Which of the two have almost similar size

Options

A. `_(22)Ti` and `_(40)Zr`
B. `_(41)Nb` and `_(73)Ta`
C. `_(39)Y` and `_(57)La`
D. `_(20)Ca` and `_(31)Ir`

Answer

To determine which two elements have almost similar sizes, we need to understand the concept of lanthanoid contraction and its effects on atomic sizes in the d-block elements.

### Step-by-Step Solution:

1. **Understanding Lanthanoid Contraction**:
- Lanthanoid contraction refers to the gradual decrease in the size of the lanthanide series elements (atomic numbers 57 to 71) as you move from lanthanum (La) to lutetium (Lu). This occurs due to the poor shielding effect of the 4f electrons.

2. **Effect on Atomic Size**:
- As we move across the lanthanide series, electrons are added to the 4f orbitals. Although the nuclear charge increases, the shielding effect provided by the 4f electrons is not very effective. This results in the outer electrons (in the 6s orbital) experiencing a stronger effective nuclear charge, leading to a decrease in atomic radii.

3. **Transition Series Elements**:
- The elements in the d-block are divided into transition series. The first transition series includes elements from Scandium (Sc, atomic number 21) to Zinc (Zn, atomic number 30). The second transition series includes elements from Yttrium (Y, atomic number 39) to Cadmium (Cd, atomic number 48). The third transition series includes elements from Hafnium (Hf, atomic number 72) to Mercury (Hg, atomic number 80).

4. **Identifying Similar Sizes**:
- Due to lanthanoid contraction, the sizes of elements in the second transition series and the third transition series become similar. Specifically, elements like Niobium (Nb, atomic number 41) from the second transition series and Tantalum (Ta, atomic number 73) from the third transition series exhibit this similarity in size.

5. **Conclusion**:
- Therefore, the two elements that have almost similar sizes are Niobium (Nb) and Tantalum (Ta).

### Final Answer:
Niobium (Nb) and Tantalum (Ta) have almost similar sizes due to lanthanoid contraction.