Question

Explain what you understand by covalent radius, Vander Waals radius, ionic radius, and atomic radius. How do they vary in a period and a group?

Answer 1

Hint: Periodic trends are observed in atomic sizes, ionization enthalpies, electron gain enthalpies, electronegativity, and valence. There are numerous physical properties of elements such as melting point and boiling points, heat of fusion and evaporation, etc.

Complete step by step solution:
Atomic radius: it is defined as “the distance from the centre of the nucleus to the outermost shell of electrons”. The determination of the exact radius of the atom is not possible because of the very small size of an atom. Depending upon the nature of combining atoms, the atomic radius can be following types.
(A) covalent radius
(B) metallic radius
(C) Van der Waals radius
(D) ionic radius
- Covalent radius: it is one half of the distance between the nuclei of two covalently bonded atoms in a homo-diatomic molecule which is called covalent radius atom.
- Metallic radius: It is taken as half of the internuclear distance separating the metal cores in the metallic crystal.
For example, the distance between two adjacent copper atoms in solid copper is 256pm, hence, the metallic radius of copper value is128pm.
- Van der Waals radius: It is half of the distance between two similar atoms in a separate molecule in a solid.
-Ionic radius: A neutral atom charges to a cation by the loss of one or more electrons and an anion by the gain of one or more electrons. The number of charges on cation and anion is equal to the number of electrons lost or gained respectively.
The covalent and Van der Waals radii decrease with increase in atomic number as they move from left to right in a period.
The atomic radius decreases from left to right within a period. Increase in ionic character leads to shortening of bond and so decrease in atomic radii.

Note: Chemical reactivity is highest at the two extremes of a period because of the ease of electron loss. Highly reactive elements do not occur in nature in Free State, they are in combined state. Occurrence in nature is not a periodic property for the elements.