d-Block Elements
The d-block of the periodic table contains the elements of the group 3-12 in which the d-orbitals are progressively filled in each of the four long periods.
In these elements, the last electron enters (n – 1) d-subshell (d-orbitals of the penultimate orbit), i.e., (n-1) d-subshell is gradually filled up. The configuration varies from (n-1)d^{1}ns^{2} to (n-1)d^{10}ns^{2}
These are present between s-block and p-block elements. The properties of these elements are intermediate between the properties of s-block and p-block elements. Forty elements belong to d-block. Fourth, fifth, sixth and seventh periods consists of ten elements each.
There are mainly three series of the transition metals.
(1) 3d series (Sc-Zn)
(2) 4d series (Y-cd)(3)5d series (La-Hg)
(4) 6d series (Ac-Uub)
Transition Elements:
They are often called “Transition elements” because their position in the periodic table between s-block and p-block elements.
Typically, the transition elements have an incompletely filled d-level. Since 12th group has do configuration and are not considered as transition elements but they are d-block elements.
d-block elements show horizontal as well as vertical relationship.
Zn, Cd and Hg are generally not regarded as transition elements.
Ex: Zn, Cd and Hg
Zn
Cd
Hg
3d 4s
4d5s
5d6s
So out of 40 d-block elements there are 37 transition elements Electronic Configuration: General E.C. of d-block (n-1)d^{i-10}ns^{0-1}
For 3d series For 4d series For 5d series For 6d series 3d^{1-10}4s^{1-2} 4d^{1-m}5s^{n-1} 5d^{1-10}Ee^{1-2} 6d^{1.10}7s^{1-2}
Most of the elements possess two electrons in the outermost orbital, i.e., ns^{2} . However, some of the elements such as Cr, Cu, Nb, Mo, Ru, Rh, Ag, Pt, Au and Rg have one electron in the outermost orbital, i.e., ns’ while one element palladium has no electron on ns orbital General Properties of d-Block Elements (Transition elements):
(1) Variation in atomic and ionic sizes of transition
metals:
On moving left to right in the period, generally atomic and ionic radius value
decreases. The atomic radii decrease from Sc to Cr because the effective nuclear charge (ENC) increases.
The atomic size of Fe, Co, Ni is almost same due to increase in nuclear charge is cancelled by the repulsion between the electrons and increase in shielding effect.
Cu and Zn have bigger size because the shielding effect decreases and electron- electron repulsion increases.
Order of size of 3d series:
(i) Covalent radius:
Sc Ti V Cr Mn Fe Co Ni Cu Zn
(ii) Metallic radius:
Sc Ti V Cr Mn Fe Co Ni
The variation in covalent and metallic radius of Cr and Mn is due to the fact that in case of Mn two electrons participate in metallic bond formation whereas in case of Cr three electrons participate in formation of metallic bond, therefore metallic bond strength of Cr is high and its metallic radius is low.
Ionic Radii:
The ionic radii follow the same trend as the atomic radii. For the same oxidation state, the ionc radii generally decreases as the atomic number increases in a particular transition series
The ionic radii decreases with increase of charge on the ion.
Lanthanod contraction
There is regular decrease in the atomic and ionic radii of the transition metals as the atomic number increases. This is because of filling of 4f orbitals before the 5d orbitals. This contraction in size is quite regular. This is called lanthanoid contraction,
The conclusion of the Lanthanoid contraction is that the 4d and 5d series exhibit similar radii and have very similar physical and chemical properties much more than the expected on the usual family relationship. 14 lanthanides between La and Hf, [there is continuous decrease in size from Ce(58) to Lu(71)) Hf size becomes nearly equal to the
size of Zr.
) Density:
Density of these elements generally increases with decrease in metallic radius coupled with Increase in mass. Thus, from Ti to Cu the significant increase in density may be noted.
Order of density (3d series)
) Ionisation Enthalpy:
There is slight and irregular variation in ionization energies of transition metals due to irregular variation of atomic size. The I.Ε. of 5d transition series is higher than 3d and 4d transition series because of lanthanoid contraction.
Order of IP of d-block Elements is
3d < 5d > 4d
There are various exceptions in the IP of d-block elements which is due to the following reasons.
(1) Exceptional electronic configuration.
(2) Irregular variation in size and Zer
(3) When e is removed from ns orbital then remaining e is shifted to (n-1) orbital due to which the no. of exchanges are changed and IP also changes.
On moving from top to bottom in d-block Z is dominating factor.
Order of IP of 3d series will be:-
Sc Ti V Cr Mn Fe Co Ni Cu zn(4) Oxidation States:
d-Block elements show variable oxidation states due to tendency of (n-1)d as well as ns electrons to take part in bond formation (except 3rd and 12th group elements).
Absorption of Energy
ns subshell
↑
(n-1) d subshell
Minimum Energy Gap
Characteristics features of oxidation state:
Sc[III B) and Zn[II B] don’t show variable
oxidation state.
Most common oxidation state of d-block metals is +2.
Highest oxidation state of 3d-series element is +7, while those of d-block element is +8.
Highest oxidation state of d-block element is found in mid of the series [Stability of highest oxidation state is increases upto mid of the series and then start decreasing]
d-block metals can also show zero oxidation state (low oxidation state) when a complex compound has ligands capable of -acceptor character in addition to the o-bonding. For example: [Ni(CO),], [Fe(CO),]
Highest oxidation state of d-block metals is found in their compounds with oxygen and fluorine.
The high value of oxidation state of above elements is due to very small size and very high electronegativity of oxygen and fluorine.
Highest oxidation state of d-block metals with respect to halogen is found to be +6.(5) Standard electrode potential (E’) and
chemical reactivity:
The stability of the compounds in solution depends upon standard electrode potentials rather than I.Ε.
Electrode potential values depend upon factors such as enthalpy of sublimation (or atomisation) of the metal, the ionisation enthalpy and the hydration enthalpy, i.e.,