Formula Building: Ions Criss-Cross

Also called

chemical formula · criss-cross method · writing formulae · valency · ions · cation · anion · sodium chloride NaCl · magnesium chloride MgCl2 · aluminium oxide Al2O3 · calcium carbonate CaCO3 · polyatomic ion · carbonate ion · charges balance · ionic compound · रासायनिक सूत्र · आयन · संयोजकता

Topics

• Metals lose electrons to form positive ions (cations): Na⁺, Mg²⁺, Al³⁺, Ca²⁺. Non-metals gain them: Cl⁻, O²⁻.
• A compound has no overall charge, so the total positive charge must equal the total negative charge.
• Na⁺ and Cl⁻ pair one to one: NaCl. One plus cancels one minus.
• Mg²⁺ carries two positive charges, so it needs two Cl⁻ to cancel them: MgCl₂.
• Criss-cross: the charge on each ion becomes the subscript of the other. Al³⁺ and O²⁻ give Al₂O₃.
• A polyatomic ion such as carbonate, CO₃²⁻, acts as one unit: Ca²⁺ and CO₃²⁻ give CaCO₃.
• Check a formula by adding the charges: 2 × (3+) + 3 × (2−) = 0 for Al₂O₃.
• When the criss-crossed subscripts share a factor, divide them: Ca₂(CO₃)₂ is written CaCO₃.

About this model

A GLB file of about 378 KB, shown in Lably's 3D viewer and, on a phone, placed in your room with AR. Credit: Model, science and animation: Lably (built procedurally with three.js; no downloaded meshes). | CC BY 4.0. Original geometry and animation made for Lably.

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