General characteristics of d-block elements
1 Atomic and Ionic Radii
The atomic and ionic radii of transition elements are smaller than those of p- block elements and larger than those of s-block elements. The atomic and ionic radii of first transition elements are given in the List.
Element and Atomic (pm) radii
Sc - 144
Ti - 132
V - 122
Cr - 118
Mn - 117
Fe - 117
Co - 116
Ni - 115
Cu - 117
Zn - 125
The atomic radii of first transition series decreases from Sc to Cr and remains almost constant till Cu and then increases towards the end. This can be explained based on two effects namely screening and the nuclear charge effect. These two effects oppose each other resulting in increase in nuclear charge. Hence very slight variation in the atomic radii from Cr to Cu is observed.
It has been observed that Zirconium and Hafnium have almost equal atomic radii. This is because of lanthanide contraction.
2 Metallic character
All the transition elements are metals, since the number of electrons in the outermost shell is very small, being equal to 2. They are hard, malleable and 3. Formation of coloured ions
Most of the transition metal compounds are coloured in their solid or solution form. The colour of transition metal ions is due to the presence of unpaired electrons in it and the energy gap between two energy levels in the same d-subshell beingsmall. Hence very small amount of energy is required for excitation of electrons from one energy level to the other. The energy can be easily provided by the visible light. The colour observed corresponds to the complementary colour of the light absorbed.
It may be noted that Zinc, Cadmium and Mercury salts do not form any coloured compounds because of the absence of vacant d orbitals to which electrons can be excited. Sc3+ ions are also colourless because of the absence of d-electrons.
4 Catalytic Properties
Most of the transition metals and their compounds are used as catalyst.
The catalytic activity of transition metals is due to the following reasons.
i) They show a variety of oxidation states and thereby can form intermediate products with various reactants.
ii) They are also capable of forming interstitial compounds which can adsorb and activate the reacting species.
Some examples of catalyst are
i) Iron / molybdenum act as catalyst in the synthesis of ammonia by Haber's Process.
ii) Vanadium pentoxide (V2O5) is used for catalytic oxidation of SO2 to SO3
iii) TiCl4 is employed as a catalyst in the manufacture of polythene.
5 Variable oxidation states
Alltransitionelementsexhibitvarietyofoxidationstates(or)variablevalencies in their compounds. This property is due to the following reasons.
i) These elements have several (n - 1) d and ns electrons.
ii) The energies of (n - 1)d and ns orbitals are fairly close to each other.
Salient features of oxidation states of transition elements
1. The elements which exhibit the maximum number of oxidation states occur either in or near the middle of the series. For example , in the first transition series manganese exhibits maximum number of oxidation states (+2 to +7).
2. The elements in the beginning of the series exhibit fewer oxidation states because they have less number of d-electrons which they can lose or contribute for sharing. The elements at the end of the series exhibit fewer oxidation states, because they have too many d electrons and hence fewer vacant d-orbitals can be involved in bonding.
3. The transition elements in lower oxidation states (+2 and +3) generally form ionic bonds and in higher oxidation state form covalent bonds.
4. The highest oxidation state shown by any transition metal is +8. For example, ruthenium and osmium show highest oxidation states of +8 in some of their compounds.
5. Some transition metals show oxidation state of zero in their compounds. Ni(CO)4 and Fe(CO)5 are common examples.
6 Magnetic Properties
The magnetic properties of a substance are determined by the number of unpaired electrons in it. There are two main type of substances.
i) Paramagnetic substances
The paramagnetic character arises because of the presence of unpaired electrons. Paramagnetic substances are the substances which are attracted by magnetic field.
ii) Diamagnetic Substances
Diamagnetic character arises because of the absence of unpaired electrons.
Diamagnetic substance are the substances which are repelled by the magnetic field.
Most of the transition elements and their compounds are paramagnetic and are attracted by magnetic field. Greater the number of unpaired electrons in the substance greater is the paramagnetic character, The magnetic character of a substance is expressed in terms of magnetic moments. The magnetic moment can be calculated using the relation
µ = n(n + 2)BM (Bohr Magneton)
n = number of unpaired electrons
Ti3+ - The number of unpaired electrons is 1. Hence
µ = 1(1+ 2)BM = 3 = 1.732 B.M
Larger the value of magnetic moment, the greater is the paramagnetic character.
In addition to paramagnetic and diamagnetic substance, there are a few substances such as iron which are highly magnetic as compared to ordinary metals. These substances are called ferromagnetic substances
7 Complex formation
The cations of d-block elements have strong tendency to form complexes with certain molecules (e.g. CO, NO, NH3....etc) or ions (e.g. F-, Cl-, CN- ....etc) called ligands. Their tendency to form complexes is due to two reasons.
Small size and high positive charge density.
Presence of vacant (n-1)d orbitals which are of appropriate energy to accept lone pair and unshared pair of electrons from the ligands for bonding with them.
Examples of some complex compounds are,
[Cu(NH3)4]2+, [Ag(NH3)2]+, [Fe(CN)6]4-,....etc.
8 Formation of alloys
Transition metals form alloys with each other. This is because they have almost similar size and the atoms of one metal can easily take up positions in the crystal lattice of the other. Eg. Alloys of Cr-Ni, Cr-Ni-Fe, Cr-V-Fe, Mn-Fe......etc.
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