electrochemistry - Electronegativity

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.

Answer

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.