Acidic order of Halogens- hydrogen halides acidc strength

Acids are the substance which can donate  Hydrogen ions in solution or the substance which accept electrons .

But now the question is how to know which acid is more acidic and which one is less . 

 For this you have to understand that Strong acid can easily donate H+ This state is valid for the molecules having Hydrogen atom in their molecules .

acidic order/ Strength of halogens ( Hydrogen halides )

Hydrogen halides are the molecules having hydrogen and halide group(Fluorine , Chlorine , Bromine and Iodine ) .Bond length between Hydrogen halides increases down the group due to increase in the size of halide atom . 

Increasing order of bond length :

HI >HBr >HCl> HF

Due to increase in bond length , bond strength become weaker down the group .

Order of Bond strength :

HF >HCl >HBr> HI

It means that Hydrogen Iodide(HI) can easily donate proton ( due to weak bond ) into the solution and therefore it is most acidic hydrogen halide 

whereas Flourine due to  strong hydrogen bond  between Hydrogen and flourine , its difficult to break this bond and therefore it is least acidic in nature .

Order of acidic strength :

HI >HBr >HCl> HF

So , In simple acidic strength of hydrides of p-block  elements increases down the group .

Other example of Acidic strength order :

BiH3 >SbH3>AsH3>PH3>NH3