An element X has a valency 3. Write the formula of its oxide?

Hint: The word valency defines an element’s capacity to undergo combination with other elements. For example, Chlorine possesses 7 valence electrons in it. To complete an octet requires only one electron. So, its valency is 1.

Complete Step by Step Solution:
Let's first understand oxide ions. The Oxygen element has an atomic number of 8. So, its electronic configuration is 2,6. To complete an octet, an Oxygen atom requires two more electrons. So, the Oxygen atoms form an oxide ion whose valency is 2. The oxide ion is written chemically as \[{{\rm{O}}_{\rm{2}}}^{2 - }\] . So, the valency of the oxide ion is 2.

Now, we will discuss how to write the formula of a compound. To know the formula, we first need to write the chemical symbols of the elements of the compound. For example, in the aluminium chloride compound, two elements aluminium and Chlorine present.

So, chemical symbols of the elements are\[{\rm{Al}}\] and Cl.
Now, we have to write the valencies of the elements of the compound.
Al → +3
Cl → -1
Now, we have to interchange the valencies of each other and write the formula of the compound. So, the formula of Aluminum chloride compound is \[{\rm{AlC}}{{\rm{l}}_{\rm{3}}}\] .
Here, element X has a valency of 3 and the valency of the Oxide ion is -2.
So,
X → 3
O → 2
Hence, the given compound has the formula of \[{{\rm{X}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}\].

Note: It is to be noted that valency is not the same as valence electrons. Valence electrons represent those electrons that occupy an atom's valence shell. And valency is an atom's combining capacity.