NEET Chemistry Notes The d- and f- Block Elements – General Properties of the Transition Elements

NEET Chemistry Notes The d- and f- Block Elements – General Properties of the Transition Elements

General Properties of the Transition Elements

General Properties of the Transition Elements

The general properties of d-block elements and their trends are discussed below:

 Atomic and Ionic Radii

•  In transition metals, on moving left to right in a period net nuclear charge increases due to poor shielding effect. Due to this, the atomic and ionic radii for transition elements for a given series show a decreasing trend for first five elements and then becomes almost constant for next five elements of the series. At the end of the period, there is a slight increase in the atomic radii.
• The atomic and ionic radii of the elements of 4 d-series are higher than corresponding element of 3d-series due to increase in number of shells. However, the elements of 4d-and 5d-series have almost constant values e.g. Zr radii -160 pm, Hf radii 159 pm. It is due to lanthanoid contraction.
  Nb and Ta, Mo and W also have nearly same size due to lanthanoid contraction.

 Enthalpies of Atomisation

• Transition elements exhibit higher enthalpies of atomisation because of large number of unpaired electrons in their atoms. Transition elements have very high melting and boiling points.

 Ionisation Enthalpies

• Ionisation enthalpy values for 5d-series are higher than that of 3d- and 4d-series due to weak shielding effect of 4f electrons present in 5d-series transition elements.
•  Ionisation enthalpy values of Zn, Cd and Hg are abnormally higher on account of greater stability of s-subshell.

Oxidation States

• Transition metals show variable oxidation states due to two incomplete outermost shells. Only stable oxidation states of the first row transition metals are
  
•  In each period, the highest oxidation state increases with increase in atomic number, attains a maximum value in the middle and then decreases, for example, Mn (3d-series), Ru (4 d-series) and Os (5d-series) has maximum value for oxidation state as + 7, + 8, + 8 respectively.
• The transition elements in their lower oxidation states (+2 and + 3) usually forms ionic compounds. In higher oxidation state compounds are normally covalent. Ni and Fei n Ni(CO)₄and Fe(CO)₅ show zero oxidation state.

Trends in the Standard Electrode Potentials

• Transformation of the solid metal atoms to M²⁺ ions in solution is their standard electrode potentials. If sum of the first and second ionisation enthalpies is greater than hydration enthalpy, standard potential
  will be positive and reactivity will be lower and vice-versa.

Trends in Stability of Higher Oxidation States

•  The’highest oxidation numbers are achieved in in TiX₄. VF₅ and CrF₆. The +7 state for Mn is not represented in simple halides but MnO₃F is known and beyond Mn no metal has a trihalide except FeX₃ and CoF₃. The increasing order of oxidising power in the series is
  
•  The highest oxidation states of transition metals are found in their compounds with fluorine and oxygen only because of their higher electronegativity and smaller atomic size.

Magnetic Properties

• The magnetic properties of d-block elements are due to only spin of unpaired electrohSiahe magnetic moment is determined by the numbers bf unpaired electrons (n) which is given by
•  When a magnetic field is applied to substances, mainly two types of magnetic behaviour are observed : diamagnetism and paramagnetism. Paramagnetism is due to the presence of unpaired electrons. Each such electron having a magnetic moment associated with its spin angular momentum. If all electrons are paired, substance will be diamagnetic and magnetic moment will be zero.

Formation of Coloured Ions

• The d-orbitals are non degenerated in the presence of ligands. When an electron from a lower energy d-orbital is excited to a higher energy d-orbital (d-d* transition) the energy of required wavelength is absorbed and rest light is transmitted out. Therefore, the colour observed corresponds to the complementary colour of the light absorbed.
•  In V₂0₅, V is in +5 oxidation state. In spite of d° it is coloured compound. This colour arises due to defects in the crystal lattice and charge transfer.

 Formation of Complex Compounds

•  Transition metals have small size and irigli nuclear charge which facilitate the acceptance of lone pair of electrons from ligands.
• They have vacant d-orbitals of appropriate energy in order to accommodate the lone pair of electrons.

Catalytic Properties

• Transition metals have two incomplete outermost shells and ability to adopt multiple oxidation states as Well as forming complexes, therefore used as a catalyst.
•  Transition metals also provide larger surface area for the reactant to be adsorbed.

Formation of Interstitial Compounds

• Small atoms of non-metals (H, C, N) fit into the voids of crystalline solid of transition metals and form interstitial compounds. The principal physical and chemical characteristics of these compounds are as follows:
•  They have high melting points, higher than those of pure metals.
• They are very hard. Some borides approach diamond in hardness.
• They retain metallic conductivity.
• They are chemically inert.

 Alloy Formation

•  Alloy is the homogeneous solid solution of two or more metals. Transition metals have approximately the same size, therefore in molten form they can fit to each other’s crystalline structure and form homogeneous mixture and the alloy, e.g. brass (copper-zinc) and bronze (copper-tin) etc.

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