1) The polar character of a covalent bond is defined by the difference in the electronegativities of the atoms. The higher the diffrerence the greater the polar character of the bond.
2) The electronegativities of the elements are found in tables.
These are the electronegativities of the four elements of the question:
H: 2.20 ← lowest
I: 2.66
Br: 2.96
Cl: 3.16 ← highest
3) Since Cl has the greatest electronegativity of the four elements, and H has the lowest one, it is the bond H - Cl which will have the greatest difference of electronegativities, and it is the most polar bond.
For the question given above, option 2 which is H-Cl pair of atoms has the most polar bond among the four of them.
The larger the value of the electronegativity, the greater the atom’s strength to attract a bonding pair of electrons. <span>Hydrogen has an electronegativity of 2.1, and chlorine has an electronegativity of 3.0. The electron pair that is bonding HCl together shifts toward the chlorine atom because it has a larger electronegativity value.</span>
The experimenter observed this experiment in a lab rather than natural world because it might be dangerous to the atmosphere if he does the experiment in the natural world and it was still an hypothesis so that's why he did it in the lab.
