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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?

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There isn't a clear limit between ionic and covalent bonds; it's more useful to think of covalent and ionic as two extreme models for bonding, with all real bonds falling somewhere in between in a continuum. We classify cases that fall close to one of those ends as "ionic" or "covalent", but in cases that fall somewhat in-between, it may not be easy to ascribe a label to them. Any limits (such as your values for electronegativity) are arbitrary and should be taken as a guideline and not as a strict definition.

In fact, as you advance in your understanding of chemistry, it makes more sense to talk about the ionic character of bonds than to talk about ionic and covalent bonds - with bonds close to the ionic ideal having a high percentage of ionic character and bonds close to the covalent ideal having a low percentage of ionic character. There are schemes to quantify ionic character from electron densities (either experimental or calculated); for instance, there's a classic expression proposed by Linus Pauling for diatomic molecules in the 1960s, as a function of electronegativities:

$$ f_{AB} = 1 - \exp{[-0.25 (X_A - X_B)]}$$

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.

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