Why `Zr` and `Hf` or `Nb` and `Ta` exhibit similar properties?
Or Zirconium (atomic number40) and hafnium number 72) occur together in minerals and they exhibit similar properties. Give reasons.

Answer

To understand why zirconium (Zr) and hafnium (Hf), as well as niobium (Nb) and tantalum (Ta), exhibit similar properties despite being in different periods of the periodic table, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Identify the Elements and Their Positions**:
- Zirconium (Zr) has an atomic number of 40 and is located in the 4th period.
- Hafnium (Hf) has an atomic number of 72 and is located in the 6th period.
- Similarly, niobium (Nb) has an atomic number of 41 and is in the 5th period, while tantalum (Ta) has an atomic number of 73 and is in the 6th period.

2. **Understand the Concept of Atomic Size**:
- As we move down a group in the periodic table, atomic size generally increases due to the addition of electron shells. However, the increase in size is not uniform across periods.

3. **Introduce the Lanthanide Contraction**:
- The lanthanide contraction refers to the phenomenon where the atomic and ionic sizes of the lanthanide series (elements from cerium to lutetium) are smaller than expected. This occurs because the f-orbitals, which are being filled, do not shield the nuclear charge effectively.

4. **Effect of Lanthanide Contraction on Zr and Hf**:
- The presence of 14 lanthanide elements between lanthanum (La, atomic number 57) and hafnium (Hf) leads to a significant decrease in size. This contraction results in hafnium being similar in size to zirconium, despite being in a different period.

5. **Similar Properties Due to Similar Size**:
- When two elements have similar atomic sizes, they tend to exhibit similar chemical and physical properties. This is because size influences factors like ionization energy, electronegativity, and the ability to form similar types of bonds.

6. **Conclusion**:
- Therefore, the similar properties of zirconium and hafnium, as well as niobium and tantalum, can be attributed to the lanthanide contraction, which causes their atomic sizes to be comparable, leading to similar chemical behavior.