What is meant by the 'lanthanide contraction'? Mention one important fact that can be considered as a consequence of the lanthanide contraction.

Answer

### Step-by-Step Solution:

1. **Definition of Lanthanide Contraction**:
- Lanthanide contraction refers to the gradual decrease in the size of the lanthanide ions as we move from cerium (Ce) with atomic number 58 to lutetium (Lu) with atomic number 71. Despite the increase in atomic number, the ionic radii of the lanthanides do not increase as expected but rather decrease.

2. **Reason for Lanthanide Contraction**:
- As we move across the lanthanide series, the nuclear charge (the positive charge of the nucleus) increases due to the addition of protons. This increased positive charge pulls the electrons closer to the nucleus.
- The general electronic configuration for lanthanides is [Xe] 4f¹-¹⁴ 5d⁰-¹ 6s². The 4f orbitals are more diffused and have poor shielding effect on the outer electrons. This poor shielding means that the outer electrons experience a stronger effective nuclear charge, leading to a decrease in size.

3. **Consequence of Lanthanide Contraction**:
- One important consequence of lanthanide contraction is that the ionic radii of the transition metals in the 4d and 5d series become very similar. For example, zirconium (Zr) from the 4d series and hafnium (Hf) from the 5d series have nearly the same ionic size. Similarly, niobium (Nb) and tantalum (Ta) also exhibit comparable sizes. This similarity in size leads to similar chemical properties among these elements.

### Summary:
- **Lanthanide Contraction**: The decrease in size of lanthanide ions from Ce to Lu due to increased nuclear charge and poor shielding by 4f electrons.
- **Consequence**: Similar ionic sizes and chemical properties of transition metals in the 4d and 5d series (e.g., Zr and Hf, Nb and Ta).