2.1.2Additional lessons
Binary Ionic Compounds — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 20 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6What Binary Means
What Binary Means
Two elements
- Two elements A binary ionic compound contains ions of only two different elements, one metal and one non-metal.
Cation first
- Cation first The metal cation is named and written first, followed by the non-metal anion, as in sodium chloride NaCl.
Anion ending
- Anion ending The non-metal part ends in -ide, so oxygen becomes oxide and nitrogen becomes nitride.
Neutral overall
- Neutral overall The total positive charge exactly balances the total negative charge, so the compound has no net charge.
Part 2 of 6Deducing the Formula
1
Write the symbol and charge of the cation and the anion, such as Al³⁺ and O²⁻.
2
Find the lowest common multiple of the two charge magnitudes; for 3 and 2 this is 6.
3
Choose the number of each ion so that the total positive charge equals the total negative charge: two Al³⁺ (6+) and three O²⁻ (6−).
4
Write the formula with the cation first and the ion ratio as subscripts, giving Al₂O₃.
5
Check that the charges sum to zero and that the ratio is the simplest whole-number one.
| Cation | Anion | Formula | Name |
|---|---|---|---|
| Na⁺ | Cl⁻ | NaCl | sodium chloride |
| Mg²⁺ | Cl⁻ | MgCl₂ | magnesium chloride |
| Ca²⁺ | O²⁻ | CaO | calcium oxide |
| Al³⁺ | O²⁻ | Al₂O₃ | aluminium oxide |
| Li⁺ | N³⁻ | Li₃N | lithium nitride |
Part 3 of 6Formula Unit and Charge Balance
Formula unit
- Formula unit The formula gives the simplest ratio of ions in the lattice, not the formula of a separate molecule.
Ratio of ions
- Ratio of ions In MgCl₂ there are two chloride ions for every magnesium ion in the crystal.
Using brackets
- Using brackets When a polyatomic ion is needed more than once its formula is bracketed, as in Mg(NO₃)₂ or (NH₄)₂SO₄.
Transition metals
- Transition metals Some metals form more than one ion, so a Roman numeral shows the charge, for example iron(III) oxide Fe₂O₃.
Part 4 of 6Ratio and State
Ratio and State
Why ratios differ
- Why ratios differ Ions of higher charge need fewer partners to cancel the overall charge, so the ratio depends on the charges involved.
Solid, not molecular
- Solid, not molecular Ionic compounds exist as huge lattices of ions and are not made of separate molecules.
State symbols
- State symbols Ionic solids use (s), and when dissolved the separated ions are shown with (aq).
Dissociation
- Dissociation Dissolving can be written as NaCl(s) → Na⁺(aq) + Cl⁻(aq) to show the ions separating.
Part 5 of 6Worked Example
Question Deduce the formula and name of the compound formed between calcium (Group 2) and nitrogen (Group 15).
- Calcium forms Ca²⁺ and nitrogen forms N³⁻.
- The lowest common multiple of 2 and 3 is 6, so three Ca²⁺ (6+) balance two N³⁻ (6−).
- Write the formula with the cation first: Ca₃N₂.
Answer Ca₃N₂, calcium nitride.
Part 6 of 6Exam Checklist & Tips
2.1.2 – Key Points for the Exam
- Binary compound A compound made of just two different elements.
- Formula unit The simplest whole-number ratio of ions in an ionic compound.
- Charge balance The condition that total positive charge equals total negative charge in a compound.
- Roman numeral A numeral in brackets showing the charge of a metal ion, as in copper(II) oxide.
Common Mistakes
- Writing the charges in the formula, such as Mg²⁺Cl₂, instead of MgCl₂.
- Swapping the charges incorrectly so that Al³⁺ and O²⁻ become AlO instead of Al₂O₃.
- Not simplifying the ratio, for example writing Mg₂O₂ for magnesium oxide.
- Calling ionic formulae molecular formulae; there are no individual molecules in the lattice.
Last updated:
\n