Chemistry · Periodic Classification Of Properties · NEET
Because they measure two different situations. Covalent radius is measured when two atoms are BONDED and pulled close together, so their electron clouds overlap and the distance is small. Van der Waals radius is measured when two atoms are NOT bonded (just touching in a solid), so there is no overlap and no pull, and the atoms stay farther apart. Half of a bigger gap gives a bigger radius. So Van der Waals radius > covalent radius, always, for the same element.
Noble gases are monoatomic. They already have a full outer shell, so they do not form covalent bonds. That means you can never measure a covalent radius for them. The only radius you can measure is the Van der Waals radius, which is a non-bonded radius and is always larger. So when a table lists noble gas radii next to covalent radii of other elements, the noble gas value looks 'too big'. NCERT warns you: compare noble gas radii with Van der Waals radii of other elements, not with covalent radii.
No, this is the NEET trap. A noble gas radius only looks the largest because it is a Van der Waals radius while its neighbours are given as covalent radii. You are comparing two different types of measurement. If you compared covalent radius to covalent radius, size decreases across a period due to rising effective nuclear charge. So the 'jump up' at the noble gas is a measurement effect, not a real sudden increase in atom size.
Take a solid where the atoms/molecules are packed but NOT chemically bonded to each other (for example solid argon, or two chlorine molecules lying side by side). Find the distance between the centres of two nearest same atoms that belong to SEPARATE molecules. Half of that distance is the Van der Waals radius. Covalent radius, by contrast, is half the distance between two same atoms bonded in the SAME molecule.
Bonded radius = covalent radius (or metallic radius) — measured when atoms are chemically joined and squeezed close. Non-bonded radius = Van der Waals radius — measured when atoms only touch due to weak Van der Waals forces, with no bond. Non-bonded atoms are farther apart, so the non-bonded (Van der Waals) radius is larger.
Yes. Like other radii, Van der Waals radius decreases across a period (nuclear charge increases, pulling the outer cloud in) and increases down a group (new shells are added). The type of measurement changes the absolute number, but the same trend logic — effective nuclear charge and number of shells — still applies.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Not exactly. 'Atomic radius' is a general word. Van der Waals radius is one specific type of atomic radius, measured for non-bonded atoms. The other main types are covalent radius (for covalently bonded atoms) and metallic radius (for metals).
Because they are monoatomic and chemically inert. They do not normally form covalent bonds, so there is no bonded pair to measure a covalent radius from. Only non-bonded (Van der Waals) contact exists, so only that radius can be measured.
The Van der Waals radius of chlorine is larger. The covalent radius is measured within the bonded Cl2 molecule (atoms squeezed together), while the Van der Waals radius is measured between chlorine atoms of two separate Cl2 molecules (atoms only touching, farther apart).
NEET periodic table questions often give a radius table and ask for the correct order. If a noble gas value looks oddly large, remember it is a Van der Waals radius and should not be mixed with covalent radii. This one idea stops you from picking the trap option.
