Amount of energy released when an electron is added to an isolated gaseous atom.
x( g ) + e- → x- ( g ) + E.A.
Units : k J mol-1 , k Cal mol-1 and eV per atom.
Cl has the highest E.A.
Electron gain Enthalpy When expressed in terms of enthalpy change (ΔH) then it is termed as E.G.E. Remember that
ΔH = - ve for exothermic change.
For EA1, energy is released ∴ΔHEA1 = - ve
For EA2, onwards is required ∴ ΔHEA2 = + ve
EA1 + EA2 ∵ ΔH = +ve
Factors affecting Electron Affinity
(i) Nuclear Charge: Varies directly As nuclear charge increases, the incoming e- is more strongly attracted and more amount of energy is liberated.
(ii) Atomic Size: Varies inversely
Less amount of energy is liberated.
So, larger atoms have lower E.A. value.
(iii) E.C.: Stable electronic configurations have less tenoency to accept electron, so they have lower E.A. values.
(i) In a Period : Along a period, electron affinity increases [with exception] as Zeff .
(ii) In a group : Along a group, electron affinity decreases after 3rd period. Between 2nd and 3rd period in p block electron affinity of 2nd period is lesser due to high electron density.
F = 328, Cl = 349, Br =324, I = 295
(1) A fully filled and half filled which have low values or even sometimes energy is required rather than getting released.
(2) 2nd period has lower value than 3rd owing to repulsion between electrons.
Unexpected E.A.: EA of chlorine is higher than that of F.
In general: The third period elements have higher EA values than 2nd period elements.
Factor affecting electronegativity
(1) Nuclear attraction : varies directly
(2) Atomic radius : varies inversely
(3) Change on ions : More positive charge more electronegativity and more –ve change less electronegativity. (4) Hybridisation : to be discussed later in bonding.
⇒ When ENC increases more EN, also increases.
Across the Period : The E.N. values increase along a period from left to right.
Down the Group:
Most Electronegative Atom of P.T.:
Scale of measurement
(1) Pauling’s Scale (1932):
XA = E.N. of A
XB = E.N. of B. Bond energies of A - A, B - B, A - B molecules.
(2) Mulliken Scale (1934) : According to this scale, EN. of an atom is the average sum of I E. and EA. of an atom.
In Pauling Scale
(If I.P., E.A. are taken in eV)
(I.P., E.A. = Kcal mol-1)
Factors dependent on electronegativity
(1) % ionic character varies directly.
(2) Strength of bond varies directly.
Application of E.N.
(1) Polarity of a molecule : As the DE increases between two bonded atoms; it polarity also increases.
Eg. H - I < HBr < H - Cl < H - F .....dipole moment The direction of dipole is also determine by
The more electronegative atom constitutes the negative pole.
(2) Nature of the bond of a molecule
If the Δ E.N. is less (< 1 .7) the bond the has more covalent character..
If the Δ E.N is more (> 1.7) the bond the has more ionic character..
Allred Scale for measurement of % ionic character of a covalent bond : % ionic character of a bond =
(3) Determination of Bond Strength: As the ΔE = X A - X B increases, a stronger covalent bond is formed.
Table : Hydrides MHx - E.N. of M and Nature
|S.NO.||Δ E.N .(M .H .)||Nature||Eg.|
|1.||M < H||Basic||LiH < NaH < KH < RbH < CsH BeH2 < MgH2 < CaH2 < SrH2 < BaH2 < RaH2 AlH3 < GaH3 < InH3 < TiH3 NH3 < PH3 < AsH3 < SbH3 < BiH3 |
Transition metal hydrides
|2.||M = H||Neutral||CH4 < SiH4 < GeH4 < SnH4 < PhH4|
|3.||M > H||Acidic||H2O < H2S < H2Se < H2Te < H2PO HF < HCl < HBr < HI|
INERT PAIR EFFECT
Inert pair effect term refers to the resistances of a pair of s electrons to be lost or to participate in covalent bond formation.
(1) Group III A :
(A) TI(3+): 2, 8; 18, 32, 18
Tl(+) : 2,8, 18, 32, 18, 2
Tl(1) compounds are more stable than Tl(III) compounds.
(B) The mean bond energy for chlorides are GaCl3 = 242, InCl3 = 206 and TlCl3 = 153 kJ mol-1.
Thus the s electrons are most likely to be Inert in thallium and TICl3 is the least stable among these chlorides.
(2) Group IVA:
(A) Sn2+ : 2, 8, 18, 18, 2 (outer 5s2 electrons not lost)
Sn4+: 2, 8, 18, 18
Sn(II) exist as a simple ion Sn2+ and is a strong reducing agent whereas Sn(IV) is covalent.
(B) Pb2 : 2. 8, 18, 32. 18, 2 (outer 6s2 electrons not lost)
Pb4: 2,8, 18, 32, 18
Pb2 is Ionic, stable and more common than Pb (IV) which is oxidizing.
(A) Arsenic and atomic show a variable valence of 3 and 5 while Bismuth is only trivalent.
Bi3+ : 2, 8, 18, 32, 18, 2 (outer 6s2 electron not lost)
Bi3+ is stable and Sb3+ is a reducing agent.