Which of the following best helps to account for the fact that the F- ion is smaller than the O2- ion?

Question

Which of the following best helps to account for the fact that the F- ion is smaller than the O2- ion?

BlackTom AI · Accepted Answer

The question asks us to explain why F− is smaller than O2−. 
Option: 'F− has a larger nuclear charge than O2− has.' This statement is key to understanding the size difference. When comparing ions that have the same number of electrons, the one with more protons (and thus a higher nuclear charge) exerts a stronger attractive force on the electron cloud. 
For F−, the nucleus has 9 protons, while O2− has 8 protons. Since both ions effectively contain 10 electrons, the greater positive charge in F− increases the effective nuclear charge (Zeff) experienced by the electrons. This stronger pull draws the electron cloud closer to the nucleus, resulting in a smaller ionic radius for F−. 
Conversely, in O2− the lower nuclear charge means the electrons experience a weaker attraction to the nucleus, allowing the electron cloud to spread out more and giving a larger radius. 
In short, the key mechanism is the difference in effective nuclear charge: F− (higher Zeff) pulls electrons in more tightly than O2− (lower Zeff), accounting for the smaller size of F−. 
Note: While factors such as shielding and electron–electron repulsion influence radii, the core comparison here rests on the proportional relationship between nuclear charge and electron cloud contraction when the electron count is the same.

AP Chemistry A Unit 1 quiz

Which of the following best helps to account for the fact that the F- ion is smaller than the O2- ion?

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