9701 · 3.7
Dot-and-cross diagrams
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).
Need to know
What you need to know
- 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.
Explanation
Electron Accounting for Bonds
- Only show outer shell (valence) electrons. Use dots for one atom and crosses for another to track their origin.
- For ionic bonds, show the transfer of electrons. Place ions in square brackets with the charge shown top-right.
- 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.
- 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.