Trends in Periodic Properties- 2 | Inorganic Chemistry PDF Download

Electron  Affinity

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.

• EA measures the tightness with which an atom can hold the additional e^- .
• Larger value of E.A. reflects the greater tendency of an atom to accept the e^- .

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 E_A1, energy is released              ∴ΔH_EA1 = - ve

For E_A2, onwards is required         ∴ ΔH_EA2 = + ve

E_A1 + E_A2                                               ∵ 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

• As atomic size increases, The distance between nucleus and incoming electron also increases. The e- is less stabilized.

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.

Periodic Trends
 (i) In a Period : Along a period, electron affinity increases [with exception] as Z_eff ­.

• X have highest E.A. : In corresponding periods (Halogens). By accepting 1e^-, achieve ns²p⁶ configuration.
• Noble gase have zero E.A. :As they have stable configuration, so they do not have any tendency to accept e.
• E.A., Be, Mg → EA almost zero : They have all paired e^-s. So they have stable configuration.
• E.A. of N, P → extremely low : These have stable half filled E.C. Their p orbitals one half filled in their valence shells.

(ii) In a group : Along a group, electron affinity decreases after 3^rd period. Between 2^nd and 3^rd period in p block electron affinity of 2^nd period is lesser due to high electron density.

F = 328,  Cl = 349,  Br =324,  I = 295

Exception 

(1) A fully filled and half filled which have low values or even sometimes energy is required rather than getting released.

(2) 2^nd period has lower value than 3^rd 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 E_A values than 2nd period elements.

Electronegativity (E.N.)

• It is defined as, the tendency of an atom to attract the shared pair of electron in a molecule.

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.

Periodic Trends 

Across the Period : The E.N. values increase along a period from left to right.
Down the Group:

• The E.N. values decrease down the group.

Most Electronegative Atom of P.T.: 

• The Black Sheep is (F).
• Least EN. atom → (Fr)
• 1st Group elements (Alkali metals) have lowest EN. in a period.
• The Halogens are most electronegative in a period.
• Non metals have high EN. values.

Scale of measurement

(1) Pauling’s Scale (1932):

• Pauling assigned an arbitrary EN value of 4.0 to the most electronegative atom F.
• The E.N. of other elements were as by keeping this value as the standard reference.
• The E.N. difference between two atoms is determined by the expression,

X_A = E.N. of A

X_B = 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 MH_x - E.N. of M and Nature

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.
Example: 

(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.

(3) GroupVA:

(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.