Which of the following is favorable condition for the formation of ionic bond?

If the electro negativity difference (Δ E.N ) between cation and anion is greater than 1.7 then the ionic bond is formed between two atoms of elements.

Ex: NaCl ,NaF and CsF etc…

Here, E.N of Na is 0.93 and E.N of Cl is 3.16 and for F , E.N is 3.98

For NaF electro negativity difference in na and F is

$∆\mathrm{E}.\mathrm{N}=3.98-0.93=3.05>1.7$