Chemistry · Chemical Bonding · NEET
Both are measured between two SAME atoms, but the situation is different. Covalent radius is used when the two atoms are joined by a covalent BOND inside one molecule. You take the distance between their two nuclei and divide by 2. Van der Waals radius is used when the two atoms are NOT bonded — they belong to different molecules sitting next to each other in a solid and only touch. Again you take the distance between the two nuclei and divide by 2. Same math (half the distance), different bonding situation.
When two atoms form a covalent bond, they overlap and get pulled close together, so the nuclei come nearer and the half-distance (covalent radius) becomes small. When two atoms are only touching (non-bonded), there is no bond pulling them in — only weak van der Waals contact stops them. So the nuclei stay farther apart and the half-distance (van der Waals radius) is larger. Rule for NEET: van der Waals radius > covalent radius for the same atom.
NCERT uses chlorine as the standard example (Fig 4.2). Inside a Cl2 molecule the two chlorine atoms are bonded; half of that Cl–Cl bond distance is the covalent radius (about 99 pm). Between two chlorine atoms of DIFFERENT Cl2 molecules lying side by side in solid chlorine, there is no bond, only touching; half of that distance is the van der Waals radius (about 180 pm). So the same chlorine atom has a small covalent radius and a much bigger van der Waals radius.
Yes, for two identical atoms. Bond length is the distance between the nuclei of the two bonded atoms. For a bond between two SAME atoms (like Cl–Cl), each atom contributes equally, so covalent radius = bond length ÷ 2. For a bond between two different atoms, you add the two covalent radii to estimate the bond length (covalent radius is roughly additive).
Yes. NCERT says the van der Waals radius represents the OVERALL size of the atom including its valence (outer) shell in a non-bonded situation. That is why it is bigger — it measures the atom as a whole, undisturbed by bonding. Covalent radius is more like the radius of the atom's core when it is squeezed into a bond.
If the question mentions atoms joined by a bond, a molecule, or bond length — use covalent radius. If it mentions atoms of neighbouring molecules, packing in a solid, non-bonded contact, or the overall size of a noble gas atom — use van der Waals radius. Remember the order van der Waals > covalent for the same atom; questions often test just that comparison.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Van der Waals radius is greater for the same atom, because non-bonded atoms are held farther apart than atoms joined by a covalent bond.
It is half the distance between the nuclei of two identical atoms joined by a single covalent bond in the same molecule.
It is half the distance between the nuclei of two identical, non-bonded atoms belonging to separate molecules that touch each other in a solid.
Yes. Noble gas atoms do not form normal covalent bonds, so their size is given by the van der Waals radius, which measures their overall non-bonded size.
Covalent radius adds up to give bond length, and comparing covalent vs van der Waals radius is a common one-mark Chemical Bonding fact tested in NEET.
