So I’m student-teaching chemistry and my master teacher and the book tend to say that if the difference in electronegativities between two elements is 1.7 or greater, the bond is considered ionic, 0.1 to 1.7 is polar-covalent, and 0 is covalent. The book also say that ionic compounds are between metals and non-metals. Now, I understand that there’s a continuum here, and not strict “lines.” BUT, the first example the textbook gives of a polar covalent bond is the (classic?) H–F bond. They even show that the difference in electronegativities is 1.9 (4.0-2.1). So the only way I can justify this is to say that ionic bonds are ALWAYS metal to non-metal. Is this true? What about, say NaH? Is this an ionic bond? The difference in electronegativities is only 1.2 (2.1-0.9). Since aluminum has an electronegativity of 1.5, then you would need a 3.2 to really be conidered ionic, which only leaves Al oxygen and fluorine to make ionic bonds with. Would AlCl really not be considered ionic? Or is metal to non-metal ALWAYS ionic, and non-metal to non-metal ALWAYS covalent (polar or not). Thanks for the help!
Matt
you know, the whole electronegativity thing is only a primitive model and all this metal or nonmetal is too. i dont think its worth it to try and pick it apart.
looking at electron orbital shapes and their hybridizations and all that crap is closer to the reality of what is happening.
NaH ? AlCl ? for some reason i have never heard of such compounds … but i am no chemist so i wouldnt know if this is something that tends to hold together or what.
I’ve heard of AlCl[sub]3[/sub], but not AlCl =] Unfortunately I have very little time to post, but no, it’s not as simple as all metal-nonmetal bonds being ionic and all nonmetal-nonmetal bonds being covalent. A lot of compounds contain a different kind of bond, a ‘dative’ or ‘coordinate’ bond where groups of atoms called ‘ligands’ donate both electrons to a metal. A lot of metal-nonmetal compounds contain covalent bonds, and there are even compounds where unusual numbers of electrons participate in a bond (such as a ‘three-center, two-electron bond’). Beryllium chloride is one example of a metal-nonmetal compound with covalent bonds, although the difference between the Pauling electronegativities of Be and Cl is 1.57, so you’d expect it that way. In beryllium chloride you get each beryllium atom bonded to four chlorines, and each chlorine is bonded to two berylliums and gives, on average, half an electron to each beryllium. The result is a ‘chain polymer’ with the formula BeCl[sub]2[/sub]. There are a lot of other examples, but… no time. Anyone with a copy of Cotton and Wilkinson’s Advanced Inorganic Chemistry will be able to give a few hundred examples, most of which I was required to memorize at one point. =P
I’m surprised you don’t recall being taught that H can act as either a metal or a non-metal. Some periodic charts show H in the middle of the top row with, IIRC, a dotted line to its other position in the upper left. In your example, I’d have to say that it is an ionic bond, and H is acting as a non-metal. Your book may give you the nitty gritty on bonds, but surely you realize Na cannot act as a non-metal, so Na can never form a covalent bond. …Unless the rules have changed! 
- Jinx
I failed to suggest that, perhaps, the numeric analysis of bonding mentioned in the OP may not always work for H compounds since hydrogen’s properties do not follow the general structure of the Periodic Chart. As you know, there is a method to the apparent madness to how the Periodic Chart is arranged.
Extra Credit: Hmm, when do we find H acting as a metal???
- Jinx