The lanthanide contraction refers to:

Options

A. radius of the series
B. valence electrons of the series.
C. the density of the series
D. electronegativity of the series.

Explanation

**Step-by-Step Solution:**

1. **Understanding Lanthanide Contraction:**
- The lanthanide contraction refers to the phenomenon observed in the lanthanide series (elements 57-71, from lanthanum to lutetium) where there is a gradual decrease in atomic and ionic sizes as we move from left to right across the series.

2. **Role of 4f Electrons:**
- The contraction is primarily due to the presence of 4f electrons. These electrons are added to the 4f subshell as we move across the lanthanide series.

3. **Poor Shielding Effect:**
- The 4f electrons exhibit poor shielding of the nuclear charge. This means that they do not effectively shield the outer electrons from the attractive force of the nucleus.

4. **Increased Effective Nuclear Charge (Z effective):**
- Due to the poor shielding, the effective nuclear charge (Z effective) experienced by the outer electrons increases. This leads to a stronger attraction between the nucleus and the outer electrons.

5. **Resulting Smaller Atomic Size:**
- As a result of the increased Z effective, the atomic size of the lanthanides decreases more than what would be expected based on periodic trends. This is known as the lanthanide contraction.

6. **Impact on Electronegativity:**
- The smaller atomic size and increased Z effective also lead to an increase in electronegativity across the lanthanide series.

7. **Conclusion:**
- Therefore, the lanthanide contraction refers to the greater than expected decrease in atomic size and the associated increase in electronegativity due to the poor shielding effect of the 4f electrons.

**Final Answer:**
The lanthanide contraction refers to the greater than expected contraction in atomic size of the lanthanoid series due to poor shielding by 4f electrons, leading to an increased effective nuclear charge and increased electronegativity.

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