Chemistry Help?

The only evidence is the electronegativity of the elements that are forming the bond. Electronegativity measures the relative attraction that the atom has for the pair electrons that form the bond. According to my chemistry book, the following equation is used to determine the percentage of ionic character.

% = (a – b) ÷ a

a is for nonmetals These usually form negative ions.b is for metals or metalloids. These usually form positive ions. If this number is greater than 50%, the bond is ionic. If this number is between 5% and 50%, the bond is polar covalent. If the number is less than 5%, the bond is nonpolar. Let’s use chlorine as the nonmetal.

B = 2.0

Al = 1.5

Cl = 3

For BCl3, % = (3 – 2) ÷ 3

This is 33⅓ %. Since this is between 5% and 50%, the bond is polar covalent.

For AlCl3, % = (3 – 1.5) ÷ 3

This is 50%. So the bond is ionic.

I disagree with electron1's interpretation of the question. I take it mean: "What are the physical properties of compounds of boron and aluminum that would lead you to conclude that they are covalent and ionic respectively?"

I'd say the principal evidence would be that compounds of aluminum in solution conduct electricity and compounds of boron do not. Less conclusive would be the fact that aluminum compounds are crystalline and compounds of boron tend not to be crystalline.