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1.6Principles of Chemistry

Ionic Bonding — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 7 parts Lesson 14 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Ionic Bonding: Ions, Formulae, Dot-and-Cross Diagrams and Properties — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Explain how ions form, deduce ionic formulae, draw dot-and-cross diagrams and explain the properties of ionic compounds.An ion is an electrically charged atom or group of…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Giant lattice
  • Swap-and-drop

Part 1 of 5How ions form

An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons, so that the atom obtains a full outer shell.

Cations
Metals lose electrons to form positive ions (cations, more protons than electrons); non-metals gain electrons to form negative ions (anions).
Groups 1, 2, 3, 5, 6 and 7 form ions with the same electronic structure as a noble gas.
The charge equals the number of electrons lost or gained: Mg loses 2 to form Mg2+; Br gains 1 to form Br-.
To deduce a charge, find the outer electrons; atoms with fewer than four usually give electrons away, those with more than four usually gain.
Positive ionsNegative ions
Group 1: 1+ (Na+); Group 2: 2+ (Mg2+); Group 3: 3+ (Al3+)Group 5: 3- (N3-); Group 6: 2- (O2-); Group 7: 1- (Cl-)
Ag+, Cu2+, Fe2+, Fe3+, Pb2+, Zn2+, H+, NH4+OH-, NO3-, CO32-, SO42-

Part 2 of 5Formulae of ionic compounds

An ionic compound has no overall charge, so the positive and negative charges must cancel.

1
Direct comparison: iron(II) sulfate, Fe2+ and SO42-, one of each, FeSO4.
2
Swap-and-drop: copper(II) chloride, Cu2+ and Cl-; swap the charges to give CuCl2.
3
Iron(III) chloride is FeCl3.
4
Use brackets for complex ions: magnesium hydroxide Mg(OH)2, aluminium sulfate Al2(SO4)3.
5
If the charges are the same size, simplify to the simplest ratio (Mg2+ and O2- give MgO, not Mg2O2).

Part 3 of 5Dot-and-cross diagrams

Show only outer electrons, use dots for one atom and crosses for the other, put square brackets round each ion and the charge outside at the top right.

Dot-and-cross diagrams
Sodium chloride: sodium (2,8,1) transfers its outer electron to chlorine (2,8,7). Na+ is 2,8 and Cl- is 2,8,8.
Magnesium oxide: magnesium (2,8,2) transfers two electrons to oxygen (2,6) giving Mg2+ (2,8) and O2- (2,8).
Magnesium chloride: Mg transfers one electron to each of two chlorine atoms, giving Mg2+ and two Cl-.

Checking a diagram

  • count the electrons in each bracketed ion and make sure they match the noble gas configuration (2,8 or 2,8,8), that the charge equals the electrons lost or gained, and that the total number of electrons on the diagram equals the number in the original atoms.
  • For example in sodium chloride the sodium ion shows eight electrons in the second shell and the chloride ion eight in the third shell, giving a charge of 1+ and 1- respectively.

Part 4 of 5Structure and properties

An ionic compound is a giant lattice of alternating positive and negative ions with strong electrostatic forces of attraction in all directions. Therefore:

PropertyExplanation
High melting and boiling pointsMuch energy is needed to overcome the strong forces; greater ion charges give stronger forces and higher melting points (MgO higher than NaCl)
Does not conduct when solidThe ions are in fixed positions in the lattice
Conducts when molten or dissolvedThe ions are free to move and carry the charge (not electrons)

Part 5 of 5Worked examples

  1. Draw or describe the dot-and-cross diagram for magnesium oxide. [3]3 marks

  2. Give the formulae of calcium nitrate and aluminium oxide. [2]2 marks

  3. Why does solid sodium chloride not conduct but its solution does? [2]2 marks

  4. Explain why MgO has a higher melting point than NaCl. [2]2 marks

Key terms

TermMeaning
CationPositive ion.
AnionNegative ion.
Ionic bondElectrostatic attraction between oppositely charged ions.
Giant latticeRegular 3D arrangement of ions.
Swap-and-dropMethod to find a formula from ion charges.

Avoid these mistakes

1
Saying electrons carry the current in molten salts.
2
Forgetting square brackets and charges on ions.
3
Using the wrong ion charge (Fe2+ versus Fe3+).
4
Showing all inner shells in dot-and-cross diagrams.

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