Classification of Elements and Periodicity in Properties Test - 5

The two rows of elements at the bottom of the Periodic Table, called the Lanthanoids, Ce(Z = 58) – Lu(Z = 71) and Actinoids, Th(Z = 90) – Lr (Z = 103) are characterised by the outer electronic configuration (n-2)f^1-14 (n-1)d^0-1ns². The last electron added to each element is filled in f- orbital.

In a group movig 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.

In a period moving from left to right, the effective nuclear charge increases because the next electron fills in the same shell. So the atomic size decreases.

The elements of Group 1 (alkali metals)  which have ns¹ outermost electronic configuration belong to the s-Block Elements.

Elements of s block are more matallic than p block elements

The two rows of elements at the bottom of the Periodic Table, called the Lanthanoids, Ce(Z = 58) – Lu(Z = 71) and Actinoids, Th(Z = 90) – Lr (Z = 103) are characterised by the outer electronic configuration (n-2)f^1-14 (n-1)d^0-1ns². The last electron added to each element is filled in f- orbital. These two series of elements are hence called the Inner-Transition Elements (f-Block Elements).

The distance from centre of the nucleus to the outermost shell containing the electrons is called atomic radius. There are four types that are widely used: covalent radii, metallic radii, van der Waal's radii and ionic radii.

For the same element ,

• Metallic radius of atoms are always greater than corresponding  covalent radius.
• Metallic radius is always lesser than van der Waal's  radius

  Hence , the correct increasing order   of atomic radii  of an element  is covalent <  Metallic < van  der Waal's radius.

The bond lengths depend upon  the number and type of bonds  present  in  a molecule . Greater the number of bonds shorter would be the bond length.

Thus , the given  molecules  are  all diatomic molecules where ,

(i)  N₂ has the smallest bond length as it is triple bonded.

(ii)  O₂  is double bonded so its bond length is greater than   N₂  .

(iii) F₂ and Cl₂F₂ and Cl₂   both , though single bonded , the bond length of  Cl₂  is  greater than that of  F₂   due to  its larger  atomic size.

Hence ,  the correct increasing order of given  molecules is   N₂ < O₂ < F₂< Cl₂

In general as the we move down the group electron gain enthalpy decreases; so the expected trend should be I < Br < Cl < F  but in actual the order is I < Br < F < Cl . This is due to small size of F atom. The valence electrons of F atom provide strong repulsion to incoming electron, hence F has unexpectedly low electron affinity.