Periodic Table

The ionic radii generally increase down a group in the periodic table. However, for the lanthanide series, there are some considerations:

1. : It has the smallest ionic radius among these ions due to its position in the periodic table and the high effective nuclear charge.
2. : Although it is at the end of the lanthanide series, the ionic radius is relatively small due to the increase in nuclear charge.
3. : This ion has a larger radius than Lu3+ and Y3+ because it is located above in the lanthanide series.
4. : This ion has the largest ionic radius among the listed ions.

Therefore, the correct increasing order of ionic radii is:

• Nitrogen: Exists primarily as a diatomic molecule (N₂) and does not exhibit other allotropic forms under normal conditions.
• Phosphorus: Shows allotropy with forms such as white, red, and black phosphorus.
• Arsenic: Exhibits allotropy with gray, yellow, and black forms.
• Antimony: Shows allotropy with metallic and nonmetallic forms.