What is variable? Give two examples of elements showing variable valency?

Hint: As we know that in chemistry, periodic tables play a vital role. In the periodic table there are totally \[118\] elements. In the periodic table there are totally \[18\] columns and \[7\] rows. The columns are called groups. Hence, \[18\] groups in the periodic table. The rows are called periods. Hence, totally \[7\] period in the table.

Complete answer:
We have to know that the valency is nothing but the number of electrons that donate or accept the atom to attain the stable electronic configuration. The stable configuration means the elements donate or accept electrons to attain the noble gas or inert gas electron configuration.
In the periodic table, some elements have only one valency. Some elements have more than one valency.
The two examples of elements showing variable valency are given below.
Copper is one of the elements in the periodic table showing variable valency. The symbol of copper is\[Cu\]. The variable valency of copper is given below,
\[C{u^{ + 1}}\] and \[C{u^{ + 2}}\].
Iron is one of the elements in the periodic table showing variable valency. The symbol of Iron is \[Fe\]. The variable valency of Iron is given below,
\[F{e^{ + 3}}\] and \[F{e^{ + 2}}\].

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. The mass number of the atom is nothing but the sum of the number of protons and number of neutrons.