Explain covalent radius and van der Waals radius.

(N/A) Covalent radius:
- The covalent radius is defined as one-half of the distance between the nuclei of two identical atoms bonded by a single covalent bond in a molecule.
- For a heteronuclear molecule $AB$, the bond length $d_{AB}$ is given by $d_{AB} = r_A + r_B$, where $r_A$ and $r_B$ are the covalent radii of atoms $A$ and $B$ respectively.
- Example: The bond length in $Cl_2$ is $198 \ pm$, so the covalent radius of $Cl$ is $198 \div 2 = 99 \ pm$.
van der Waals radius:
- The van der Waals radius represents the overall size of an atom, including its valence shell, in a non-bonded state.
- It is defined as one-half of the distance between the nuclei of two identical atoms belonging to neighboring molecules of an element in the solid state.