d and f block elements for IIT JEE

d and f Block Elements - Electronic Configuration and Characteristics

d-block elements

Groups 3 to 12 elements are called d-block elements or transition elements. These elements are present between the p-block and the s-block elements in the Periodic Table. These elements' properties are intermediate between the properties of s - block and p - block elements, i.e. d - block elements represent a change or transition in properties from most electropositive s - block elements to less electropositive p - block elements. Therefore, these elements are called transition elements.

Forty elements belong to the d-block. Fourth, fifth, sixth and seventh periods consist of ten elements each.

The d-block elements include the most common metal used in construction and manufacturing, metals that are valued for their beauty (gold, silver and platinum), metals used in coins (nickel, copper) and metals used in modern technology (titanium). Copper, silver, gold and iron were known and used in early civilization. 

Certain d-block elements are particularly important in living organisms. Iron, the transition element, is present in the largest quantity in the human body. The best known biological iron containing compound is the protein haemoglobin, the red component of blood that is responsible for the transport of oxygen. Cobalt is the crucial element in vitamin Bl2, a compound that acts as a catalyst in the metabolism of carbohydrates, fats and proteins. Molybdenum and iron together with sulphur form the reactive portion of nitrogenase, a biological catalyst used by nitrogen fixing organisms to convert atmospheric nitrogen into ammonia. Copper and zinc are important in other biological catalysts. Iron, zinc, copper, cobalt, nickel, manganese and molybdenum are known to be essential components of enzymes. Vanadium and chromium are also essential for life. Some bad elements are also present in this block. For example, mercury is toxic and is a threat to the environment.

General characteristics of d-block elements

The members of a given transition series are not as different from each other as the members of the same period's representative elements. In a transition series, there is no change in the number of electrons of the outermost shell and only change occurs in the (n-l)d electrons from member to member in a period. These elements show horizontal and vertical relationships or they show similarities in a period as well as in group. The general trends of some of the important properties are discussed here first in the respective periods and then in groups. 

  • 1. Metallic Character

  • All the transition elements are metals. They exhibit most of the properties of metals. They possess high density, high melting and boiling points, ductility, high tensile strength, metallic lustre, hardness, brittleness, malleability etc. They are good conductors of heat and electricity. They exhibit all the three types of structures: face centred cubic (FCC), hexagonal closed packed (HCP) and body centred cubic (BCC). These properties reveal that both metallic and covalent bonding is present in the atoms of d-block elements.

  • 2. Atomic Radii

  • The transition metal atomic radii are smaller than the corresponding s - block elements. The atomic radii of the elements of a given series decrease with an increase in the number of atoms, but this reduces after midway. In the beginning, the decrease in atomic radii in each series is due to an increase in nuclear charge from member to member, which tends to pull in the ns electrons, i.e. it tends to decrease the size. 

  • 3. Ionic Radii

  • The ionic radii generally decrease in the same oxidation state as the atomic number increases in a given transition series.

  • 4. Atomic Volumes and Densities

  • The atomic volumes of the d- block elements are low as compared with elements in neighbouring s-block elements. This is because the nuclear charge is poorly screened and so it attracts all the electrons more strongly.

  • 5. Melting and Boiling Points

  • The transition metals have very high melting and boiling points. The melting points of these metals rise to a maximum value in each series and then decrease as the number of atoms increases. Manganese and technetium, however, have an abnormally low fusion points.Tungsten has the highest melting point (3410°C) amongst the transition elements.

  • 6. Ionisation Energies

  • Most d-block elements have ionization energy values between those of s-and p-block elements. The elements of d-block are less electropositive than elements of s-block and more electropositive than elements of p-block. The transition elements are not as readily forming ionic compounds as the s-block elements. The increase in the ionisation potential values in a given transition series is explained on the basis of increasing nuclear charge and screening effect of (n-l)d electrons on ns electrons. With the increase of electrons in (n-l) d-subshell, the outer ns electrons are shielded more and more. Thus, the effect of significantly increasing the nuclear charge is somewhat neutralized by the extra screening effect and therefor, the potential for ionization increases but quite slowly between the d - block elements.

