Q.Choose the correct order of atomic radii of fluorine and neon (in pm) out of the options given below and justify your answer.

The key idea is that atomic radius depends on the electronic structure and the type of radius measured — for noble gases, the van der Waals radius is used, which is much larger than the covalent radius of a halogen.

• Fluorine forms covalent bonds, so its atomic radius is the covalent radius, about 72 pm. …

The question asks for the order of atomic radii of fluorine (F) and neon (Ne) in picometers. At first glance, you might think that atomic radius decreases across a period, so neon should be smaller than fluorine. But that’s a classic trap — the measured atomic radius of neon is actually larger than that of fluorine. Let’s understand why.

Atomic radius is not a single fixed number; it depends on how we define it. For elements that form bonds (like fluorine), we use the covalent radius — half the distance between two bonded atoms. For noble gases like neon, which don’t form stable bonds, we use the van der Waals radius — half the distance between two non-bonded atoms in a solid or liquid. These two definitions give very different numbers.

Fluorine has a covalent radius of about 72 pm. It’s a small atom because its nuclear charge (9 protons) pulls the 2p electrons strongly inward. Neon, with 10 protons, has an even higher nuclear charge, but its outermost electrons are in a completely filled 2p⁶ shell. That full shell is very stable and resists being squeezed — the electrons are spread out more, and the atom is “softer” in terms of electron cloud. When measured as a van der Waals radius, neon’s radius is about 160 pm, more than double that of fluorine.

So the correct order is: fluorine 72 pm, neon 160 pm.

1. Identify the measurement type. For fluorine, which forms covalent bonds, the standard atomic radius is the covalent radius. For neon, a noble gas, the standard is the van der Waals radius. These are not comparable by the same ruler — but the question expects you to know the typical values. …