The question asks for the correct order of atomic or ionic radii.

Atomic and ionic radii depend on the number of electron shells and the effective nuclear charge. For isoelectronic species, the radius decreases with increasing nuclear charge. For elements in the same period, atomic radius generally decreases from left to right. For elements in the same group, atomic radius generally increases down the group. Cations are smaller than their parent atoms, and anions are larger than their parent atoms. For ions of the same element, a higher positive charge leads to a smaller radius, and a higher negative charge leads to a larger radius.

Option (B) compares the radii of isoelectronic anions: F^-, O^2-, and N^3-. All three ions have 10 electrons (like Neon). For isoelectronic species, the ionic radius decreases as the nuclear charge (atomic number) increases because the electrons are pulled more strongly towards the nucleus. The atomic numbers are: N (7), O (8), F (9). Therefore, the order of nuclear charge is N < O < F. Consequently, the order of ionic radii will be F^- < O^2- < N^3-.

• A) N < Be < B: This order is incorrect. Across a period, atomic radius generally decreases from left to right. B (atomic number 5) is to the right of Be (atomic number 4), so Be > B. N (atomic number 7) is further to the right, so B > N. The correct order would be N < B < Be.
• C) Na < Li < K: This order is incorrect. Within a group, atomic radius increases down the group due to the addition of new electron shells. Li is in period 2, Na in period 3, and K in period 4. Therefore, the correct order of atomic radii is Li < Na < K.
• D) Fe³⁺ < Fe²⁺ < Fe⁺⁴: This order is incorrect. For ions of the same element, a higher positive charge means fewer electrons and a greater effective nuclear charge per electron, leading to a smaller ionic radius. Therefore, Fe³⁺ should be smaller than Fe²⁺. The Fe⁺⁴ ion is not commonly formed and would be even smaller than Fe³⁺. The correct order for the given ions would be Fe⁺⁴ < Fe³⁺ < Fe²⁺.

Correct Answer: (B)