Chemical Formulas and Naming Compounds

Question

My teacher has assigned a worksheet in which I am given a substance's name, and I have to determine if the bond is ionic or covalent, but I don't know how to know. For example, I am given the substance name aluminum (Al 3+)  nitrate (NO3-). How can I tell if it is covalent or ionic?

Answer 1

If you are in Introductory Chemistry, they usually teach that a metal + a non-metal makes an ionic bond, and two non-metals or a metaloid + non-metal make covalent bonds.  A little more advanced course teaches to use ∆EN (difference in electronegativty values) and if ∆EN > 2 (some books use >1.7) then it is ionic.  In the example you give, Al is a metal, and NO3^- is a polyatomic anion, and this would be ionic.  The bonds forming NO3^- are, however, covalent.  So, here are a few examples:

Sodium chloride. NaCl.  Na is a metal; Cl a non-metal.  Bond is ionic

Potassium sulfide.  K₂S.  K is a metal; S a non-metal.  Bond is ionic

Sulfur dioxide.  SO₂.  Both sulfur and oxygen are non-metals. Bond is covalent.

Magnesium sulfate.  MgSO₄.  Magnesium is a metal.  Sulfate anion is polyatomic.  Bond is ionic.

NOTE: Consider H as a non-metal, so HCl would be considered covalent.  Consider any metal + a polyatomic anion as ionic.

I'm sure there are exceptions, but for an introductory course, these rules should help you. Aloha !