Question

Some elements and their valencies are given.
The chemical formula of calcium oxide is ${\text{CaO}}$. What is the valency of calcium?

Element	Valency
${\text{Ba}}$	$2$
${\text{Cl}}$	$1$
${\text{Zn}}$	$2$
${\text{O}}$	$2$

Answer 1

We also need to know that in chemistry, periodic tables play a vital role. In the periodic table totally $118$ elements. In the periodic table totally $18$ columns and $7$ rows. The columns are called as groups. Hence, $18$ groups in the periodic table. The rows are called periods. Hence, the total $7$ period in the table. Calcium and oxygen are one of the elements in the periodic table. The atomic number of calcium is $20$. The atomic number of oxygen is $8$. The atomic symbol of calcium is ${\text{Ca}}$. The atomic symbol of oxygen is ${\text{O}}$.

Complete answer:
We also remember that the valency is nothing but the number of electrons that donate or accept the atom to attain the stable electronic configuration.
The atomic number of oxygen is $8$. The valency of the oxygen is $2$. The stable electronics of the second period configuration is $10$. That’s the reason, oxygen accepts the two electrons to attain the stable electronic configuration.
The atomic number of calcium is $20$. The valency of the calcium is $2$. The stable electronics of the third period configuration is $18$. That’s the reason, calcium donates the two electrons to attain the stable electronic configuration.
Oxygen accepts the two electrons from calcium, in this way both the atoms attain the stability.
Hence, the valency of the calcium is $2$.

Note:
We have to know that the atomic number of the atom is equal to the number of protons or the number of electrons. In every atom the number of protons and the number of electrons are equal. The stable electronic configuration in the second period is $10$. The stable electronic configuration in the third period is $18$. The oxygen is present in the second period in the periodic table. Calcium is present in the third period in the periodic table.