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We know a compound could form between NaCl because they are +1 and -1 ions which make them both into a complete valence set.

Could a compound form between Al and Br, for example, and what type of bond would this be? Would such a compound never form because it is not 'stable' (both atoms having a complete octet, like NaCl)?

It is said that a bond between a metal and non-metal such as this must be Ionic. If this is the case then it violates the rule that the Electronegativity difference must be > 1.7. The EN difference is only ~1.3 between Al and Br, which would indicate a Covalent bond instead. Which is correct?

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    $\begingroup$ There are laws which are precise with absolute mathematical certainty, and there are laws which are about as precise as our measurements, and then there are laws which are very, very approximate and imprecise. Those which you mentioned are of the latter sort. $\ce{AlBr3}$ is mostly covalent, until dissolved in water, at which point it starts looking more like ionic, but not quite. $\endgroup$
    – Ivan Neretin
    Commented Jan 17, 2021 at 21:53
  • $\begingroup$ And @IvanNeretin forgot to mention that there are rules which are more like guidelines than actual rules. Welcome to the Black Pearl! $\endgroup$
    – Jan
    Commented Jan 19, 2021 at 8:54

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Neither the 1.7 electronegativity difference rule nor the metal-nonmetal rule for ionic bonding is really correct, as Ivan points out in the comments. Following are some well-documented exceptions to both rules:

  • The chart given here, illustrated below, shows potassium iodide has about 70% ionic character even though its electronegativity difference is only 1.6. The ionic character matches that of caesium chloride, which has an electronegativity difference of 2.3. Also note the electronegativity difference of 1.8 but less than 50% ionic character in hydrogen fluoride. enter image description here

  • The low solubility of most transition metal sulfides in water may be correlated with covalent character in their bonding, which goes along with sulfur having a relatively low electronegativity for a nonmetal. Alkali and alkaline earth sulfides which have more ionic character dissolve readily in water, and also react with it through hydrolysis of the strongly basic sulfide ion.

You need a better rule. One such rule considers the structure of the compound. When you are dealing with elements in Groups 12-17, they have only four valence orbitals; thus a coordination number of four or less serves as an indicator of covalent bonding while higher coordination numbers require at least some ionic contribution. Thus the dimeric molecular structure of aluminum bromide heralds covalent bonding in the neat aluminum bromide. enter image description here From Wikimedia Commons

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