Question

Which of the following is most stable?
A. $HF$
B. $HI$
C. $HCl$
D. $HBr$

Answer 1

We need to study the stability of the given hydrogen halides. Hydrogen halides are inorganic compounds which are diatomic and have the formula HX where H is hydrogen and X can be any halogen out of Fluorine, Chlorine, Bromine, Iodine or Astatine. The main characteristics of halogens are that they have seven valence electrons in their outermost shells, combine with metals to form salts and are highly reactive and electronegative.

Complete step by step answer:
We must have to remember that the halogens have the capacity to gain one electron by reacting with atoms of other elements. Hence they are highly electronegative hence unstable. Electronegativity of an atom is the ability of the atom to attract an electron from a more electropositive element. Higher the electronegativity means a stronger is ability to attract electrons of that atom.
We need to know that the stability of a hydrogen halide means the ability to hold the atoms in such a manner so that it does not dissociate easily. We know that electronegativity of an atom decreases as we go down the group in the periodic table. Hence Fluorine, F is the most electronegative element. The most electronegative element holds the gained electrons more tightly and hence more stable. Hence we can say that the electronegativity of the halogens above will make their hydrogen halides more stable. Therefore, the order of stability of the given hydrogen halides is $HF > HCl > HBr > HI$ .

So, the correct answer is Option A.

Note: We must note that not only does electronegativity contribute to the stability of hydrogen halides but bond strength, bond length and the size of the atoms also provides thermal stability to hydrogen halides. We must have to know that the thermal stability is directly proportional to the strength of the H-X bond. Bond strength is inversely proportional to bond length and bond length is directly proportional to size of the halogens.