Question

what is strange about block "d"?

Answer 1

Atomic radii:

D-block elements:

Sc:160 pm

Ti : 140 pm

V : 135 pm

Cr : 140 pm

Mn: 140 pm

Fe : 140 pm

Co : 135 pm

Ni : 135 pm

Cu :135 pm

Zn : 135 pm

Normally across the period atomic radii should decrease. But d-block elements do not follow this trend. It increases From V to Cr and them remains constant and decreases and remains constant again.

Answer 2

Normally across the period atomic radii should decrease. But d-block elements do not follow this trend. It increases From V to Cr and them remains constant and decreases and remains constant again.

Properties
D-block elements show typical metallic behaviour. These are characterised by high tensile strength, malleability, ductility, electrical and thermal conductivity as well as metallic lustre. Except Zinc (Zn), Cadmium (Cd), Mercury (Hg) and Manganese (Mn), they have one or more typical metallic lattices. Due to the larger extend of metallic bonding by virtue of d-electrons, they are hard and have high melting and boiling points. The group-12 elements (Zn, Cd and Hg) shows exception in this regard also.

Melting and Boiling point
The high melting and boiling points of d-block elements can be attributed to the involvement of d-orbital electrons in addition to the s-electrons in metallic bonding. Melting and boiling points increases as the d-orbital gets filled. This trend goes till d5 configuration and then decreases regularly as the orbital gets further filled and the electrons gets paired in the orbital. A point to be noted here is that Manganese (Mn) and Technetium (Tc) have abnormally low melting and boiling points.

Exceptional case of Mercury – the liquid metal: Mercury is one among the two elements (other being Bromine(Br) ) and the only metal that exist in its liquid state at room temperature. This is unexpected for the general outlook of a metal and can be explained by the fact that 6s valence electrons of Mercury are more closely pulled by the nucleus, rendering those outer s-electrons less involved in metallic bonding. Still, Mercury shows good electrical conductivity.
Atomic and Ionic sizes
The ionic sizes shows gradual decrease as we move right across a series. This is because as we number of electrons increase, the nuclear charge too increase. Due to poor shielding of nuclear charge by d-electrons, the net increase in nuclear charge outweighs the effect of added electron, thereby reducing the size.

Similar trend and reasons can be observed for case of atomic radii as well, though the decrease is much gradual. In case of going down the series, the atomic radii shows increase. There is a huge jump from 3d to 4d in terms of atomic size, but the 4d and 5d series have a small difference only. This is explained on the basis of Lanthanoid contraction. In case of 5d series, the inner 4f orbitals are filled before 5d orbitals. The poor shielding ability of 4f electrons renders the outer electrons greater nuclear pull, causing lanthanoid contraction. Due to this, the expected increase in atomic size is compensated by increased nuclear pull, keeping the size nearly same.

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