1.6.4Principles of Chemistry
Ionic Bonds- Dot and Cross Diagrams — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 38 of 50
Part 1 of 6Drawing Conventions
Dot-and-cross diagrams show where each outer electron ends up after a transfer.
Two electron colours
- Use solid dots for electrons from one atom and crosses for electrons from the other, so the origin of each electron is clear.
Outer shell only
- Show only the outer-shell electrons, because inner shells take no part in forming the ionic bond.
Square brackets
- Enclose each ion in square brackets and place its charge as a superscript outside the brackets at the top right.
Full outer shell
- Most ions in these diagrams show eight outer electrons, matching the noble gas that the ion resembles.
Larger compounds
- When more than two atoms are involved, hollow circles or other symbols can distinguish each atom's electrons.
Part 2 of 6Sodium Chloride
1
Sodium has one outer electron, and it transfers that electron to a chlorine atom, which has seven outer electrons.
2
The sodium atom becomes a sodium ion with a 1+ charge and a full outer shell of eight electrons.
3
The chlorine atom gains the electron and becomes a chloride ion with a 1− charge and eight outer electrons.
4
The oppositely charged ions are attracted together, and one sodium ion balances one chloride ion, giving the formula NaCl.
Part 3 of 6Other Examples
Other Examples
Magnesium oxide
- Magnesium gives two electrons, both moving to an oxygen atom with six outer electrons, forming Mg²⁺ and O²⁻ in a 1:1 ratio.
Calcium chloride
- Calcium gives two electrons, one to each of two chlorine atoms, forming Ca²⁺ and two Cl⁻ ions, so the formula is CaCl₂.
Aluminium oxide
- Each aluminium gives three electrons and each oxygen takes two, so six electrons move to form 2Al³⁺ and 3O²⁻, giving Al₂O₃.
Lithium fluoride
- Lithium loses one electron to fluorine, forming Li⁺ and F⁻ in a 1:1 ratio, so the formula is LiF.
Part 4 of 6Electron Balance
Electron Balance
Metal atoms
- Metal atoms lose electrons to form positive ions, and the number of electrons lost equals the positive charge.
Non-metal atoms
- Non-metal atoms gain electrons to form negative ions, and the number gained equals the negative charge.
Overall balance
- The total number of electrons lost by the metal atoms must equal the total number gained by the non-metal atoms.
Part 5 of 6Worked Example
Question Draw a dot-and-cross diagram for calcium oxide and state its formula.
- Calcium is in Group 2, so each calcium atom has two outer electrons and forms Ca²⁺.
- Oxygen is in Group 6, with six outer electrons, and gains two electrons to form O²⁻.
- Both electrons from one calcium atom move to one oxygen atom, so one ion of each type forms.
- Show Ca²⁺ and O²⁻ in square brackets, with the eight outer electrons on the oxide ion and the charges outside the brackets.
Answer
- Ca²⁺ and O²⁻ ions in a 1:1 ratio;
- formula CaO.
Part 6 of 6Exam Checklist & Tips
1.6.4 – Key Points for the Exam
- Dot-and-cross diagram A diagram that shows the outer electrons of each atom or ion, using dots and crosses to show their origin.
- Electron transfer The movement of one or more electrons from a metal atom to a non-metal atom.
- Ionic bond The electrostatic attraction between oppositely charged ions formed by electron transfer.
- Cation A positively charged ion, formed when an atom loses electrons.
- Anion A negatively charged ion, formed when an atom gains electrons.
Common Mistakes
- Drawing the transferred electron as a shared pair, which describes covalent bonding rather than complete transfer.
- Writing the charge inside the square brackets instead of as a superscript outside them.
- Drawing the inner electron shells, which is unnecessary for exam dot-and-cross diagrams.
- Giving a wrong formula ratio, such as NaCl₂, when one sodium ion balances one chloride ion.
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