Periodicity in atomic radius and ionic radius

For normal elements

(a) In a period from left to right effective nuclear charge increases because the next electron fills in the same shell. So the atomic size decreases. For example the covalent radii of second period elements in A° are as follows −

Li       Be       B         C       N       O         F

1.23   0.89   0.80   0.77   0.74   0.73   0.72

(b) In a group moving from top to bottom the number of shells increases. So the atomic size increases. Although the effective nuclear charge increases but its effect is negligible in comparison to the effect of increasing number of shells. For example the covalent radii of IA group elements in A° are as follows −

Li      Na        K       Rb       Cs

1.23   1.57   2.03   2.16   2.35

For inert gases:

The atomic radius of inert gas (zero group) is shown largest in a period because of its Vander Waal’s radius which is generally larger than the covalent radius. The vander Waal’s radius of inert gases also increases in moving from top to bottom in a group.

For transition elements:

There are three series of transition elements :

3d − Sc (21) to Zn (30)

4d − Y (39) to Cd (48)

5d − La (57), Hf (72) to Hg (80)

(a) From left to right in a period

(i) The atomic size decreases due to the increase in effective nuclear charge.

(ii) In transition elements, electrons are filled in the (n-1)d orbitals. These (n-1)d electrons screen the ns electrons from the nucleus. So the force of attraction between the ns electrons and the nucleus decreases.

This effect of (n−1)d electrons over ns electrons is called shielding effect or screening effect. The atomic size increases due to shielding effect and balance the decrease in size due to increase in nuclear charge to about 80%.

iii) Thus moving from left to right in a period, there is a very small decrease in size and it may be considered that size almost remains the same.

iv) In the first transition series the atomic size slightly decreases from Sc to Mn because effect of effective nuclear charge is stronger than the shielding effect. The atomic size from the Fe to Ni almost remains the same because both the effects balance each other. The atomic size from Cu to Zn slightly increases because shielding effect is more than effective nuclear charge due to d10 structure of Cu and Zn. The atomic radii of the elements of 3d transition series are as under.

Sc      Ti        V      Cr      Mn      Fe      Co    Ni    Cu    Zn

1.44   1.32   1.22   1.18   1.17   1.17   1.16   1.15   1.17   1.25

Inner transition elements:

As we move along the lanthanide series, there is a decrease in atomic as well as ionic radius. The decrease in size is regular in ions but not so regular in atoms. This is called lanthanide contraction. The atomic radii in A° are as under:

La        Ce       Pr       Nd        Pm     Sm     Eu          Gd

1.88    1.82    1.83    1.82     1.81    1.80    2.04    1.80

Tb     Dy        Ho        Er     Yb     Lu

1.78    1.77    1.76    1.75    1.94    1.73

There are two peaks one at Eu (63) and other at Yb (70). This is due to the difference in metallic bonding. Except Eu and Yb other lanthanides contribute three elctrons in metallic bond formation. These two atoms contribute two electrons in the bond formation leaving behind half filled and completely filled 4f−orbitals respectively.

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