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Why is the Electronegativity difference for atoms in bonding uncertain while determining what the compound will be?

According to the IB(International Baccalaureate) they say that the Electronegativity difference required for Ionic bonds is 1.7 though compounds like Hydrogen Fluoride are covalent even though they have a Electronegativity difference of 1.9. If the IB scale of above 1.7 being ionic and below 1.7 being covalent then why are there many exceptions to this rule? Also, why does Electronegativity not change, since having more electrons in atom would probably cause repulsion of new electrons? If you search for what the Electronegative difference to predict bonding and were to look at Yahoo.answers you will see 3 different answers to how to predict these reactions at 1.7,1.8 or 1.9 electronegative difference Yahoo question. Another exception is Calcium Carbide which forms a ionic bond at 1.5 electronegative difference.

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  1. There are several methodics of assgning Electronegativity, each giving slightly different results. There is no strict border between ionic and covalent bound, and it usually have some weigh of covalent bonding too.

  2. Electronegativity of atom in molecule changes in response to its neighborhood

  3. Hydrogen without electrons will be naked proton. Because of its size it produces so strong electrostatic field, that it ruptures electronic shell of other atoms. So, hydrogen never forms positive ions, it is always attached to some other atom(s) covalently.

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Lots of materials show a bonding character different from what would be expected from electronegativity difference alone. Here are a few:

• Hydrogen fluoride has an electronegativity difference greater than 1.7, yet it is generally considered polar covalent.

• Borides and hydrides of alkali and most alkaline earth metals are found to be predominantly ionic when charge separation is measured. The case of magnesium diboride ($\ce{MgB2}$), with an elecronegativity difference less than $1.0$, is discussed here.

• Alkyl fluorides and alkyl lithium have nearly the same electronegativity difference in the bond between carbon and the respective other element, but the alkyl lithium compounds show much more ionic character as evidenced by their reactivity.

The crux of the matter is that ionic character is determined not by electronegativity difference but by molecular orbital structure. Electronegativity difference can, of course, have a major influence on this structure but is not the sole determinant. Magnesium diboride is more ionic than hydrogen fluoride bcause in the former, the boron atoms associate to produce molecular orbitals that accept electrons from the magnesium (or any other strongly electropositive element) more readily than individual boron atoms. Largely covalent alkyl fluorides have good carbon-fluorine covalent overlap, whereas more ionic alkyl lithiums have poor overlap between the carbon and the bulky, diffuse valence orbitals on lithium.

The balance between ionic and covalent bonding is thus not a simple matter of looking up table valued, despite what many an introductory textbook might say. A good grasp of molecular orbital theory (which itself can get complicated), and preferably some experimental verification, should be brought to bear.

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