Introduction:
Copper is a chemical element with the symbol Cu and atomic number 29. It is a transition metal that belongs to Group 11 on the periodic table. One of the unique properties of copper is its variable valency, which means it can exhibit multiple oxidation states or valence numbers.

Explanation:
Copper has variable valency due to the presence of different energy levels or orbitals in its electronic structure. The electronic configuration of copper is [Ar] 3d^10 4s^1. Let's understand the reasons behind its variable valency:

1. Electronic Configuration:
Copper has one electron in its outermost 4s orbital and ten electrons in its 3d orbitals. The 4s orbital has a lower energy level compared to the 3d orbitals. During chemical reactions, copper can easily lose the 4s electron to achieve a stable configuration with a completely filled 3d orbital (3d^10). This results in a valency of +2.

2. Exceptional Stability:
The completely filled 3d orbital in copper provides exceptional stability, making it difficult for copper to lose additional electrons and form a +3 valency. The energy required to remove an electron from the 3d orbital is significantly higher than removing it from the 4s orbital. Therefore, copper predominantly exhibits a valency of +2.

3. Copper(I) Compounds:
Copper can also exhibit a valency of +1 in some compounds, known as copper(I) compounds. In these compounds, copper loses both the 4s and one of the 3d electrons, resulting in a partially filled 3d orbital (3d^9). However, copper(I) compounds are relatively unstable and tend to oxidize quickly in the presence of air or moisture.

4. Factors Influencing Valency:
The valency of copper can also be influenced by other factors such as the nature of the reacting species, ligands, and the overall redox potential of the reaction. These factors can determine whether copper will exhibit a valency of +1 or +2 in a particular chemical reaction.

Conclusion:
In conclusion, copper has variable valency due to its electronic configuration, exceptional stability of the completely filled 3d orbital, and the ability to lose either one or two electrons from its outermost energy levels. The valency of copper can be +1 or +2, depending on the specific compound and reaction conditions. Understanding the variable valency of copper is essential in predicting its chemical behavior and reactions.