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

The species listed are isoelectronic, meaning they have the same number of electrons (10 electrons). The size of isoelectronic species is determined by the nuclear charge (number of protons). With a higher nuclear charge (number of protons), the electrons are pulled more tightly towards the nucleus resulting in a smaller size.
• Na+: Sodium has 11 protons, 10 electrons. It has the highest nuclear charge among these species and therefore the smallest size.
• Ne: Neon has 10 protons and 10 electrons, and is neutral.
• F−: Fluorine has 9 protons and 10 electrons, resulting in a larger size than Na+ or Ne because of less nuclear charge and also because it is an anion, with added electron repulsions.
• O2−: Oxygen has 8 protons and 10 electrons. It has the smallest nuclear charge among the isoelectronic species and is also a doubly charged anion which causes strong electron repulsions leading to the largest size.
Thus, the order of increasing size is Na+<Ne<F− <O2−