Question: how to calculate the difference of electronegativity?
Answer: Subtract the smaller electronegativity from the larger one to find the difference. For example, if we're looking at the molecule HF, we would subtract the electronegativity of hydrogen (2.1) from fluorine (4.0). 4.0 - 2.1 = 1.9. If the difference is below about 0.5, the bond is nonpolar covalent.