Final answer:
The size of a species in each pair (atom vs ion) is determined by its electronic configuration and the effective nuclear charge. For each pair, the atom or ion with more electrons will generally be larger due to increased electron-electron repulsion. Therefore, Li, I-, Cr, and O2- are the larger species in their respective pairs.
Step-by-step explanation:
The student is asking which species is larger in each given pair. The size of an ion or atom in these pairs is determined by the number of electrons and the nuclear charge. When an atom loses an electron to form a cation, it gets smaller due to the reduced electron-electron repulsion and increased effective nuclear charge. Conversely, when an atom gains an electron to form an anion, it gets larger because of the added electron-electron repulsion and decreased effective nuclear charge.
- Li or Li⁺: Li is larger because Li⁺ has lost an electron, and the effective nuclear charge pulling on the remaining electrons is greater.
- I⁻ or Cs⁺: I⁻ is larger because it has gained an electron, resulting in a greater electron-electron repulsion than the Cs⁺ cation which has lost an electron.
- Cr or Cr³⁺: Cr is larger because losing three electrons to form Cr³⁺ increases the effective nuclear charge, leading to a smaller ionic radius.
- O or O²⁻: O²⁻ is larger because it has gained two electrons, thereby increasing electron-electron repulsion and decreasing the effective nuclear charge per electron.