Chemistry / Unit 2 Chem 0294 worked example, 60 to 90 seconds
Predicting Ionic vs. Molecular Bonding from the Periodic Table
The idea
Predicting whether a compound is ionic or molecular means checking whether its elements combine a metal with a nonmetal, or combine two nonmetals with each other.
In practice
Potassium iodide, KI, pairs potassium, a metal, with iodine, a nonmetal, so it is predicted to be ionic. Hydrogen peroxide, H2O2, pairs two nonmetals, hydrogen and oxygen, so it is predicted to be molecular. Lithium carbonate, Li2CO3, pairs a metal with the polyatomic carbonate ion, so it too is predicted ionic. Magnesium oxide, MgO, pairs magnesium, a metal, with oxygen, a nonmetal, so it too is predicted ionic, matching its known high-melting, crystalline behavior.
Where people slip
This metal-plus-nonmetal guideline is a simplification with real exceptions, such as aluminum chloride, which behaves as a molecular compound despite pairing a metal with a nonmetal.
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