Define valency. Give one example.

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 as groups. Hence, \[18\] groups in the periodic table. The rows are called periods. Hence, totally \[7\] period in the table. Copper is one of the metallic elements in the periodic table. It is present in the first transition series. Each element in the periodic table has a certain group and period. In that case copper is one of the elements in the period. The atomic number of copper is \[29\] and the symbol of copper is \[{\text{Cu}}\]. The mass number of copper is \[63.5\].

Complete answer:
We need to remember that the total number of valence electrons is dependent on the valence electrons in the atoms in the molecules or ions.
The total number of valence electrons is equal to the sum of the valence electrons in the atoms in the molecules or ions. The general electronic configuration of copper in ground state is\[1{s^2},2{s^2}2{p^6}3{s^2}3{p^6}3{d^9}4{s^2}\]. Because of stability we can change the electronic configuration of copper is \[1{s^2},2{s^2}2{p^6}3{s^2}3{p^6}3{d^{10}}4{s^1}\].
Copper has two oxidation states. There are \[C{u^{ + 2}}\] and \[C{u^{ + 1}}\]. The stability wise \[C{u^{ + 2}}\] is more stable than \[C{u^{ + 1}}\]. The molecular formula of cupric oxide is \[CuO\]. The valency of \[Cu\] in \[C{u_2}O\] is \[1\]. Because the oxidation of copper in cupric oxide is \[C{u^{ + 1}}\].

Note:
We have to know that the atomic number of the element is nothing but the number of electrons or number of protons. The mass number of the atom is nothing but the sum of the number of protons and the number of neutrons. Copper is one of the transition metals in the period. The nature of the copper is metallic solid. In the modern period, copper plays a vital role in industrial development. In our body also copper plays a vital role through haemoglobin in the blood.