The absolute value of the difference in electronegativity of two bonded atoms gives us a rough measure of the polarity to be expected in a bond. When this difference is small, the bond is non-polar. When it is large, the bond is polar, or if very large perhaps ionic. The electronegativity differences for the bonds H—H, H—Cl ,and Na—Cl are 0 0.9, and 2.1, respectively, following the expected order. Electronegativity differences explain why ionic bonds usually form between a metal atom and a nonmetal atom; the electronegativity difference would be largest between these elements. On the other hand, covalent bonds primarily form between two nonmetals because the electronegativity differences are small.