  • 7. Oxidation States

  • The variable oxidation states of a transition metal is due to the involvement of (n-1)d and outer ns electrons in bonding as the energies of ns and (n - 1)d subshells are nearly equal. The lower oxidation state is generally shown by when ns-electrons participate in bonding and higher oxidation states are exhibited when ns and (n - 1)d electrons take part in bonding.

  • 8. Magnetic Properties

  • Majority of substances show magnetic nature. These are either paramagnetic or diamagnetic. A paramagnetic substance is one which is weakly attracted in a magnetic field and a diamagnetic substance is one which is repelled by a magnetic field. The paramagnetic behaviour arises due to the presence of one or more singly occupied atomic orbitals, while diamagnetic behaviour is due to presence of paired electrons in the atomic orbitals.

    f-block elements

    28 elements from atomic number 58 to 71 (14 elements) and from atomic number 90 to 103 (14 elements) have been arranged in two horizontal rows below the Periodic Table. These elements are collectively called the f- block elements as the lastor differentiating electron in the atoms of these elements is accommodated on one of the seven f-orbital of the next penultimate energy shell. These elements have also been called inner transition elements because the ante-penultimate energy shell, i.e., (n -2)f-orbitals, lie comparatively deep within the kernel.

    f-block consists of two series of elements known as Lanthanides or Lanthanons and Actinides or Actinons.The lanthanide series follows lanthanum (at. No. 57) a member of the 5d-series. Similarly, actinide series comes after actinium (at. No. 89), a member of 6d-series. The 14 members of lanthanide series have been placed along with lanthanum in the third group and sixth period and similarly 14 members of the actinide series have been placed with actinium in the third group and seventh period. The justification for assigning one place to these elements has been given on the basis of their similar properties. The properties are so similar that the fifteen elements from La to Lu can be considered as equivalent to one element. The same explanation can be given in the case of actinides. In case, these elements are assigned different positions in order of their increasing atomic numbers, the symmetry of the whole arrangement would be disrupted. Due to this reason, the two series of elements, i.e., lanthanides and actinides are placed at the bottom of the periodic table and constitute one block of elements, i.e. the f-block. The general electronic configuration of the f-block elements is:

    (n-2)1-14(n-l)d0, 1ns2 or 4f1-145d0,16s2

  • (a) 4f-series (Lanthanides):

  • There are fourteen elements from cerium to lutetium in this series. The 4f-orbitals are gradually filled up. In the past, these elements were called as the rare earths. This name is not appropriate because many of the elements are not particularly rare. Promethium is an artificial radioactive element.

  • (b) 5f-series (Actinides):

  • There are fourteen elements from thorium to lawrencium in this series. The 5forbitals are gradually filled up. The members of actinium are radioactive and majority of them are not found in nature. The elements from atomic number of 93 onwards are called transuranic elements and have been discovered by synthetic methods, i.e., these are man-made elements.

    Similarities of lanthanides and actinides

  • i. The elements of both the series show mainly +3 oxidation state.

  • ii. The elements of both the series are electropositive in nature. They are reactive metals and act as strong reducing agents.

  • iii. In both the series, there is a contraction in atomic and ionic size as the atomic number increases, i.e. like lanthanide contraction, them is actinide contraction. These contractions arc due to the poor shielding effect between the electrons residing in (n-2)f orbitals.

  • iv. Cations with unpaired electrons in both the series are paramagnetic.

  • v. Most of the cations of lanthanides and actinides are coloured. The cations having the same number of unpaired electrons have almost the same absorption spectra. Sharp line-like bands are observed in both the series. These are due to the jump of an electron from one energy level to another within (n- 2)f-orbitals.

  • vi. The nitrates perchlorates and sulphates of trivalent actinides as well as lanthanides are soluble while the hydroxides, carbonates, fluorides of the elements of both the series are insoluble.

  • vii. Ion exchange behaviour is exhibited by both actinides and lanthanides.