39 . Electron gain enthalpy : Halogens have maximum negative electron gain enthalpy because these elements have only one electron less than stable noble gas configuration .
Electron gain enthalpy becomes less negative down the group because atomic size increases down the group .
40 . Eelctronegativity : These elements are highly electronegative and electronegativity decreases down the group . They have high effective nuclear charge .
41 . Bond dissociation enthalpy :
Bond dissociation enthalpy follows the order Cl2 > Br2 > F2 > I2 This is because as the size increases bond length increases .
Bond dissociation enthalpy of Cl2 is more than F2 because there are large electronic repulsions of lone pairs present in
F2 .
42 . Colour : All halogens are coloured because of absorption of radiations in visible region which results in the excitation of outer electrons to higher energy levels .
43 . Oxidising power : All halogens are strong oxidisinga gents because they have a strong tendency to accept electrons .
Order of oxidizing power is F2 > Cl2 > Br2 > I2
44 . Reactivity with H2 :
Acidic strength : HF < HCl < HBr < HI Stability : HF > HCl > HBr > HI
This is because of decrease in bond dissociation enthalpy . Boiling point : HCl < HBr < HI < HF
HF has strong intermolecular H bonding
As the size increases van der Waals forces increases and hence boiling point increases .