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Thursday, August 5, 2021

Vander Waal’s radius or Collision radius:

 (1) The Vander Waal’s radius is defined as one half of the distance between the nuclei of two adjacent identical atoms  belongging to two neighbouring molecules of an elements in the solid state.


(2) The name Vander waal’s radius is used because of the force existing between yhe molecules are the vander waal’s forceof attractions. And Vander Waal’s forces are directly proportional to the molecular weight or atomic weight (in inert gases).

(3) These force of are maximum in solid state and absent in gaseous condition. In liquid state the magnitute is very small so Vander waal’s radius determine in solid state only.

(4) The Vander Waal’s radii is also known as non bonded radii.

(5) The Vander Waal’s radii is determined by X-rays difffraction studies.

(6) In molecules of non-metals both covalent and Vander Waal’s radius exists.

(7) The Vander Waal’s radii is always greater than covalent radius of a given elements. And also greater than that of all known radius.

(8) The Vander Waal’s radius is generally two times of Covalent radius

            Vander Waal’s and Covalent radii of some elements in Angstrom.

Elements

H

N

O

F

Cl

Br

Vander Waal’s radius

1.2

1.5

1.4

1.35

1.80

1.95

Covalent  radius

0.35

0.75

0.73

0.72

0.99

1.14

(9) The noble gases do not from covalent bonds. Thus in crystal of noble gases no chemical forces operated between the atoms. It is only the Vander Waal’s forces prevailing in these atoms. Thus for noble gases in the solid state the crystal radii (atomic radii) are actually Vander Waal’s radii.

(10)   Vander Waal’s radius > Metallic radius > covalent radius.

Metallic radius or Crystal radius:

Trends:

(1) In a Period:

While we move from left to right along period, atomic radius generally decreases.

(2) In a Group:

In a given  group as one moves from top to bottom atomic radius increases.

Related Questions:

(1) What are the Amphoteric metals ? gives Examples.

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