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.