Chemistry · Periodic Classification Of Properties · NEET
An atom's electron cloud has no sharp edge, so you cannot put a ruler on one atom. Instead, take two SAME atoms bonded by a single covalent bond (like Cl-Cl). Measure the distance between their two nuclei. Half of that distance is the covalent radius of each atom. Short version: covalent radius = half the bond length between two identical bonded atoms.
When two identical atoms bond, each atom contributes an equal part to the total gap between the nuclei. So the total internuclear distance is shared equally by the two atoms. Since both atoms are the same, each atom's radius is exactly half of the full distance. That is why covalent radius = (bond length) / 2.
You first find the bond length (the distance between the two nuclei) using X-ray diffraction, electron diffraction, or spectroscopic methods. For example, in Cl2 the Cl-Cl bond length is 198 pm. Then you halve it: covalent radius of chlorine = 198 / 2 = 99 pm. So the measured thing is the bond length; the covalent radius is calculated from it.
Bond length is the FULL equilibrium distance between the nuclei of two bonded atoms. Covalent radius is each atom's SHARE of that distance. For two identical atoms, covalent radius = bond length / 2. For two different atoms A-B, the bond length is roughly the SUM of their two covalent radii (bond length approximately = r(A) + r(B)).
Not exactly. 'Atomic radius' is a general word for the size of an atom. Covalent radius is ONE specific way to define that size, used for atoms held by covalent bonds (mostly non-metals). Metals use metallic radius, and non-bonded atoms like noble gases use van der Waals radius. In NEET, 'atomic radius' of non-metals usually means their covalent radius.
In a covalent bond the two atoms overlap and come close, so the shared electron pair pulls the cores near each other. This gives a SMALL covalent radius. The van der Waals radius is measured between atoms that are NOT bonded (just touching in a solid), so they stay far apart, giving a LARGER value. That is why noble gases, which do not bond, only have large van der Waals radii.
Yes, but by addition. The bond length of H-Cl is roughly the sum of the covalent radius of H and the covalent radius of Cl. So covalent radii are additive: r(A-B) approximately equals r(A) + r(B). Small differences appear when the atoms have very different electronegativities, but for NEET the additive rule is enough.
The correct order of atomic radii in group 13 elements is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The Cl-Cl bond length in a chlorine molecule is 198 pm. Since chlorine bonds to an identical atom by a single bond, its covalent radius is 198 / 2 = 99 pm.
It decreases across a period from left to right. The electrons enter the same valence shell while the nuclear charge (and effective nuclear charge) increases, pulling the electron cloud closer to the nucleus.
Yes. Going down a group, a new electron shell is added at each step, so the atom gets bigger and the covalent radius increases (with small exceptions like the Ga-Al case).
Noble gases are monoatomic and usually do not form covalent bonds. So they have no covalent radius; only their large van der Waals (non-bonded) radius is quoted.
Yes, approximately. For a bond A-B, the bond length is close to the sum of the covalent radii of A and B: r(A-B) is about r(A) + r(B). This lets us estimate bond lengths of new molecules.
