a)Atomic radius is the distance from the centre of the nucleus to the outermost shell containing electrons. In other words, it is the distance from the centre of the nucleus to the point up to which the density of the electron cloud is maximum.
b)Covalent radius is half of the internuclear separation between the nuclei of two single-bonded atoms of the same species (homonuclear). While van der Waalsradius is used to define half of the distance between the closest approach of two non-bonded atoms of a given element.
c)Metallic radius is defined as one-half of the distance between the two adjacent metal ions in the metallicstructure. This radius depends on the nature of the atom as well as its environment—specifically, on the coordination number (CN), which in turn depends on the temperature and applied pressure.