Chapter 5 · Molecules and Compounds
5.4A Molecular View of Elements and Compounds
9 min · two checks
Predict
Is oxygen gas, O₂, a compound?
The idea
- Classify atomic elements, molecular elements, and ionic compounds.
- Recognize seven common diatomic elements.
Some elements exist as single atoms, especially the noble gases (He, Ne, Ar, Kr, Xe). Some exist as molecules of one element. The common diatomic elements are H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. Memorize that list; writing “oxygen” in an equation as O instead of O₂ is a lasting source of unbalanced equations. Phosphorus and sulfur often appear as P₄ and S₈.
Compounds split into molecular compounds (discrete molecules of two or more elements, usually nonmetals: H₂O, CO₂, C₆H₁₂O₆) and ionic compounds (lattices of cations and anions: NaCl, CaF₂). Polyatomic ions such as NO₃⁻ or SO₄²⁻ are charged clusters that behave as a single ion inside an ionic compound. Naming and formula writing in the next sections depend on knowing which of these pictures you are in.
Keep these
- Diatomic elements: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
- Noble gases are atomic elements.
- Molecular compounds are neutral molecules; ionic compounds are ion lattices, sometimes with polyatomic ions.