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
