The average distance between the center of the nucleus and the electron cloud of outermost orbit is called atomic radius. The exact position of an electron is uncertain and cannot be determined accurately. Hence we take the atomic radius as the average distance.
The atomic radii are broadly divided into three types as shown below depending on the nature of substance and bonding in it etc.
The half of inter nuclear distance between two adjacent atoms in a metallic crystal is known as crystal radius or metallic radius or most of the time as atomic radius. It is applicable to metals.
Since the atoms in a metallic crystal are closely packed, the crystal radius is considered to be more accurate radius of atom.
The half of inter nuclear distance between two atoms held together by a covalent bond. This is applicable to atoms in covalent substances.
The half of inter nuclear distance between two closest atoms of different molecules attracted by van der Waal's forces. It is especially applicable to noble gas elements.
Usually van der Wall's radii are 40% greater than crystal or covalent radii.
Though it is strictly not possible to compare the radii of atoms in a given group or period since for each element the accurate atomic radii are not available and the values depend on the type of radii reported and the state of the substance as well as the nature of bonding interactions.
However, a rough comparison of reported atomic radii is helpful in understanding the trends in the periodic properties of elements.
Thus in groups, the atomic size increases down the group as the differentiating electron enters into the new quantum shell. This outweighs the increase in nuclear charge.
E.g., The order of atomic radius in IA group is Li < Na < K< Rb < Cs < Fr
In periods, the atomic radius decreases across the period from left to right as the nuclear charge and atomic number increase. The differentiating electron enters into same shell.
In a given period IA group element (alkali metal) is bigger in size and VII A group element (halogen) is smaller in size.
E.g., Order of atomic size in 2nd period Li > Be > B > C > N > O > F < Ne
Yet, it is misleading that the sizes of zero group elements are greater than those of corresponding halogens, since the only reported radii for noble gases are van der Waal's radii.
In the transition elements, the observed decrement in atomic size is very small across the period. It is due to shielding effect of inner d-electrons. In these elements, from left to right, the differentiating electron enters into the inner d-orbitals that results in the reduction of nuclear attraction over the outermost electron. This slightly offsets the increase in the positive charge in the nucleus. Hence the decrease in the radius is not as much as expected.
In Lanthanoids, the atomic radius decreases with increase in atomic number due to poor shielding effect of inner f-electrons. It is commonly known as Lanthanoid contraction.
Consequences of Lanthanoid contraction
i) Lanthanoids possesses similar crystalline structures and hence their separation is difficult.
ii) Elements of 4d and 5d series show more similarity in their properties.
E.g., Zirconium (Zr), ( 4d series) and Hafnium (Hf), ( 5d series) have almost same atomic radii.