2.1.1Additional lessons
Forming Ions — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 of 172
Part 1 of 5Why Ions Form
Ions are charged particles produced when atoms transfer electrons.
Cation
- A positive ion formed when an atom loses one or more electrons, as metals typically do.
Anion
- A negative ion formed when an atom gains one or more electrons, as non-metals typically do.
Stable configuration
- Many main-group ions end up with the same electron arrangement as the nearest noble gas.
Ion charge
- The charge equals the number of protons minus the number of electrons in the ion.
Part 2 of 5Charges from the Periodic Table
Charges from the Periodic Table
Group 1
- Atoms lose one electron to form ions with a 1+ charge, such as Na⁺.
Group 2
- Atoms lose two electrons to form 2+ ions, for example Mg²⁺.
Group 13
- Aluminium loses three electrons to form Al³⁺.
Groups 15 to 17
- Atoms gain three, two or one electron to form 3−, 2− and 1− ions respectively.
| Ion | Electron configuration | Same as |
|---|---|---|
| Na⁺ | 1s²2s²2p⁶ | Ne |
| Mg²⁺ | 1s²2s²2p⁶ | Ne |
| O²⁻ | 1s²2s²2p⁶ | Ne |
| Cl⁻ | 1s²2s²2p⁶3s²3p⁶ | Ar |
Part 3 of 5Transition Metal Ions
1
Transition elements can form more than one stable ion, so the charge is shown with a Roman numeral, as in iron(II) and iron(III).
2
When a transition metal forms a cation, the 4s electrons are removed first, before the 3d electrons.
3
Iron has configuration [Ar]3d⁶4s²; Fe²⁺ is [Ar]3d⁶ and Fe³⁺ is [Ar]3d⁵.
4
Copper commonly forms Cu²⁺, and silver forms Ag⁺ and zinc forms Zn²⁺ in almost all compounds.
Part 4 of 5Polyatomic Ions
Ammonium
- NH₄⁺ is the common polyatomic cation, with a charge of 1+.
Hydroxide and nitrate
- OH⁻ and NO₃⁻ are both singly charged anions you must recognise.
Hydrogencarbonate
- HCO₃⁻ carries a 1− charge and forms from carbonic acid losing one proton.
Carbonate and sulfate
- CO₃²⁻ and SO₄²⁻ are doubly charged and are widely met in salts.
Phosphate
- PO₄³⁻ has a charge of 3−, the highest among the common polyatomic ions.
Part 5 of 5Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Cation A positively charged ion.
- Anion A negatively charged ion.
- Isoelectronic Having the same number of electrons and the same electron configuration.
- Polyatomic ion A charged group of covalently bonded atoms, such as SO₄²⁻.
Common Mistakes
- Removing 3d electrons before 4s electrons when forming transition metal ions.
- Writing the ion with the wrong sign, for example Cl⁺ instead of Cl⁻.
- Thinking that the charge of an ion changes the number of protons.
- Saying an atom 'wants' electrons; ions form because the resulting arrangement has lower energy.
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