are negative, i.e. the processes are endothermic or the endothermic nature outweighs the exothermic nature in the formation of O2-, S2-
etc.
O + e →
O- (ΔH = -141 KJmol-1)
O- + e →
O2- (ΔH = +780 KJmol-1)
Here, EA1(O)
→ 141 KJmol-1 while the EA2(O) = -780 KJmol-1
S + e →
S- (ΔH = -200 KJmol-1)
S-
+ e → S2- (ΔH = +492 KJmol-1)
Here, EA1(S)
= 200 KJmol-1 while EA2(S) = -492 KJmol-1
First and second electron affinity of elements |
The
factors which control the ionization process come under consideration for the
process measuring the electron affinity. In general, the factors favouring the
ionization process disfavour the electron affinity process, i.e. higher
ionization energy leads to higher electron affinity. But the species having
half-filled or full-filled level possess high ionisation energy and lower
electron affinities. The important factors are:
Size of the species: If Z* for electrons at the
periphery is more or less the same for different species (e.g. in a group of
the representative elements), the electrostatic attraction towards the nucleus
experience by the electrons at the periphery is less for the largest species.
In fact, for the representative elements, in a group the effect of slight increasing trend of Z* is less important than the effect of increasing
trained of n (principle quantum number) when we move from top to bottom. For
such systems, the accommodation of an additional electron which is to be bound
at the periphery by the attractive force of the nucleus is disfavoured more for
the larger species. Thus in general, the smaller atoms in a group possess
higher electron affinities.
Effective nuclear charge: The species having highest Z*
the periphery show higher electron affinities.
Nature of the orbital into which
the new electron gets accommodation:
The orbitals which can better penetrate into the electron clouds are more
suitable to house the incoming electron. Of the incoming electron follows the
sequence: ns > np > nd > nf.
Nature of the electronic
configuration:
If the already bare and extra stability due to either the half-filled or
full-filled level, then such species are very much reluctant in accepting the
incoming electron. On the other hand, if the newly added electron creates the
half-filled or full-filled level, the process is favoured.
This
aspect disfavours the Gr IIA (2) elements (ns2), Gr II B (12)
elements [(n-1)d10ns2], Gr VA (15) elements [ns2np3],
noble gases [ns2np6] but favours the Gr VIIA (17) elements
[ns2np5]
Variation
of electron affinity in the periodic table
In a period: The effective nuclear charge
increases and size decreases with the increase of atomic number in a period.
This is why, the electron affinity, in general increases in a period. In fact
it reaches the climax for the Gr VIIA (17) elements.
Because
of the special effect of the electronic configuration, the general trend is
violated in some cases. These are:
Gr IIA
(2) metals: Be, Mg, Ca, etc. are having the ns2 orbital structure.
Hence accommodation of the incoming electron brings the configuration ns2np1.
This process is disfavoured in two ways: The addition of the new electron destroys
the full-filled level structure and accommodation of the new electron occurs in
the p level which is less penetrating. Here it is worth noting that for alkali
metals, the accommodation occurs in the ns level giving rise to ns2
configuration. Thus the process is favoured more in the Gr IA(1) elements
compared to the Gr IIA (2) elements.
Transition series: In the transition series, the
electron affinity values are given below in KJmol-1
Sc =
0 Ti = 20 V = 50
Cr = 64 Mn = 0 Fe = 24
Co = 70 Cu = 118 Zn = 0
The drop
in Mn arises as the starting species is stabilized due to its half-filed d
level. The incoming electron breaks down this extra stability. The drop in Zn
arises due to its full filled structure.
Post-lanthanides: Due to the lanthanide
contraction, among the post-lanthanides, the electron affinity is unusually
high.
In a
group: for the representative elements moving down in a group the size
gradually increases due to the opening of new principal quantum number. In
fact, the effect of slight increasing trend of Z* is less important compared to
the effect of increasing principle quantum number (n) moving down in a
group. This is why, the electron affinity falls down with the increase of
atomic number.
There
are some exceptions and these are:
F <
Cl, O < S, N < P, B < Al: Here it is interesting to note that the
electronegativity sequence is in the opposite order and it is expected from the
size sequence. To explain the observed sequence of electron affinity we are to
consider the other factors. Though the electrostatic attractive pull towards
the nucleus favours the second period elements more compared to the third
period elements The added electron creates and unfavorable effect, i.e.
electron-electron repulsion, which is more for the second period elements
because of their smaller sizes. The repulsion force is not so large in the
third period elements because of their larger size. In addition to this,
possibility of delocalization of the increased electron density in the vacant
3d orbital reduces the repulsions for the third period elements. This possibility to reduce the electron-electron
repulsion through the participation of d orbitals is significant for the
heavier species, e.g. S, Cl, in the third period. On the other hand, for the
second period elements, this mechanism does not operate because of the absence
of any suitable d orbital. Thus the reduced electron electron repulsions in the
third period elements outweigh the favour due to the larger electrostatic
attraction experienced in their corresponding second period elements.
Reference
1) Concise inorganic chemistry by J. D. Lee.
2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.
3) Shriver and Atkins Inorganic Chemistry.
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