Vriti EdustoreD and F block elements:(transition elements)
D and F block elements: The elements that lie in between S-block and P-block are the d-block elements. These elements are called transition elements as they show transitional properties between s and p-block elements. These elements contain partially filled d-orbitals and hence they are called as d-block elements. The f-block of the periodic table of the elements consists of those elements whose atoms or ions have valence electrons in f-orbitals. Actual electronic configurations may be slightly different from what is predicted by the aufbau principle. The elements are also known as inner transition elements. GENERAL CHARACTERISTICS OF TRANSITION ELEMENTS (i) Except for mercury, which is a liquid at room temperature all other elements are solid metals exhibiting all the characteristics of a metal. (ii) The show variable oxidation states unlike s and p block elements. (iii) They, and some of their compounds, show catalytic properties. (iv) Their compounds are coloured. (v) They have great tendency to form complex compounds. (vi) They form alloys and interstitial compounds. Conductivity All the transition metals are good conductors of heat and electricity. Silver is the best conductor of electricity. Density Because of small size of their atoms and strong metallic bonding the density and hardness of transition elements are high. Ionization Energy The ionization energy (IE) of transition elements are higher than those of s-block elements but lower than p-block elements. In a particular transition series, ionization energy although increases gradually as we move from left to right but this increase is not appreciable.
The increase in ionization energy is due to increase in nuclear charge, the effect of increase in nuclear charge is partly balanced by the increase in screening effect. Consequently, the increase in ionization energy along the period of d-block elements is very small
Why do transition elements show variable oxidation states?
Solution:In the transition elements, the energies of (n-1)d orbitals and ns orbitals are very close. Hence electrons from both can participate in bonding.
Illustration 6:Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number
COMPLEXES AND THEIR PROPERTIES
The transition elements have an unparalleled tendency to form coordination compounds with the Lewis bases, which are called as ligands.
CO3+ + 6NH3 ————→ [CO(NH3)6]3+
Fe2+ + 6CN– ————→ [Fe(CN)6]4–
s and p block elements form very few complexes. The reason transition elements are so good at forming complex is that they have small, highly charged ions and have vacant low energy orbitals to accept lone pairs of electrons donated by ligands.
Size of Atoms and Ions
The covalent radii of the elements decrease from left to right across a row in the transition series. This is because of the poor screening by the d electrons due to which, the nuclear charge attracts all of the electrons more strongly, hence a contraction in size occurs.
The elements in the first group in the d-block show the excepted increase (due to the addition of extra shell) in size Sc → Y → La. However in the subsequent groups there is an increase between first and second members, but hardly any increase between second and third elements. This is due to lanthanide contraction (discussed in f-block elements).
Magnetic Properties
On the basis of behaviour in a magnetic field, substance are classified as paramagnetic, diamagnetic and ferromagnetic. Those substance which are attracted by the applied magnetic field are called paramagnetic where as those which are repelled by the magnetic field are called diamagnetic. Substances which are very strongly attracted by the applied field are called ferromagnetic.
Paramagnetism is a property due to the presence of unpaired electrons. Thus most of the transition metals are paramagnetic. As the number of unpaired electrons increases, the paramagnetic character also increases.
The magnetic moment is calculated from the following formula μ = √n(n+2) BM where n is the number of unpaired electrons and B. M stands for Bohr magneton.
Illustration 10:Why does Mn(II) show maximum papamagentic character amongst the bivalent ions of the first transition series?
Solution:Mn2+ has maximum number of unpaired electrons i.e. 3d5.
Illustration 11:A substance is found to have a magnetic moment of 3.9 B.M. How many unpaired electrons does it contain?
Solution:Using the formula, μ = √n(n+2), B.M. n = 3
Catalytic Properties
Many transition metals and their compounds have catalytic properties. For e.g. V2O5, Fe, FeCl3, Ni, Pd etc.
This property of transition elements is due to their variable oxidation states. In some cases the transition metals with their variable valency may form variable unstable intermediate compounds. In other cases the transition metal provides a suitable reaction surface.
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