Question

Complete the table given below for writing the formula of a chemical compound.

Identify the valency	Criss cross the valency	Formula of the compound	Name of the compound
$\mathrm{Al}$ $\mathrm{O}$	$\mathrm{Al}$ $\mathrm{O}$

Answer 1

Part 1: Identify the valency of $\mathrm{Al}$ and $\mathrm{O}$:

• The aluminium atom $\mathbf{\left(}\mathbf{Al}\mathbf{\right)}$ loses three electrons from the outermost shell and forms an aluminium ion with three positive charges $\mathbf{\left(}{\mathbf{Al}}^{\mathbf{3}\mathbf{+}}\mathbf{\right)}$.
• Thus, the valency of $\mathrm{Al}$ is $3$.
• The oxygen atom $\mathbf{\left(}\mathbf{O}\mathbf{\right)}$ gains two electrons in the outermost shell and forms an oxide ion with two negative charges $\mathbf{\left(}{\mathbf{O}}^{2-}\mathbf{\right)}$.
• Thus, the valency of $\mathrm{O}$ is $2$.

Part 2: Criss-cross the valency:

• The valency of $\mathrm{Al}$ becomes the number of $\mathrm{O}$ atoms.
• The valency of $\mathrm{O}$ becomes the number of $\mathrm{Al}$ atoms.

Part 3: Formula of the compound:

• After criss-crossing the valencies, there are two $\mathrm{Al}$ atoms and three $\mathrm{O}$ atoms.
• Thus, the formula of the compound is ${\mathrm{Al}}_2{\mathrm{O}}_3$.

Part 4: Name of the compound:

• While naming the ionic compounds, the name of the cation is written first and then the anion is named.
• Here, the cation is aluminium $\mathbf{\left(}{\mathbf{Al}}^{\mathbf{3}\mathbf{+}}\mathbf{\right)}$ and the anion is oxide $\mathbf{\left(}{\mathbf{O}}^{2-}\mathbf{\right)}$.
• Thus, the name of the compound is Aluminium oxide.

Identify the valency	Criss-cross the valency	Formula of the compound	Name of the compound
$\mathrm{Al}$ $\mathrm{O}$ $3$ $2$	$\mathrm{Al}$ $\mathrm{O}$ $2$ $3$	${\mathrm{Al}}_2{\mathrm{O}}_3$	Aluminium oxide