Periodic Properties

From the discussion of the periodic table, it is evident that those properties which depend upon the electron configuration of an atom will vary periodically with atomic number. On the other hand, those properties which depend upon the total number of electrons will show no such variations. Some of the more common properties which depend upon electronic configurations are:

Atomic Radius :

(i) The radius of an atom may be taken as the distance between atomic nucleus and the outermost shell containing electrons of the atom.

(ii) According to the Heisenberg’s uncertainty principle the position of a moving electron can not be accurately determined. So the distance between the nucleus and the outermost electron is uncertain.

(iii) Atomic radius can be determined indirectly from the inter nuclear distance between the two atoms in a gaseous diatomic molecule. This internuclear distance between the two atoms is called bond length.

(iv) The inter nuclear distance between the two atoms can be measured by X − ray diffraction or spectroscopic studies.

(v) Covalent radius − One half of the distance between the nuclei (internuclear distance) of two covalently bonded atoms in a homodiatomic molecule is called the covalent radius of that atom. The covalent bond must be single covalent bond. The covalent radius (rA) of atom A in a molecule A2 may be given as:

$\large r_A = \frac{d_{A-A}}{2} $
i.e. the distance between nuclei of two single covalently bonded atoms in a homodiatomic molecule is equal to the sum of covalent radii of both the atoms.

dA− A = rA + rA

In a heterodiatomic molecule AB where the electronegativity of atoms A and B are different, the experimental values of internuclear distance dA-B is less than the theoretical values . According to Schomaker and Stevenson (1941) −

DA-B = rA + rB − 0.09 Δx

Where Δx is the difference of electronegativities of the atoms A and B.

According to Pauling − If the electronegativities of the two atoms A and B are xA and xB respectively then DA-B = rA + rB − (C1xA − C2xB)

C1 and C2 are the Stevenson’s coefficients for atoms A and B respectively.

Metallic Radius :

Metal atoms are assumed to be closely packed spheres in the metallic crystal. These metal atom spheres are considered to touch one another in the crystal. One half of the internuclear distance between the two closest metal atoms in the metallic crystal is called metallic radius. Metallic radius > Covalent radius

For example − Metallic radius and covalent radius of potassium are 2.3 A° and 2.03 A° respectively.

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