Polarity of a bond is determined by the difference in electronegativity between the two bonded atoms. Fluorine is the most electronegative element, and the electronegativity difference between hydrogen and fluorine is the greatest among the given pairs, making HF the most polar bond.

The second ionization energy is the energy required to remove the second electron after the first has been removed. Sodium has a very high second ionization energy because removing the second electron means disrupting a stable noble gas configuration (Ne), making it significantly more difficult than the first ionization.