Final answer:
When electrons are lost to form positive ions (cations), the ionic radius becomes smaller than the neutral atom because of decreased electron repulsion and increased effective nuclear charge. Conversely, when electrons are gained to form negative ions (anions), the ionic radius is larger than the neutral atom.
Step-by-step explanation:
The correct statement about the trend for ionic radii is: b. When electrons are lost forming positive ions, the ionic radius is smaller than its corresponding neutral atom. This occurs because the loss of electrons results in a decrease in electron-electron repulsion within the same principal shell and an increase in the effective nuclear charge experienced by the remaining electrons. Consequently, the remaining electrons are drawn in closer to the nucleus, making the ion smaller than the neutral atom.
For example, the covalent radius of an aluminum atom is 118 pm, whereas the ionic radius of Al³⁰ is significantly smaller at 68 pm. Cations with larger charges are also smaller than those with smaller charges, highlighting the relation between charge and size. Meanwhile, for anions, the added electrons result in larger radii compared to the neutral atom due to increased electron-electron repulsion and a relatively constant nuclear charge.