van der waal radius is greater than co valent radius because van der waal radius is the distance between the nucleus of two atoms of different compound whereas covalent radius is distance between nucleus of atoms of same compound.
Atomic radius is the measure from the middle of an atom to the outter edge. Usually mesured between two atom. Measure from the middle of one atom to the middle of another and then divide by two.
Atomic center is the center of the atom, also called as Nucleus. Atomic Radius is the distance between the center of the nucleus and outermost shell of the atom. It is nearly about 1.2 * 10-10 m.