Ionic bonding

From our previous tutorial on chemical bonding, we already know that in bonding, electron can either be shared between atoms (covalent) or transferred from one atom to another (ionic). Ionic or electrovalent bonding occurs when there is more than 1.7 difference in electronegativity between the two atoms.

It usually take place when a metal atom bonds with a non-metal atom. When there is a large difference in electronegativity of two atoms, one atom will attract the shared electron pair much more strongly than the other, causing electrons to be transferred from one atom to the other.

Ionic bond is based on the electrostatic forces between two oppositely-charged ions.

It is formed when  metals which are known to have low electronegativity donates an electron to form a positive ions or cations. The non-metals atom which has high electronegativity, readily received these electrons to form negative ions or anions.

The two or more ions are then attracted to each other by electrostatic forces.

Example 1:

Chemical bonding between sodium and chlorine

In the case of NaCl, the difference in electronegativity is 2.1. Sodium has only one valence electron, while chlorine has seven.

Na = 11

Cl = 17

Looking at the electronic configuration of these atoms, sodium has 1 electron in its outermost shell or valence shell while chlorine has 7.

Remember, the purpose why atoms are combining in the first place is to attain a stable state i.e to have a complete valance electrons.

Because the electronegativity of chlorine is higher than the electronegativity of sodium, chlorine will attract the valence electron in the sodium atom very strongly. This electron from sodium is transferred to chlorine.

Sodium has lost an electron and forms a Na+ ion. Chlorine gains an electron and forms a Cl− ion.

The Equation for the reaction is:

Na + Cl → NaCl

Ionic bonding in molecule of Nacl


Example 2

Let’s take a look at another example of ionic bonding between magnesium (Mg) and oxygen (O) to form magnesium oxide (MgO).

Magnesium has 2 valence electrons and an electronegativity of 1.2, while oxygen has 6 valence electrons and an electronegativity of 3.5.

Since oxygen has a higher electronegativity, it will receive two valence electrons from the magnesium atom and these electrons are transferred from the magnesium atom to the oxygen atom.

Magnesium loses two electrons to form Mg2+, and oxygen gains two electrons to form O2−.

The attractive force between the oppositely charged ions is what holds the molecule together.

The Equation for the reaction is:

2Mg + O2 → 2MgO

Ionic bonding in molecule of MgO


Since  oxygen is a diatomic molecule, 2 magnesium atoms will be required to combine with 2 oxygen atoms to produce 2 molecules of magnesium oxide (MgO).



 1   Explain the difference between a covalent and an ionic bond.

 2   Magnesium and chlorine react to form magnesium chloride.

     (a) What is the difference in electronegativity between these two elements?

     (b) Give the chemical formula for:

  • a magnesium ion
  • a chloride ion

    (c) Write a balanced chemical equation for the reaction that takes place.

3.   Draw Lewis diagrams to represent the following ionic compounds:

       (a)  sodium iodide (NaI)

       (b) calcium bromide (CaBr2)

       (c) potassium chloride (KCl)

click here for covalent bonding

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