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bigger size than N. Thus, the anom: e, : . alous behavior, nuclear charge and inter- electronic repulsions ae, iii) The si: i :, , (iii) The size of the atoms of inert gases* are, however,, , Explanation. As we move from left to right in a period,, succeeding element while the number of the shells remains the i, the electrons of all the shells are pulled little closer to th Sols thes, wis enhanced nucten h, smaller and smaller. Thi nucleus thereby making each individval shel, , n ler. This results in a decrease of the atomic radius as we move fi ‘ Fea individual shel, , An inspection of the Table 3.9 further reveals that the atomic radius ab wel . agen, from halogen (F) to the inert gas (Ne). This is due to the reason that in case of rte wane tne name, completely filled and hence the inter-electronic repulsions are maximum Monsey aed oe or, the atomic size is expressed in terms of van der Waals radius since they do not font ale Cee, in case of all other elements, the atomic size is expressed in terms of covalent radius. ‘Sine ae wale, van der Waals radii are larger than covalent radii, therefore, the atomic size of an inert a kanciord, much higher than that of the preceding halogen. Bes avaiperioa 18, , me ae ers van der Waals’ radii also decrease as we move from left to right in a period. For, example, the van der Waals radii of N, O and F are 150 pm, 140 pm and 135 pm respectively., , (b) Variation within a group. The atomic (covalent) radii of elements increase with increase in, atomic number as we move from top to bottom in a group. For example, consider the atomic (covalent) radii, of the members of the alkali metals group (Table 3.10). These are represented graphically in Fig. 3.5., , From the above table, it is clear that the atomic en, radii increase as we move down the group from lithium, to caesium among alkali metals. Similar trend is, followed by halogens from fluorine to iodine as shown, , graphically in Fig. 3.5., , Increase in atomic (metallic), UES TA ESE, radu (pm) of alkali metals, and covalent radii of halogens, , PNP eles, Atomic lalogens | Atomic, (Group 1)” 1 (Group 17) | radius, Li 152 F 72:, , ~ enone SATEEN ERY OK) RST, and F is due to the combi ee, mbined effect of j, of increaseq, larger than those of the precedir, , ng haloge;, nuclear charge increases by one unit ee nS,, each, , , , 300, , , , , , , , (262), , , , Cs, Rb (244), , 1 (133), Br (114), , =, S, S, , , , ATOMIC RADIUS (pm)=—=>, a a, o a, , gee, , , , , , , , 99 0 mrp yy tons Lat x5, I ,, , z a af 114 ATOMIC NUMBER (2) ==>, , ee 2 J 133 ©" Variation of atomic radius with atomic, Se 262 At 140 - number for alkali metals and halogens”, , , , metals or halogens, the principal quantum number (71), eeding element and the valence electrons, , Explanation. As we move down a group, alkali, traction of the nucleus for the electrons, , increases. In other words, a new energy shell is added at each succ, lie farther and farther away from the nucleus. As a result, the at, decreases and hence the atomic radius increases., , However, with increase in atomic number, the nuclear, atuaction of the nucle should increase an i ieldi {fee, the effect of the eis wlcstaioe ts Set reduced due to the screening or a a ie, the valence electrons by the electrons present in the inner shells. In other worms the et a enc the, energy shell is so large that it outweighs the contractive ‘effect of the increased nuclear charge. Hen hg, increase in atomic radii as we move down the group (from Li ta Cs among algal tnetals air fos, among halagens) is primarily due to the addition of anew energy shell., , charge also increases, As a result, the force of, d hence the atomic radii should decrease. But, , "Van Ger Waals radius