Question

why we can't find the atomic radius of an element in its isolated state define briefly​

Answer 1

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$<marquee behavior =move bgcolor =pink><h1>chanchal here ✿●‿●✿</h1></marquee >$

Covalent radius is measured as the distance (rcov=d/2) between the nuclei of two bonded atoms (covalent). But, if you try to do the same for noble gases/inert gases (good luck!), as they have fully filled np orbitals, they will repel each other, hence the closest distance between the two atoms is taken (high pressure, low temperature) as the van der Waals radius.