# if difference of electronegativity between two atoms is 0.9 then which type bond will be formed an ionic or a covalent bond?

Bonds are formed by sharing of electrons .The difference of electronegativity determines which type of bond will be formed. If difference of electronegativity between two atoms is less than 0.9, then a non polar bond will be formed .If difference is between 0.9-1.7 then a polar bond will be formed with 50 percent ionic character .If difference is greater than 1.7 then it will be ionic bond.But if difference is exactly" 0.9 ",Which type of bond will be formed?

$$f_{AB} = 1 - \exp{[-0.25 (X_A - X_B)]}$$
which gives values for ionic character, $f_{AB}$, between 0 (purely covalent) to 1 (purely ionic). More modern expressions are far more complex and depend on other electronic properties, but they all give you fractions of ionic character in a continuum from pure ionic to pure covalent.
So I'd suggest to picture cases that happen right at an arbitrary boundary simply as "grey" cases, with a mixed character; in your case, at $\Delta X=0.9$, you'll have somewhat polar covalent bonds.