In simple terms
A friendly intro before the formal notes — no formulas yet.
Electron Accounting for Bonds
Dot-and-cross diagrams are a simple way to keep track of outer shell electrons when atoms bond. We use them to show whether electrons are transferred (ionic) or shared (covalent).
Imagine two friends who both want a complete set of trading cards. In an ionic 'bond', one friend gives their spare card to the other, and they become oppositely charged 'ions' who are then attracted. In a covalent 'bond', they each put one card into a shared pile that they both count as part of their collection.
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Only show outer shell (valence) electrons. Use dots for one atom and crosses for another to track their origin.
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For ionic bonds, show the transfer of electrons. Place ions in square brackets with the charge shown top-right.
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For covalent bonds, place shared electron pairs in the overlapping region between atoms. For a dative bond, both electrons in the pair come from the same atom.
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For elements in Period 3 or below, the octet can be expanded. Draw diagrams for species like PCl₅ or SF₆ showing more than 8 electrons around the central atom.
Explore the concept
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Step-synced diagram — highlights what to look for in the simulation above.
Only outer shell electrons shown
Only outer shell electrons shown.
Full topic notes
Formal explanation with the rigour you need for the exam.
Representing Ionic Bonding
Ionic bonding involves the complete transfer of one or more electrons from a metal atom to a non-metal atom. The metal atom becomes a positive ion (cation) and the non-metal atom becomes a negative ion (anion). The strong electrostatic attraction between these oppositely charged ions constitutes the ionic bond. The goal for each atom is to achieve a stable electron configuration, usually that of a noble gas.
Draw the atoms' outer shells before transfer.
Show the movement of electrons with an arrow (optional, but good for working).
Draw the resulting ions with their new electron configurations.
Enclose each ion in square brackets.
Write the charge of each ion on the top right-hand corner of the brackets.
Ensure the overall charge of the compound is zero by using the correct ratio of ions.
Representing Covalent Bonding
Covalent bonding involves the sharing of electrons between non-metal atoms. Each atom contributes one or more electrons to form a shared pair, which is attracted to the nuclei of both atoms, holding them together. This sharing allows atoms to achieve a stable outer shell of electrons, typically an octet (8 electrons). Bonds can be single (one shared pair), double (two shared pairs), or triple (three shared pairs).
Dative (Co-ordinate) Covalent Bonds
A dative bond is a special type of covalent bond where one atom provides both of the electrons in the shared pair. The atom donating the electron pair is called the donor and must have a lone pair of electrons. The atom accepting the electron pair is the acceptor and must have a vacant orbital. Common examples include the formation of the ammonium ion (NH₄⁺) from ammonia (NH₃) and a proton (H⁺), and the structure of the aluminium chloride dimer (Al₂Cl₆).
In a displayed formula, a dative bond is sometimes shown with an arrow pointing from the donor to the acceptor atom (e.g., N → H). However, in a dot-and-cross diagram, once formed, it is indistinguishable from a normal covalent bond. The key is to use dots and crosses to show that both electrons in that specific bond originated from the same atom.
Exceptions to the Octet Rule
While the octet rule is a useful guideline, many stable molecules do not obey it. There are two main types of exceptions you need to know. 'Expansion of the octet' occurs in elements from Period 3 onwards, as they have vacant d-orbitals that can accommodate more than eight valence electrons. 'Incomplete octets' occur when the central atom has fewer than eight electrons, common for elements in Group 2 and 13.
Expansion of the Octet: The central atom has more than 8 outer shell electrons. Examples: Phosphorus pentachloride (PCl₅) where P has 10 electrons, and sulfur hexafluoride (SF₆) where S has 12 electrons.
Incomplete Octet: The central atom has fewer than 8 outer shell electrons. Example: Boron trifluoride (BF₃) where B has only 6 electrons.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Draw a dot-and-cross diagram to show the bonding in magnesium chloride, MgCl₂.
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Identify valence electrons: Magnesium (Group 2) has 2 valence electrons. Chlorine (Group 17) has 7 valence electrons.
Draw a dot-and-cross diagram for a molecule of ethene, C₂H₄.
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Valence electrons: Carbon (Group 14) has 4. Hydrogen (Group 1) has 1.
Draw a dot-and-cross diagram for sulfur hexafluoride, SF₆.
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Valence electrons: Sulfur (Group 16) has 6. Fluorine (Group 17) has 7.
How it all connects
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
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Quick check
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Revision flashcards
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What is a dot-and-cross diagram?
A diagram that represents the arrangement of outer (valence) shell electrons in atoms, ions, or molecules. Electrons from different atoms are shown as dots or crosses to indicate their origin.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
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Draw the atoms' outer shells before transfer.
- ✓
Show the movement of electrons with an arrow (optional, but good for working).
- ✓
Draw the resulting ions with their new electron configurations.
- ✓
Enclose each ion in square brackets.
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Write the charge of each ion on the top right-hand corner of the brackets.
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Ensure the overall charge of the compound is zero by using the correct ratio of ions.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Test Your Knowledge on Dot-and-Cross Diagrams
Test Your Knowledge on Dot-and-Cross Diagrams
Extra simulations & links
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Frequently asked
Checkpoint
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