Write the chemical formula for Aluminium Oxide and Magnesium Chloride. For Ca(OH)2, name the radical and state the number of atoms of each element.

Aluminium (Al) has a valency of 3.
Oxygen (O) has a valency of 2.

The valency of Aluminium becomes the subscript for Oxygen.
The valency of Oxygen becomes the subscript for Aluminium.

The chemical formula for Aluminium Oxide is $\boxed{\text{Al}_2\text{O}_3}$.

b) Magnesium Chloride

Magnesium (Mg) has a valency of 2.
Chlorine (Cl) has a valency of 1.

The valency of Magnesium becomes the subscript for Chlorine.
The valency of Chlorine becomes the subscript for Magnesium.

The chemical formula for Magnesium Chloride is $\boxed{\text{MgCl}_2}$.

a) Name the specific radical present in this compound.

The compound is $\text{Ca(OH)}_2$.
The radical present is $\text{OH}$.
This radical is known as the hydroxide radical.
The specific radical present is $\boxed{\text{Hydroxide}}$.

b) Determine the total number of atoms present in $\text{Ca(OH)}_2$.

The formula is $\text{Ca(OH)}_2$.

• Number of Calcium (Ca) atoms = 1
• The subscript 2 outside the parenthesis means there are two $\text{OH}$ groups.
• Number of Oxygen (O) atoms = $1 \times 2 = 2$
• Number of Hydrogen (H) atoms = $1 \times 2 = 2$

Total number of atoms = (Ca atoms) + (O atoms) + (H atoms)
Total number of atoms = $1 + 2 + 2 = 5$

The total number of atoms present in $\text{Ca(OH)}_2$ is $\boxed{5}$.