2.1.1Additional lessons
Forming Ions — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 19 of 98
Part 1 of 6Electron Transfer
Ionic bonding starts with the transfer of electrons from a metal to a non-metal.
1
A metal atom has few outer-shell electrons and a low ionisation energy, so it loses them readily to reach a stable electron arrangement.
2
Losing electrons leaves more protons than electrons, so the metal atom becomes a positively charged cation.
3
A non-metal atom has a strong attraction for electrons and gains them to complete its outer shell.
4
Gaining electrons leaves more electrons than protons, so the non-metal atom becomes a negatively charged anion.
5
The oppositely charged ions then attract each other strongly, and this electrostatic attraction is the ionic bond.
Part 2 of 6Predicting Ion Charges
Group 1
- These metals each lose one electron, giving ions with a 1+ charge such as Li⁺, Na⁺ and K⁺.
Group 2
- These metals lose two electrons, so ions such as Mg²⁺ and Ca²⁺ carry a 2+ charge.
Group 13
- Aluminium loses three electrons to form Al³⁺, the usual ion of this group in IB courses.
Group 15
- Nitrogen and phosphorus gain three electrons, forming nitride N³⁻ and phosphide P³⁻ ions.
Group 16
- Oxygen and sulfur gain two electrons to form oxide O²⁻ and sulfide S²⁻ ions.
Group 17
- Halogens gain one electron each, producing fluoride, chloride, bromide and iodide ions with a 1− charge.
| Atom | Electrons transferred | Ion formed | Configuration |
|---|---|---|---|
| Na | lose 1 | Na⁺ | 2,8 (as neon) |
| Mg | lose 2 | Mg²⁺ | 2,8 (as neon) |
| Cl | gain 1 | Cl⁻ | 2,8,8 (as argon) |
| O | gain 2 | O²⁻ | 2,8 (as neon) |
Part 3 of 6Isoelectronic Ions and Naming
Isoelectronic Ions and Naming
Noble gas arrangement
- Most ions formed by main-group elements have the same electron configuration as the nearest noble gas atom.
Naming cations
- A metal cation keeps the name of its element, for example sodium ion and calcium ion.
Naming anions
- A simple non-metal anion changes the ending of the element name to -ide, such as chloride and oxide.
Equation form
- Formation is shown with electrons as a species, for example Mg → Mg²⁺ + 2e⁻ and Cl + e⁻ → Cl⁻.
Part 4 of 6Polyatomic Ions
Ammonium
- NH₄⁺ is a positive ion containing several atoms joined by covalent bonds, with an overall charge of 1+.
Hydroxide and nitrate
- OH⁻ and NO₃⁻ are common polyatomic anions that each carry a single negative charge.
Carbonate and sulfate
- CO₃²⁻ and SO₄²⁻ carry a 2− charge, so they combine with ions such as Na⁺ or Ca²⁺ in a fixed ratio.
Phosphate
- PO₄³⁻ has a 3− charge and is one of the polyatomic ions you should be able to recall.
Part 5 of 6Worked Example
Question Write the ion formed when a magnesium atom (Z = 12) reacts with oxygen, and give its electron configuration.
- Magnesium has configuration 2,8,2, so it has two outer electrons.
- It loses both to reach a full second shell: Mg → Mg²⁺ + 2e⁻.
- Mg²⁺ has 10 electrons, giving configuration 2,8, the same as neon.
Answer Mg²⁺ with configuration 2,8 (isoelectronic with neon).
Part 6 of 6Exam Checklist & Tips
2.1.1 – Key Points for the Exam
- Cation A positively charged ion formed when an atom loses one or more electrons.
- Anion A negatively charged ion formed when an atom gains one or more electrons.
- Ionic bond The electrostatic attraction between oppositely charged ions.
- Polyatomic ion A charged group of covalently bonded atoms, such as NH₄⁺ or SO₄²⁻.
- Isoelectronic Having the same number of electrons, such as Na⁺ and Ne.
Common Mistakes
- Writing the charge in the wrong form, for example Mg2+ or Mg⁺² instead of Mg²⁺.
- Saying that an anion loses electrons; anions are formed by gaining electrons.
- Forgetting that the nucleus does not change, so the ion has the same protons as the atom.
- Believing that atoms transfer electrons simply because they want a full shell;
- the transfer happens because the resulting ionic lattice is lower in energy.
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