The molecules of non metal atoms are generally gases. On cooling, the gaseous state changes to solid state.

In the solid state, the non metallic elements usually exist as aggregations of molecules are held together by Van der Waal forces. One half of the distance between the nuclei of two adjacent atoms belonging to two neighbouring molecules of a compound in the solid state is called Van der Waal’s radius.

It may also be defined as half of the inter nuclear distance of two non bonded neighbouring atoms of two adjacent molecules.

The Vander Waal’s radius and Covalent radius of Chlorine atom are 1.80 A° and 0.99 A° respectively

A neutral atom changes to a cation by the loss of one or more electrons and to an anion by the gain of one or more electrons. The number of charge on cation and anion is equal to the number of electrons lost or gained respectively. The ionic radii of the ions present in an ionic crystal may be calculated from the internuclear distance between the two ions (i) Radius of a Cation: Radius of a cation is invariably smaller than that of the corresponding neutral atom

————Na       Na+
Number of e = 11       10
Number of p =  11       11
1s22s22p63s1       1s22s22p6

Reasons

(a) The effective nuclear charge increases. For example in Na atom 11 electrons are attracted by 11 protons and in Na+, 10 electrons are attracted by 11 protons. Thus in the formation of cation number of electrons decreases and nuclear charge remains the same.

(b) Generally the formation of cation results in the removal of the whole outer shell.

(c) Interelectronic repulsion decreases. The interelectronic repulsion in Na is among and in Na+ among .

(ii) Radius of an anion: Radius of an anion is invariably bigger than that of the corresponding atom.

Cl       Cl
Number of = 17          18
Number of p = 17        17

Reasons

(a) The effective nuclear charge decrease in the formation of anion. Thus the electrostatic force of attraction between the nucleus and the outer electrons decreases as the size of the anion increases.

(b) Interelectronic repulsion increases.

iii) Isoelectronic series: A series of atoms, ions and molecules in which each species contains same number of electrons but different nuclear charge is called isoelectronic series

N3−    O2−    F   Ne    Na+   Mg2+
Number of e      10    10   10   10    10    10
Number of p        7     8     9     10     11     12
Number of p        7     8     9     10     11     12
——————————————->

(a) Number of electrons is same.

(b) Number of protons is increasing

(c) So the effective nuclear charge is increasing and atomic size is decreasing. In an isoelectronic series atomic size decreases with the increase of charge. Some of the examples of isoelectronic series are as under

(i) S2- , Cl , K+ , Ca2+ , Sc3+

(ii) SO2 , NO3 , CO32−

(iii) N2 , CO , CN

(iv) NH3 , H3O+

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