The statement that covalent radius is always greater than ionic radius is not universally true. In fact, the comparison between covalent and ionic radii depends on the specific elements involved and their bonding characteristics.
Covalent radius refers to the size of an atom when it is participating in a covalent bond, which involves the sharing of electrons between atoms. In a covalent bond, the atomic radii of the bonded atoms overlap to form a bonding region. The covalent radius is typically measured as half the distance between the nuclei of two bonded atoms.
Ionic radius, on the other hand, refers to the size of an ion, which is formed when an atom gains or loses electrons to achieve a stable electron configuration. In an ionic bond, electrons are transferred from one atom to another, resulting in the formation of oppositely charged ions that attract each other. The ionic radius is typically defined as the distance between the nucleus and the outermost electron shell of an ion.
In general, when an atom forms an ion, its ionic radius may differ from its covalent radius. The formation of an ion involves changes in the electron configuration, which can lead to variations in the effective nuclear charge and electron-electron repulsions. These factors can cause the ionic radius to be different from the covalent radius of the same atom.
There are cases where the covalent radius is larger than the ionic radius. For example, when oxygen forms an O^2- ion, it gains two electrons and increases its electron-electron repulsions, causing the ionic radius to be larger than the covalent radius of oxygen when it forms covalent bonds. However, there are also instances where the ionic radius is larger than the covalent radius, such as when a metal atom loses electrons to form a cation.
Therefore, the relative sizes of covalent and ionic radii depend on the specific elements involved, the nature of their bonds, and the changes in electron distribution that occur during bond formation. It is essential to consider these factors when comparing covalent and ionic radii.