Define valency. Give valency to copper and iron?

Valency:

• Valency is defined as the number of electrons lose or gain by an atom to attain noble gas electronic configuration.
• Sometimes because of different conditions under which a chemical reaction takes place, the valency of the same element changes. Thus, metals like lead, tin, and copper exhibit variable valency.
• Elements that exhibit more than one electropositive valency are called to have variable valency.
• Iron and copper have variable valency because they show involvement of vacant d-orbitals and electrons from outermost and penultimate shells are involved in the reaction

Variable valency:-

• Sometimes, because of different conditions under which a chemical reaction takes place, the valency of the same element changes. Thus metals like lead, tin, copper, mercury, iron etc exhibit variable valency.

Valency of copper:

• The atomic number of copper is $29$ and copper has an exception in electronic configuration.
• The electronic configuration of the copper is:$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{10}$.
• Therefore, the valency of copper ions is$1\mathrm{and}2$.

Valency of Iron:

• The atomic number of iron is $26$.
• The electronic configuration of the element is:$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^6$.
• Therefore, the valency of iron is $2\mathrm{and}3$.

Iron and copper have variable valency because they show involvement of vacant d-orbitals and electrons from outermost and penultimate shells are involved in the reaction.