Variable valency is exhibited by

Options

A. Sr
B. Be
C. Al
D. Cu

Answer

To solve the question regarding which element exhibits variable valency, we will analyze each option provided:

1. **Understanding Valency**:
- Valency refers to the ability of an atom to bond with other atoms, which is determined by the number of electrons in its outer shell.
- Variable valency means that an element can exhibit more than one valency or oxidation state in its compounds.

2. **Analyzing the Options**:
- **Option A: Strontium (Sr)**:
- Strontium is an S-block element with the electronic configuration of [Kr]5s².
- It typically exhibits a valency of +2, as it loses its two outermost electrons. Therefore, it does not show variable valency.

- **Option B: Beryllium (Be)**:
- Beryllium is also an S-block element with the electronic configuration of [He]2s².
- Similar to strontium, it usually exhibits a +2 oxidation state and does not show variable valency.

- **Option C: Aluminium (Al)**:
- Aluminium is a P-block element with the electronic configuration of [Ne]3s²3p¹.
- It typically exhibits a +3 oxidation state, but it can also show +1 in some compounds. However, it is not known for a wide range of variable valencies.

- **Option D: Copper (Cu)**:
- Copper is a D-block element with the electronic configuration of [Ar]3d¹⁰4s¹.
- Copper exhibits variable valency, commonly +1 and +2, and can show even higher oxidation states in some compounds (e.g., +3, +4). This is due to the involvement of d-orbitals in bonding, which allows for multiple oxidation states.

3. **Conclusion**:
- Based on the analysis, the element that exhibits variable valency is **Copper (Cu)**.

**Final Answer**: Variable valency is exhibited by **Copper (Cu)**.