Short Answer Type
Long Answer TypeHow do atomic radius vary in a period and in a group? How do you explain the variation?
Short Answer TypeWhy do the noble gas elements have exceptionally high atomic radii?
Or
In a period, generally the size of the atom decreases with rise in atomic number, but at the end of each period, atomic size of the noble gas increases abruptly. Why is it so?
Long Answer TypeVander Waal's radius is defined as one half of the distance between the nuclei of two adjacent atoms belonging to two neighbouring
molecules of an element in the solid state, vander Waal’s radii are determined by X-ray diffraction method.
Comparison of covalent radius and vander Waal’s radius.
vander Waal’s radii are always larger than covalent radii. This is because for the formation of covalent bond, atoms move to come closer to each other due to the overlapping of orbitals.
On the other hand, vander Waal’s forces operating between atoms are weak, therefore, adjacent atoms belonging to two neighbouring molecules are at relatively larger distances. Clearly, one half of internuclear distance between adjacent atoms belonging to two neighbouring molecules will be more than one half of the distance between the nuclei of two like atoms forming a single covalent bond. Consequently, vander Waals’ radii are always larger than covalent radii. For example, covalent radius of chlorine is 99 pm whereas vander Waal’s radius of chlorine is 180 pm.
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