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2.1.2Additional lessons

Binary Ionic Compounds — IB Chemistry HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 23 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 6The Ionic Bond

2.1.2 Binary Ionic Compounds

Binary ionic compounds are built from one type of cation and one type of anion.

Definition
  • An ionic bond is the electrostatic attraction between oppositely charged ions.
Formation
  • It usually forms between a metal, which loses electrons, and a non-metal, which gains them.
Direction
  • The attraction acts equally in all directions around an ion, so it is non-directional.
Binary
  • A binary compound contains just two elements, such as NaCl or MgO.

Part 2 of 6Writing Formulae

1
Write the symbol and charge of the cation first, then the symbol and charge of the anion.
2
The compound must be neutral, so total positive charge must equal total negative charge.
3
Use the charge number of each ion as the subscript of the other ion, then simplify to the lowest ratio.
4
Do not write a subscript of 1, and put brackets round a polyatomic ion if more than one is needed.
5
Check the answer by adding the charges: for Al₂O₃ that is 2 × (3+) + 3 × (2−) = 0.

Part 3 of 6Naming Rules

Naming Rules
Cation first
  • The metal name comes first and is unchanged, for example sodium or calcium.
Anion ending
  • A simple non-metal anion has its name ending changed to -ide, such as chloride or oxide.
Roman numerals
  • Add a numeral in brackets when the metal has several ions, as in copper(II) oxide.
No prefixes
  • Ionic names do not use di- or tri-, since the formula is fixed by the charges.
CationAnionFormulaName
Na⁺Cl⁻NaClSodium chloride
Mg²⁺N³⁻Mg₃N₂Magnesium nitride
Al³⁺O²⁻Al₂O₃Aluminium oxide
Fe³⁺O²⁻Fe₂O₃Iron(III) oxide

Part 4 of 6Common Patterns

Group 1 and 17
  • These combine in a 1:1 ratio, as in KBr.
Group 2 and 17
  • These combine in a 1:2 ratio, as in CaCl₂.
Group 2 and 16
  • These combine in a 1:1 ratio, as in MgO.
Group 1 and 16
  • These combine in a 2:1 ratio, as in Na₂O.

Part 5 of 6Worked Example

Question Deduce the formula and name of the compound formed by magnesium and nitrogen.

  1. Magnesium forms Mg²⁺ and nitrogen forms N³⁻.
  2. The lowest common multiple of 2 and 3 is 6, so 3 Mg²⁺ (6+) balance 2 N³⁻ (6−).
  3. Write the cation first with the numbers as subscripts.

Answer Mg₃N₂, magnesium nitride.

Part 6 of 6Exam Checklist & Tips

2.1.2 – Key Points for the Exam

  • Ionic bond The electrostatic attraction between oppositely charged ions.
  • Binary compound A compound made of exactly two elements.
  • Formula unit The simplest whole-number ratio of ions in an ionic compound.
  • Stock name A name using a Roman numeral to give the charge of a metal ion.

Common Mistakes

  • Writing ionic formulae as molecules, such as 'one NaCl molecule'.
  • Failing to simplify the ratio, for instance writing Mg₂O₂.
  • Using prefixes such as magnesium dioxide in a name.
  • Forgetting the Roman numeral when the metal is a transition element.

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