Question

Why is MgO a Lewis base?

Answer 1

Let us first understand that a Lewis base is an alkali that has a pair of non-bonding electrons available for donation. This base can donate the pair of nonbonding electrons to a receiver molecule. MgO is an ionic compound formed when Mg donates two electrons to Oxygen.

Complete answer:
In MgO, Magnesium loses the two electrons to form a stable configuration whereas oxygen gains those 2 electrons to complete its octet. Thus, an Ionic compound MgO is formed.
MgO is the oxide of Magnesium. Magnesium is an alkaline earth metal placed in the second group of the periodic table. We know the property of Alkaline earth metals which states that the oxides of metals form alkalis i.e. bases.
In MgO, ${{O}^{-2}}$ has 3 nonbonding electron pairs. When ${{H}_{2}}O$ is added to MgO, the ${{O}^{-2}}$ donates a pair of nonbonding electron pair to ${{H}^{+}}$ and it forms $O{{H}^{-}}$ . Thus it results in the ionization of ${{H}_{2}}O$ .
Let us see that in a reaction form.
Water forms two ions ${{H}^{+}}$ and $O{{H}^{-}}$ .
${{H}_{2}}O\text{ }\text{ }{{\text{H}}^{+}}\text{ + O}{{\text{H}}^{-}}$
This ${{H}^{+}}$ is converted to $O{{H}^{-}}$ by the donation of an electron pair by MgO.
${{O}^{-2}}\text{ + }{{\text{H}}^{+}}\text{ }\to \text{ O}{{\text{H}}^{-}}$
The formation of $O{{H}^{-}}$ indicates that MgO has donated a nonbonding electron pair. Hence, it acts as a Lewis base.

Note:
The Lewis theory states that a Lewis base is formed when a pair of nonbonding electrons are donated. Similarly, a Lewis acid is formed when the pair of nonbonding electrons are accepted by an ion or molecule. But remember that the compounds of metals, mostly oxides, are basic in nature.