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D-Block Elements - General Characteristics
The elements with partly filled d-subshell are termed as D-block elements. They are also termed as transition elements. The incomplete subshells include the (n-1) d subshell. All the d-block elements have the same number of electrons in the outermost shell. Hence, they show similar chemical properties.
The transition elements are placed between S and P block elements in the periodic table. They start from the fourth period of the long form of periodic table.
The transition elements can be either typical transition elements or the non-typical transition elements. Elements of II B group Zn, Cd and Hg have a completely filled (n-1) d subshell and hence, cannot be included in the D-block. But, they show properties such as formation of complexes with ligands such as ammonia, amines and halide ions and are hence, studied along with d-block elements. Similarly, the elements of group III A Sc, Y, La and Ac differ from other d-block elements but have an incompletely filled (n-1)d subshell and are hence, studied along with d-block elements.
These elements i.e. elements of group II B Zn, Cd and Hg and III A Sc, Y, La and Ac are termed as non-typical transition elements and the other transition elements are termed as typical transition elements.
The d-block includes three series each of ten elements. These series are characterised by the completely filled 3d, 4d, and 5d subshells and are named as 3d-(first series) Sc - Zn, 4d series (second series ) Y-Cd and the 5d series (third series) La- Hg respectively. There is an incomplete fourth series consisting of only three elements namely Ac, Ku, and Ha. In these elements, the 6d subshell starts to fill at Ac.
General Characteristics of D-Block elements:
All the elements of the D-block show similar properties due to the presence of similar electronic configuration of the outermost shell. The outermost shell configuration is ns2. Here is a list of general properties such as the atomic and ionic radii, electronic configuration and the ionisation potentials observed among the D-block elements.
- Metallic nature: as the number of electrons in the outermost shell is very less i.e. All the transition elements are metals. They show the characteristics of metals such as malleability and ductile in nature and form alloys with several other metals. They also serve as good conductors of heat and electricity. Except for Mercury which is liquid and soft like alkali metals all the transition elements are hard and brittle unlike the non-transition elements. The hard and brittle nature of these elements indicates the presence of covalent bond which is due to the presence of unfilled d-orbitals. However, their property such as good conductivity is an indication for the presence of metallic bonding. Hence, they are said to form covalent bonding as well as the metallic boding.
- Melting and boiling points: They show very high melting and boiling points. This can be attributed to the presence of strong metallic bonding due to the overlapping of (n-1) d orbitals and covalent bonding of the unpaired d orbital electrons. Since Zn, Cd and Hg have completely filled (n-1)d orbitals they are not expected to form covalent bonds. Hence, they show comparatively lower melting point than other d-block elements.
- Atomic radii: a great degree of variation is seen in the atomic radii across each
transition series. The atomic radii of the d-block elements within a given series decreases with increase
in the atomic number. This is due to the increase in the nuclear charge that attracts the electron cloud
inwards resulting in decrease in size. However, the decrease a uniform decrease in atomic radius is not
observed across a period. The decrease in atomic radii is small compared to the S and P block elements.
This is due to the screening effect caused by the electrons of the (n-1)d subshell on the outermost shell.
As a result the nucleus cannot pull the outermost electrons. Thus, the size of the atom does not alter much
in moving from Cr to Cu.
The atomic radius increases on descending the group. In a given series, the atomic radius decreases to a minimum for the group VIII elements and then it increases towards the end of the series. This increase in radius towards the end of the series is due to the force of repulsion among the added electrons. A close similarity is observed in the radii of the elements of the second and third transition series due to the filling of 4f subshells.
- Ionic radii: The ionic radius is similar to the pattern of atomic radii. Thus, for ions of a given charge the radius decreases slowly with increase in atomic number.
- Atomic volume and Densities: the atomic volume of transition elements is
much lower than those of S and P block elements. This is because of the filling of the (n-1)d orbitals
that cause an increase in the nuclear charge and pulls the electrons inward. This results in decrease
in atomic volume. With the decrease in the atomic volume, the atomic density of these elements increases.
Osmium is having a maximum density.
In a given transition series, the density increases in moving across the period and reaches a maximum value at group VIII.
The density increases as we move down the group. The atomic sizes of elements of the second and third transition series are nearly same but their atomic weights increase nearly two fold and the densities of elements of third transition series are generally twice of the corresponding second transition series.
- Ionization potentials: Transition elements have high ionization energy due to
their small size. Their ionization potentials lie between those of S and P block elements. Thus, they
are less electropositive than the s-block elements. Hence, they do not form ionic compounds readily
like the alkali and alkaline earth metals. They also have the ability o form covalent compounds.
The ionisation potentials of d-block elements increase as we move across each series from left to right. However, the increase is not as much as in case of S and P blocks elements. This is due to the screening effect caused by the new electrons that are added into the (n-1) d subshell.
The second ionisation energies of the first transition series also increases with the increase in atomic number. However, Cr and Cu are sufficiently higher than those of their neighbours. This is due to their stable electronic configuration.
- Electronic configuration: the outer electronic configuration remains constant.
But, a electron is added to the penultimate shell till the d-sub shell reaches its full capacity. There
are three series of elements depending on the n-1 d orbital that is being filled. The orbitals are filled
in order of their increasing energy i.e. an orbital of lower energy is filled first. Thus 4s orbital with
lesser energy is filled first to its full extent then the 3d orbital with higher energy is filled.
The exactly half-filled and completely filled d-orbitals are extra stable.
The electronic configuration of the first series is given as 1s22s2p6 3s2p6d1-10 4s2
The electronic configuration of the second series is given as 1s22s2p6 3s2p6d1-10 4s2p6d1-10 5s2
The electronic configuration of the third series is given as 1s22s2p6 3s2p6d1-10 4s2p6d1-10 5s2p6d1-10 6s2
Transition elements also show variable oxidation states, tendency to form complexes, magnetic nature and other properties.
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