On moving, down the group the size increases and electronegativity decreases, thus tendency to show catenation decreases. Decrease in catenation property is linked with $$M - M$$  bond enthalpy which decreases from carbon to tin.

There is a decrease in bond enthalpy $$(H - E)$$   as the size of the element increases on moving down the group hence, the stability decreases on moving down the group.

$$HCOOH$$  on dehydration gives $$CO.$$  This method is used for producing $$CO$$  at a small scale.

Hybridisation $$ = \frac{1}{2}\left( {4 + 4 + 0 - 0} \right) = 4,$$     $$s{p^3}$$  tetrahedral.

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Chlorine cannot displace $$F$$  from $$NaF.$$  The reactivity follows the order $$F > Cl > Br > I$$

The $$H - X$$  bond strength decreases from $$HF\,{\text{to}}\,HI$$.   i.e. $$HF > HCl > HBr > HI.$$      Thus $$HF$$  is most stable while $$HI$$  is least stable. This is evident from their decomposition reaction: $$HF\,{\text{and}}\,HCl$$    are stable upto $$1473K,$$  $$HBr$$  decreases slightly and $$HI$$  dissociates considerably at $$713K.$$  The decreasing stability of the hydrogen halide Oxidation is also reflected in the values of dissociation energy of $$H - X$$   bond
$$\mathop {H - F}\limits_{135kcal\,mo{l^{ - 1}}} \,\,\mathop {H - Cl}\limits_{103kcal\,mo{l^{ - 1}}} \,\,\,\,\,\,\,\mathop {H - Br}\limits_{87kcal\,mo{l^{ - 1}}} \,\,\,\,\,\,\mathop {H - I}\limits_{71kcal\,mo{l^{ - 1}}} $$

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In liquid state $$Br{F_3}$$  dissociates into $$BrF_2^ + $$  and $$BrF_4^ - \,ions$$   most easily.

Acidic strength of oxoacids of a particular halogen atom increases with increase in oxidation number, thus the order of acidic strength is
$$\mathop {HClO}\limits^{ + 1} < \mathop {HCl{O_2}}\limits^{ + 3} < \mathop {HCl{O_3}}\limits^{ + 5} < \mathop {HCl{O_4}}\limits^{ + 7} .$$