An atom of which element reacts with an atom of hydrogen to form a bond with the greatest degree of polarity?

Fluorine is the atom.

Explanation:

A bond will have highest polarity if the electro-negativity difference between the two bonded atoms is maximum.

As we have fixed the electro-positive element to be hydrogen, so to have maximum polarity in the bond we should take the most electronegative element, Fluorine.

The electro-negativity of fluorine is maximum and is equal to 4.

An atom which has highest electronegativity when combined with hydrogen atom will result in highest degree of polarity.

Therefore, fluorine is the atom with highest electronegativity and when it combines with hydrogen atom then it will attract the shared pair of electrons more towards itself.

As a result, there will be formation of HF molecules which has the highest degree of polarity.