Dot & Cross Diagrams (AQA A-Level Chemistry): Revision Notes

Dot and cross diagrams are a visual representation of the electrons involved in bonding between atoms in a molecule or compound. They are particularly useful for showing the transfer or sharing of electrons in ionic and covalent bonds.

Example: Water ($H₂O$) In a water molecule:

• Oxygen has 6 valence electrons, needing 2 more to complete its outer shell.
• Each hydrogen atom has 1 valence electron, needing 1 more. The diagram shows two shared pairs of electrons (one between each hydrogen atom and oxygen atom), forming single covalent bonds:

Here, dots represent oxygen's electrons, and crosses represent hydrogen's electrons.

Example: Sodium Chloride (NaCl) In the ionic bond between sodium (Na) and chlorine (Cl):

• Sodium (a Group 1 metal) loses one electron to become $Na⁺$.

• Chlorine (a Group 7 non-metal) gains this electron to complete its outer shell, forming $Cl⁻$. The dot and cross diagram shows:

• The sodium ion ($Na⁺$) with no valence electrons.

• The chloride ion ($Cl⁻$) with a full outer shell of eight electrons.

  

Example: Ammonium Ion ($NH₄⁺$) In ammonium ($NH₄⁺$), nitrogen donates a lone pair of electrons to bond with a hydrogen ion ($H⁺$):

The electrons from the hydrogen atoms are shown as crosses and the electrons from the nitrogen atom are shown as dots. As it is an ion, square brackets are drawn around the diagram with the charge of the ion shown on the outside.

Summary

Dot and cross diagrams provide a clear way to represent the transfer and sharing of electrons in chemical bonding. They illustrate the formation of ionic and covalent bonds and help visualise lone pairs and dative bonds. This technique is essential for understanding the molecular structure and bonding in compounds.