An ionic bond can be defined as follows:
Ionic Bond – the electrostatic attraction between oppositely charged ions
This type of bond forms between the atoms of metal and non-metal elements. An ion is an atom that has either lost or gained electrons.
As electrons have a negative charge an atom that gains electrons becomes a negative ion (anion). When an atom loses electrons it becomes a positive ion (cation).
Important Note: The net charge of an ionic compound must be zero.
To learn about covalent bonding click here.
The Formation of Ionic Bonds
Electrons are transferred from a metal atom to a non-metal atom. This produces a positively charged metal ion and a negatively charged non-metal ion.
As the metal and non-metal ions are oppositely charged there is an electrostatic attraction between them that forms the ionic bond.
Atoms are more stable when they have a full outer energy level – like all noble gases do. During the formation of ionic bonds the number of electrons lost or gained helps the atoms of each element to become more stable.
For metals this means losing the electrons in their outer energy level (the next full level will become the outer level). For non-metals this means gaining enough electrons to fill their outer energy level.
Let’s look at magnesium oxide (MgO) as an example. Magnesium has the following electronic configuration: 2, 8, 2.
If magnesium loses the 2 outer electrons it will form an Mg2+ ion and have a full outer energy level (making it more stable).
The electronic configuration of oxygen is 2, 6. To have a full outer energy level oxygen needs to gain two electrons. It can gain these electrons by accepting the 2 electrons magnesium loses to form an O2-.
These two ions are oppositely charged and so they can be held together by the strong electrostatic attractions between them.
Sometimes more than one atom from each element is required. Examples of these can be found in the worked examples below.
Dot and Cross Diagrams
Dot and cross diagrams can be used to show how electrons are transferred in the formation of an ionic bond. Usually only the outer shells of the atoms are shown. The electrons of one element are shown as dots and the other elements are shown as crosses (if there are more than two elements another symbol is used).
For the ions formed square brackets are drawn around the shell with the charge written on the outside. The element is usually written in the centre.
The diagram below shows the dot and cross diagram of magnesium oxide.
IMPORTANT: Quite often only the final ions are shown and not the formation.
Giant Ionic Lattices
The compounds formed by the ionic bonding of metals and non-metals are called giant ionic lattices:
Giant Ionic Lattice – a 3D structure composed of oppositely charged ions held together by ionic bonds.
Each ion is surrounded by oppositely charged ions which attract attract each other in all directions. For example, in magnesium oxide multiple magnesium ions are surrounded by oxygen ions and they form a lattice.
The ions form a regular, repeating arrangement and this structure gives them a high melting and boiling point.
For a comparison of ionic and covalent bonding click here.
Worked Examples
Example 1
Draw the dot and cross diagram of calcium oxide (CaO). The electronic configurations are given below:
Ca: 2, 8, 8, 2
O: 2, 6
Example 2
Draw the dot and cross diagram of sodium oxide (Na2O). The electronic configurations are given below:
Na: 2, 8, 1
O: 2, 6
Example 3
Draw the dot and cross diagram for calcium chloride (CaCl2). The electronic configurations are given below:
Ca: 2, 8, 8, 2
Cl: 2, 8, 7
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