But this “huge” space is occupied primarily by electrons, because the nucleus occupies only about 1721×10 −45 m 3 of space. Assuming spherical shape, the uranium atom have volume of about 26.9 ×10 −30 m 3. The Van der Waals radius, r w, of an atom is the radius of an imaginary hard sphere representing the distance of closest approach for another atom. For uranium atom, the Van der Waals radius is about 186 pm = 1.86 ×10 −10m. The volume of an atom is about 15 orders of magnitude larger than the volume of a nucleus.

As a result, the electron cloud contracts and the atomic radius decreases. Therefore, the effective nuclear charge towards the outermost electrons increases, drawing the outermost electrons closer. The atomic radii decrease across the periodic table because as the atomic number increases, the number of protons increases across the period, but the extra electrons are only added to the same quantum shell. Consequently, the smallest atom is helium with a radius of 32 pm, while one of the largest is caesium at 225 pm. On the periodic table of the elements, atomic radius tends to increase when moving down columns, but decrease when moving across rows (left to right). A metallic radius is one-half the distance between the nuclei of two adjacent atoms in a crystalline structure, when joined to other atoms by metallic bonds. Covalent radius is the nominal radius of the atoms of an element when covalently bound to other atoms. An ionic radius is one-half the distance between the nuclei of two ions in an ionic bond. Van der Waals radius. In principle, Vana der Waals radius is half the minimum distance between the nuclei of two atoms of the element that are not bound to the same molecule.Therefore, there are various non-equivalent definitions of atomic radius. However, this assumes the atom to exhibit a spherical shape, which is only obeyed for atoms in vacuum or free space. The atomic radius of a chemical element is a measure of the distance out to which the electron cloud extends from the nucleus. It must be noted, atoms lack a well-defined outer boundary. The atomic radius of Silicon atom is 111pm (covalent radius).

Atomic radius of aluminum