Question

The following species have an equal number of electrons. Arrange them in increasing order of their size:
(A). Ne
(B). Na+
(C). O2−
(D). F−
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D)
• B. (B), (C), (A), (D)
• C. (B), (A), (D), (C)
• D. (D), (C), (A), (B)

Hint:

Cations are smaller than their corresponding neutral atoms, due to the increase in the effective nuclear charge, whereas anions are bigger due to an increase in electron repulsion.

Answer 1

The Correct Option is C

Isoelectronic species, such as those listed, share the same number of electrons (10 electrons). Their size is inversely proportional to the nuclear charge (number of protons); a higher nuclear charge pulls electrons more tightly, resulting in a smaller size.
• Na+: With 11 protons and 10 electrons, sodium has the highest nuclear charge, leading to the smallest size.
• Ne: Neon is neutral, possessing 10 protons and 10 electrons.
• F−: Fluorine has 9 protons and 10 electrons. Its lower nuclear charge and electron repulsions from being an anion contribute to a larger size compared to Na+ or Ne.
• O2−: Oxygen, with 8 protons and 10 electrons, has the lowest nuclear charge. As a doubly charged anion, it experiences strong electron repulsions, resulting in the largest size.
Consequently, the increasing order of size is Na+<Ne<F− <O2−