Deducing Dot & Cross Diagrams for Ionic Compounds
- Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons
- A positive sodium ion with the charge +1 is formed
- Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell of electrons
- One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom
- A chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of -1
Formula of ionic compound: NaCl
For exam purposes you need only show the outer electrons in dot & cross diagrams.
You should be able to draw dot & cross diagrams for combinations of ions from groups 1,2,3,5,6 and 7.
Magnesium Oxide Dot & Cross Diagram
- Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons
- A positive ion with the charge +2 is formed
- Oxygen is a group 6 non-metal so will need to gain two electrons to have a full outer shell of electrons
- Two electrons will be transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom
- Oxygen atom will gain two electrons to form a negative ion with charge -2
Formula of ionic compound: MgO
Diagram showing the dot-and-cross diagram of magnesium oxide
When writing about ions, we use the notation 1-, 2+ etc. to describe the charge of the ion, with the number first followed by the sign (+/-). It is incorrect to write them the other way around as this refers to the oxidation state, not the charge.