K-02: Formulae of common ions and compounds

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S2.1 The ionic model
S2.2 The covalent modelHL

Knowing that sulfate is SO₄²⁻, nitrate NO₃⁻, carbonate CO₃²⁻, ammonium NH₄⁺, and being able to combine them into neutral formulae with brackets, is used constantly and is never re-taught. Directly load-bearing and worth over-learning.