Representing Ionic Bonding (Edexcel A Level Chemistry): Revision Note
Ionic Dot-and-Cross Diagrams
Dot and cross diagrams are diagrams that show the arrangement of the outer-shell electrons in an ionic or covalent compound or element
The electrons are shown as dots and crosses
In a dot and cross diagram:
Only the outer electrons are shown
The charge of the ion is spread evenly which is shown by using brackets
The charge on each ion is written at the top right-hand corner
Ionic compounds
Ionic bonds are formed when metal atoms transfer electrons to a non-metal to form a positively charged and negatively charged ion
The atoms achieve a noble gas configuration
Dot-and-cross diagrams of ionic compounds in which one of the atoms transfers their valence electrons to the other
Calcium fluoride
Calcium is a Group 2 metal
It loses its 2 outer electrons to form a calcium ion with a +2 charge (Ca2+)
Fluorine is a Group 7 non-metal
It gains 1 electron to form a fluoride ion with a -1 charge (F-)
As before, the positive and negative ions are attracted to each other via an ionic bond
However, to cancel out the 2+ charge of the calcium ion, 2 fluorine atoms are needed
Each fluorine atom can only accept 1 electron from the calcium atom
2 fluoride ions will be formed
Calcium fluoride is made when 1 calcium ion and 2 fluoride ions form ionic bonds, CaF2
The final ionic solid of CaF2 is neutral in charge
Worked Example
Draw a dot cross diagram for lithium nitride
Answer
Lithium is a Group 1 metal
It loses its outer electron to form a lithium ion with a +1 charge (Li+)
Nitrogen is a Group 5 non-metal
It gains 3 electrons to form a nitride ion with a -3 charge (N3-)
To cancel out the -3 charge of the nitride ion, 3 lithium atoms are needed and 3 lithium ions will be formed
Lithium nitride is made when 1 nitride ion and 3 lithium ions form ionic bonds
The final ionic solid of Li3N is neutral in charge
Dot and cross diagram to show the ionic bonding in lithium nitride
Worked Example
Draw a dot cross diagram for aluminium oxide
Answer
Aluminium is a Group 3 metal
It loses its outer electrons to form an aluminium ion with a +3 charge (Al3+)
Oxygen is a Group 6 non-metal
It gains 2 electrons to form an oxide ion with a -2 charge (O2-)
To cancel out the negative and positive charges, 2 aluminium and 3 oxygen atoms are needed
Aluminium oxide is made when 2 aluminium ions and 3 oxygen ions form ionic bonds
The final ionic solid of Al2O3 is neutral in charge
Dot and cross diagram to show the ionic bonding in aluminium oxide
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