The greater the difference in electronegativity (ΔEN) between two atoms, the greater the degree in ionic character in the bond.
If you have a bond between atoms A and B (A-B),
ΔEN = |EN_A - EN_B|
The rules for the type of bond are:
• If ΔEN < 0.5, the bond is nonpolar covalent
• If 0.5 ≤ΔEN < 1.7, the bond is polar covalent
• If ΔEN ≥ 1.7, the bond is ionic
Examples:
C-H: ΔEN = |2.55 – 2.20| = 0.35; nonpolar covalent
C-F: ΔEN = |2.55 – 3.98| = 1.43; polar covalent
Na-F: ΔEN = |0.93 – 3.98| = 3.05; ionic
